Recent Progress in Lead-Free Solder Technology: Materials Development, Processing and Performances (Topics in Mining, Metallurgy and Materials Engineering) 3030934403, 9783030934408

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Topics in Mining, Metallurgy and Materials Engineering Series Editor: Carlos P. Bergmann

Mohd Arif Anuar Mohd Salleh Mohd Sharizal Abdul Aziz Azman Jalar Mohd Izrul Izwan Ramli   Editors

Recent Progress in Lead-Free Solder Technology Materials Development, Processing and Performances

Topics in Mining, Metallurgy and Materials Engineering Series Editor Carlos P. Bergmann, Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil Editorial Board Jorge R Frade, Escola Superior Aveiro-Norte, Universidade de Aveiro, Oliveira de Azemeis, Portugal Juan Bautista Carda Castelló, Departament de Química Inorgànica i Orgànica, Universitat Jaume I, Castellón de la Plana, Valencia, Spain Raul Bolmaro, Rosario, Argentina Vincenzo Esposito, Kgs. Lyngby, Denmark

“Topics in Mining, Metallurgy and Materials Engineering” welcomes manuscripts in these three main focus areas: Extractive Metallurgy/Mineral Technology; Manufacturing Processes, and Materials Science and Technology. Manuscripts should present scientific solutions for technological problems. The three focus areas have a vertically lined multidisciplinarity, starting from mineral assets, their extraction and processing, their transformation into materials useful for the society, and their interaction with the environment. ** Indexed by Scopus (2020) **

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Mohd Arif Anuar Mohd Salleh · Mohd Sharizal Abdul Aziz · Azman Jalar · Mohd Izrul Izwan Ramli Editors

Recent Progress in Lead-Free Solder Technology Materials Development, Processing and Performances

Editors Mohd Arif Anuar Mohd Salleh Faculty of Chemical Engineering Techonology Universiti Malaysia Perlis Jejawi, Perlis, Malaysia Azman Jalar Institute of Micro Engineering and Nanoelectronics Universiti Kebangsaan Malaysia UKM Bangi, Malaysia

Mohd Sharizal Abdul Aziz School of Mechanical Engineering Universiti Sains Malaysia Penang, Malaysia Mohd Izrul Izwan Ramli Center of Excellence Geopolymer and Green Technology Universiti Malaysia Perlis Jejawi, Malaysia

ISSN 2364-3293 ISSN 2364-3307 (electronic) Topics in Mining, Metallurgy and Materials Engineering ISBN 978-3-030-93440-8 ISBN 978-3-030-93441-5 (eBook) https://doi.org/10.1007/978-3-030-93441-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

This book highlights recent research progress in lead (Pb)-free solder technology, focusing on materials development, processing, and performances. It discusses various Pb-free solder materials’ development, encompassing composite solders, transient liquid phase sintering, and alloying. The book also details various Pbfree solder technology processing and performances, including flux modification for soldering, laser soldering, wave soldering, and reflow soldering, while also examining multiple technologies pertaining to the rigid and flexible printed circuit board (PCB). Some chapters explain the characterization and modeling techniques using computational fluid dynamics (CFD). Recent progress in lead-free solder technology book offers detailed chapters on recent research works in the area of lead-free solder technology. The book is divided into two parts: Part I: Development of Materials and Part II: Processing and Performances. This book serves as a valuable reference for researchers, industries, and stakeholders in advanced microelectronic packaging, emerging interconnection technology, and those working on lead-free solder. Authors hope that the chapters of this book will provide readers with valuable insight into the lead-free solder technology. In the other hand, it is also possible that some topics are not included due to the constraints of the book size. Perlis, Malaysia Penang, Malaysia Bangi, Malaysia Perlis, Malaysia

Mohd Arif Anuar Mohd Salleh Mohd Sharizal Abdul Aziz Azman Jalar Mohd Izrul Izwan Ramli

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Acknowledgements

We would like to acknowledge the support provided by the Tin Solder Technology Researchers Group (TSTRG) Malaysia, Universiti Malaysia Perlis, Universiti Sains Malaysia, and Universiti Kebangsaan Malaysia. We would also like to express our sincere gratitude to people providing support directly and indirectly in writing and editing the book. The support from all of our research collaborators including institutions and industries are very much appreciated. We would also want to thank our parents, wife, children, and friends, who supported and encouraged us in spite of the hard time during the Covid-19 pandemic. Thank you for all your support, it really means a lot to us. Lastly, we would like to apologize if any names we have failed to mention.

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Contents

Development of Materials Recent Studies in the Development of Ceramic-Reinforced Lead-Free Composite Solder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Norainiza Saud, Mohd Arif Anuar Mohd Salleh, Rita Mohd Said, Flora Somidin, Nur Syahirah Mohamad Zaimi, and Mohd Izrul Izwan Ramli Development of Geopolymer Ceramic-Reinforced Solder . . . . . . . . . . . . . . Mohd Mustafa Al Bakri Abdullah, Mohd Arif Anuar Mohd Salleh, Nur Nadiah Izzati Zulkifli, Nur Syahirah Mohamad Zaimi, Romisuhani Ahmad, Noorina Hidayu Jamil, Ikmal Hakem Aziz, and Mohd Izrul Izwan Ramli Surface Modifications on Ceramic Reinforcement for Tin-Based Composite Solders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Leong Wai Keong, Ahmad Azmin Mohamad, and Muhammad Firdaus Mohd Nazeri Molecular Dynamic of the Nanoparticle Reinforcement in the Pb-Free Solder During Reflow Soldering Process . . . . . . . . . . . . . . . Mohd Sharizal Abdul Aziz, I. N. Sahrudin, M. S. Rusdi, M. H. H. Ishak, C. Y. Khor, and Mohd Arif Anuar Mohd Salleh Recent Progress in Transient Liquid Phase (TLP) Solder for Next Generation Power Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flora Somidin, Rita Mohd Said, Norainiza Saud, and Mohd Arif Anuar Mohd Salleh

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Advanced Assembly of Miniaturized Surface Mount Technology Components Using Nano-reinforced Solder Paste . . . . . . . . . . . . . . . . . . . . . 113 F. C. Ani, A. A. Saad, A. Jalar, C. Y. Khor, M. A. Abas, and Z. Bachok

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Properties of Sn0.7Cu Solder Alloys Bearing Fe and Bi . . . . . . . . . . . . . . . . 133 Mohd Faizul Mohd Sabri, Mohd Faiz Mohd Salleh, Syed Hassan Abbas Jaffery, and Mohammad Hossein Mahdavifard Processing and Performances The Effect of Isothermal Ageing Treatment on Different PCB Surface Finishes: Simulation and Experimental . . . . . . . . . . . . . . . . . . . . . . 171 F. Muhamad Razizy, N. Zhen Zhang, M. S. Hashim, O. Saliza Azlina, and O. Shahrul Azmir Flux Modification for Wettability and Reliability Improvement in Solder Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 N. Ismail, A. Jalar, M. A. Bakar, and A. Atiqah Advancement of Printed Circuit Board (PCB) Surface Finishes in Controlling the Intermetallic Compound (IMC) Growth in Solder Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 A. Atiqah, A. Jalar, M. A. Bakar, and N. Ismail Significance of Intermetallic Compound (IMC) Layer to the Reliability of a Solder Joint, Methods of IMC Layer Thickness Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 M. A. Bakar, A. Jalar, A. Atiqah, and N. Ismail The Effect of Laser Soldering onto Intermetallic Compound Formation, Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Siti Rabiatull Aisha Idris, Nabila Tamar Jaya, and Muhammad Asyraf Abdullah Reliability Analysis on the Flexible Printed Circuit Board After Reflow Soldering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Muhammad Iqbal Ahmad, Mohd Sharizal Abdul Aziz, and C. Y. Khor Solder Paste’s Rheology Data for Stencil Printing Numerical Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 M. S. Rusdi, M. Z. Abdullah, Mohd Sharizal Abdul Aziz, S. A. H. A. Seman, and M. H. Hassan Tin Whiskers Growth in Electronic Assemblies . . . . . . . . . . . . . . . . . . . . . . . 311 M. S. Chang, Mohd Arif Anuar Mohd Salleh, D. S. C. Halin, and N. Z. Mohd Mokhtar

About the Editors

Mohd Arif Anuar Mohd Salleh (Ph.D.) is an Associate Professor with the Materials Engineering Programme at the Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis. He graduated with a B.Eng (Hons) in Mechanical Engineering (2006), followed by M.Eng in Mechanical Engineering majoring in Materials (2007) at the Universiti Tun Hussein Onn Malaysia. He received his Ph.D. in 2016 from the University of Queensland, Australia, in Materials Engineering, specializing in the development of advanced solder materials. He is also a certified Professional Engineer and a corporate member of the Institute of Engineers, Malaysia (IEM). He is currently the President of Tin Solder Technology Research Malaysia under the Tin Industry Board (Research and Development), Malaysia. He worked in and lectured on the subject of electronic packaging materials for more than 13 years. Before joining Universiti Malaysia Perlis, he was employed as a Failure Analysis Engineer at Intel Malaysia. He also worked as a part-time research officer for a few research projects on solder materials development at the University of Queensland Australia (2013–2015) and Imperial College London (2015). He has published more than 180 articles, encompassing proceedings, journals, books, and modules as primary author and co-author, and has an H-index of 16. His research endeavors were significant and impactful, as reflected in his receiving grants worth RM3 million. Mohd Sharizal Abdul Aziz received the B.Eng., M.Sc., and Ph.D. degrees from the Universiti Sains Malaysia, in 2006, 2012, and 2015. In June 2016, he became a Senior Lecturer at the School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Penang, Malaysia. Sharizal is the author or co-author of more than 100 papers in the international journal, book chapter, and conference proceeding with the current H-index 9. He has given numerous invited/ keynote talks at international conferences. Sharizal also organized workshops on Soldering and SMT processes for academics and professionals in collaboration with the microelectronics industries in Malaysia. His research interest is in Thermofluids and Computational Fluid Dynamics, focusing on Soldering, Surface Mount Technology, and Advanced Packaging. He is also registered with the Board of Engineers Malaysia and a fellow Electronics Packaging Research Society, Malaysia. xi

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About the Editors

Azman Jalar (Ph.D.) is a professor of applied metallurgy from the Faculty of Science and Technology, and Principal Research Fellow in electronics packaging at the Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia. Graduated with Ph.D. degree in metallurgy and materials from School of Metallurgy and Materials, University of Birmingham, UK in 2001. His interest in microstructure-properties-performance or materials science paradigm motivates him to conduct research in electronic packaging since 2002. He also interested in electronic materials, nanomaterials, materialography and stereometry. He has significantly contributed in solving many industrial-related semiconductor packaging problems through industry-driven research activities. Past and current research projects include packaging materials, solder materials and soldering, wire bonding and package architecture. Past projects collaborated with Freescale, OnSemi, AIC semiconductor, Celestica, Jabil electronics, and Infineon, and ongoing projects with Nexperia, Western Digital, and RedRing Solder. Mohd Izrul Izwan Ramli received his Ph.D. from the Universiti Malaysia Perlis (UniMAP) Malaysia in materials engineering majoring in the development of solder materials. Now he works as a postdoctoral researcher in Universiti Malaysia Perlis (UniMAP) working on several projects related to the development of solder materials. His current research interest is in materials engineering and focusing on the development of lead-free solder alloys for electric/electronic interconnects. His research activities include using advance material characterizations techniques such as the synchrotron imaging, synchrotron XRF, and synchrotron tomography and have contributed to several leading discoveries in solder alloy development. He has published 57 journals in leading journals in his research area with total citation of 179 and H-index: 9. He also received several international research grants as co-researcher during his studies and postdoctoral period. He has now more than 5 patents and copyrights in the lead-free solder alloy area.

Development of Materials

Recent Studies in the Development of Ceramic-Reinforced Lead-Free Composite Solder Norainiza Saud, Mohd Arif Anuar Mohd Salleh, Rita Mohd Said, Flora Somidin, Nur Syahirah Mohamad Zaimi, and Mohd Izrul Izwan Ramli Abstract Until today, there have been abundant research studies conducted by various researchers to improve the existing lead-free solders’ properties with the current electronics packaging desired performances. One of the leading choices in upgrading the existing lead-free alloys is by ceramic composite technology approach. The addition of ceramic particles into lead-free solder has altered and subsequently improved monolithic solder’s microstructural, physical, thermal, electrical and mechanical properties. The ideal way to reincorporate ceramic powder particulates into the monolithic solder matrix is by utilising powder metallurgy (PM) routes under a solid state, consisting of mixing, compaction and sintering. Regarding the sintering step, microwave-assisted sintering technology consumes much lower energy with lesser heating time compared to the conventional sintering method. Generally, finer microstructure distribution enhanced the mechanical properties of the ceramic-reinforced composite material. Self-generated heat produced by sintered material makes microwave technology a great assistant in sintering processing. Some of the limitations and advantages in ceramic composite lead-free solder’s development are also described. Keywords Lead-free solder · Composite solder · Powder metallurgy · Properties

1 Introduction The present work continues to find the replacement for lead solders known for their excellent properties. However, despite the characteristic of minimal-cost material, corrosion resistance, marvellous thermo-mechanical fatigue resistance and good wettability, lead (Pb) had become the top on the list of prohibited hazardous substances. Therefore, the prohibition of lead (Pb) solders necessitated researchers N. Saud (B) · M. A. A. Mohd Salleh · R. Mohd Said · F. Somidin · N. S. Mohamad Zaimi · M. I. I. Ramli Electronics Packaging Materials Research Group, Centre of Excellence Geopolymer and Green Technology (CEGeoGTech), Universiti Malaysia Perlis, Arau, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 M. A. A. M. Salleh et al. (eds.), Recent Progress in Lead-Free Solder Technology, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-3-030-93441-5_1

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in electronic packaging manufacturers to scrutinise the alternative lead-free solders. Apart from that, researchers also need to face the scenario where electronics equipment tends to be miniaturised. Thus, a number of Joule heat will be generated on circuit chips, impacting the thermal reliability. Due to that, the new lead-free solder alternative should possess a good thermal reliability characteristic. Many studies have been carried out to find a new candidate for lead-free solder with good reliability, especially related to thermal stability. Among them, composite solder has been introduced. Composite solder is defined as the solder that is produced from two or more constituents of materials, and the properties of the resulted solder will be unlike those individual constituents. Interestingly, the properties of the composite solder could overcome the disadvantageous properties of the individual constituents. The constituents incorporated into lead-free solder could be metal particles, intermetallic compound particles (IMC), ceramic particles and so on. While the methods to incorporate the constituents into lead-free solder would be manual mixing, mechanical mixing, powder metallurgy mixing and melting technique. Out of all the promising constituents, lead-free solder incorporated ceramics, also known as ceramic composite solder, was reported to have good thermal stabilities, service temperature, as well as excellent mechanical properties (Vaidya & Pathak, 2019; Ma & Suhling, 2009). Researchers have studied a variety of ceramic composite solder. A wide range of reinforcement materials has been applied to a number of promising lead-free solders. Whether in nano size, micron size or even larger type of reinforcement, they have improved in so many ways when incorporated into lead-free solders. Therefore, this chapter will discuss the roles of the reinforcing ceramic materials, the fabrication method and the current development of the ceramic composite solder in electronics industries.

2 Materials Selection The main objective of the ceramic composite solder approach is to engineer and improve the properties of existing lead-free solder alloys. Therefore, proper selection of reinforcement particles should be considered. Moreover, the method used to develop composite solders should not alter the current metallurgical process in electronic manufacturing. Therefore, this topic will cover the factors and properties of solder matrix and the reinforcement particles that need to be considered in developing ceramic composite solders.

2.1 Physical Properties In developing a composite solder, the most important criteria to consider is choosing a compatible reinforcement particle to the solder alloy matrix. Here, the reinforcement

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particle should be bond to the solder alloy matrix. The bonding between the reinforcement particles and solder alloy matrix can be either strong or weak, depending on the final applications of the composite solder itself. Moreover, the reinforcement particles should have no or minimal solubility in the molten solder alloy subjected to reflow soldering. This is important in maintaining the stability of the reinforcement particles under the normal reflow temperature. In addition, the density of reinforcement particles and solder alloy should be taken into account. The uniform distribution of reinforcement particles in the solder alloy matrix depends on the density values. The density of reinforcement particles should be closely matched with the density of the solder alloy matrix. The bigger the differences between reinforcements implies that solder alloy density could settle or float the reinforcement particles during the mixing process. This causes the segregation of reinforcement particles within the solder alloy matrix, resulting in non-uniform distribution. Moreover, the reinforcement particles size should be optimal and uniform. The addition of micron size of reinforcements particles had been reported vastly by many researchers (Wang et al., 2009; Guo et al., 2001a, 2001b). It was found that the addition of micron-size particles could improve the creep properties as compared to non-reinforced lead-free solder alloy (Shi et al., 2008). Researchers have attempted to incorporate nano-size particles with the improvement proved by the micron size reinforcement particles, especially with the ceramic particles. Tsao et al. reported that the addition of nanosize of titanium dioxide (TiO2 ) particles had improved the grain size and morphology of Ag3 Sn. This could lead to an increase in the microhardness, yield strength and ultimate tensile strength. Besides that, the other criteria that need to be satisfied in choosing compatible reinforcement particles are the wettability of the reinforcement particles to the solder matrix (Shen & Chan, 2009). The interfacial strength between reinforcement particles and solder matrix could modify the overall mechanical behaviour of the composite solders. Therefore, the reinforcement particles added should be wetted by the solder matrix with the formation of reliable bonding. The reliable bonding between the reinforcement particles and solder matrix was vital to avoid the formation of gas pores (Shen & Chan, 2009). Moreover, internal cracks commonly occur during the pressing, where the rolling process can be avoided with reliable bonding between reinforcement particles and solder matrix. Apart from that, the wettability of composite solder to the substrate should also be considered. The molten composite solder alloy needs to be wetted well on the solid substrate to form a reliable metallurgical bonding with the ideal solder joint. The wettability can be measured through the contact angle between solder and substrate, as shown in Fig. 1. Generally, if the contact angle lies between 0° and 90°, the system is wetted. However, if the contact angle is between 90° and 180°, the system is said to be non-wetted (Abtew & Selvaduray, 2000). Besides that, in the study, Sabri et al. (2015) suggested that if the wetting angle between solder and substrate is 0° ≤ 8 ≤ 20°, the wetting quality is considered very good (Sabri et al., 2015). Meanwhile, if the wetting angle is between 20° ≤ 8 ≤ 40°, the wetting quality is considered good and still acceptable. On the other hand, if the wetting angle formed exceeded 40°, it is not acceptable in the solder system.

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Fig. 1 Schematic diagram of wetting properties

In the development of composite solder, excess addition of the reinforcement in the lead-free solder could deteriorate the wettability of the solder (Zhang & Tu, 2014). In terms of solder processing and operating conditions, the reinforcements particles added in the solder alloy should not modify the existing processing and operating temperature in electronic manufacturing. In choosing compatible reinforcement particles, the reinforcement particles should not significantly increase the melting temperature of solder alloys. Owing to the fact that the melting temperature of solder alloy could affect the soldering process and the performance of the electronic components. For instance, during reflow soldering, the soldering process temperature would increase by 25 °C higher than the melting temperature. Thus, a higher melting temperature needs a higher reflow soldering process temperature. However, a high soldering temperature beyond the existing soldering parameter could damage the typical components in the electronics. Therefore, composite solder developed should work at a similar temperature as the existing solder alloy to avoid damages and high operating costs.

2.2 Mechanical Properties Mechanical properties of a material can be defined as those that affect the mechanical behaviour when a material exhibits the application of forces. Besides that, mechanical properties also refer to the mechanical strength and the ability of a material to be moulded in a suitable shape (Murugan, 2020). The application of the force causes changes in the physical appearance of the material. Different type of material displays different behaviour once the load was released. Therefore, the fundamental knowledge of the selected reinforcement’s mechanical properties is crucial to produce a solder that suits their mechanical properties requirement. Recently, electronic packaging industries are facing the demand for higher density interconnections in electrical or electronic products. These higher density interconnections give rise to the decrement in the size of the solder interconnections. Solder interconnects in the electronic devices play a role by providing electrical and mechanical connections

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between components and the substrate. Therefore, very reliable solder interconnections were demanded, which determines the overall functionality of the electronic products. The electronic devices also were likely exposed to drop or impact during manufacturing, handling and services. Thus, maintaining and improving the solder joint strength or interconnections in a lead-free solder before applying to targeted applications needs to be concerned. In addition, the solder interconnects in the electronic devices were linking to the formation of the intermetallic compound (IMC) layer. The strength of the solder joint depended on the thickness of the IMC layer. As the IMC layer was getting thicker, the strength of the solder joints can be decreased. Moreover, the IMC layer can be a crack initiation site. Therefore, by controlling the thickness of the IMC layer in the solder joints, reliable solder interconnects in the electronic devices can be produced. One viable way to improve the mechanical strength and properties of the solder joints was by introducing reinforcement particles, forming composite solder. Therefore, this subtopic will discuss the improving mechanical properties of the lead-free solder alloy through composite technology. The purpose of employing the composite technology approach was to enhance the mechanical properties of existing lead-free solder alloy through the strengthening effect imposed by reinforcements particles added. The reinforcement particles should be fine and homogeneously distributed in the solder alloy to have an effective strengthening effect in the solder alloy. The reinforcement particles incorporated in the solder alloy should be able to lock the grains boundary of the solder from grain boundary sliding and obstruct the dislocation movement. Shen and Chan (2009) stated that the reinforcement particles should have low interfacial energy and low diffusivity in the solder matrix to ensure the dimensional stability of the reinforcement particles in the solder alloy. Moreover, to effectively obstruct the dislocation motions, the reinforcement particles added should have sufficient fine size particles, greater flow resistance than the solder matrix and be able to resist the fracture. Besides that, in dealing with the mechanical properties, various forces or stresses normally act on the solder connection, including tensile, shear, bending, compressive and shear stresses, as illustrated in Figs. 2 and 3. Therefore, to evaluate and elucidate the mechanical Fig. 2 Various applied forces that are subjected to materials during real application

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Fig. 3 The stresses that are acting on the materials

properties of the composite solder, several mechanical testings, including shear test, tensile test and microhardness test, can be performed.

2.3 Cost To develop a composite solder, the cost of solder materials and reinforcement particles selected should be taken into consideration. A comparison of several reinforcement particles’ costs is shown in Table 1. The cost for the reinforcement particles given is based on the price from (“Chemicalbook,”; “MiliporeAldrich,”). However, the cheapest cost for the reinforcement particles would not be the major criteria Table 1 Reinforcement particles price as of 2021 (“Chemicalbook,”; “MiliporeAldrich,”)

Reinforcements

Price ($US)

Lanthanum oxide (La2 O3 )

279/25g

Iron (III) oxide (Fe2 O3 )

119/25g

Titanium carbide (TiC)

164/25g

Zinc oxide (ZnO)

92.20/50g

Silicon dioxide (SiO2 )

100/50g

Titanium oxide (TiO2 )

233/100g

Aluminium oxide (Al2 O3 )

283/100g

Silicon carbide (SiC)

553/100g

Cerium oxide (CeO2 )

265/100g

Silicon nitride (Si3 N4 )

233/100g

Zirconium oxide (ZrO2 )

48/100g

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in determining and selecting the type of reinforcement particles used in composite solder.

3 Fabrication Technique 3.1 Casting The manufacturing process for making complex shapes that other methods would be difficult or uneconomical to make is known as casting. Generally, casting is a process in which the materials are mixed and heated in a furnace. Then, in the form of molten metal they are usually poured into a mould, which contains a hollow cavity and normally has a variety of shapes. Thus, casting involves pouring molten metal into a mould, letting the metal cool and solidify, and finally removing the part from the mould (Kosky et al., 2013). The basic steps in the casting process include patternmaking, coremaking, moulding, melting and pouring, and finishing (Fig. 4). Solder composite fabrication starts with the preparation of parent alloy and reinforcing particle. The process is quite similar to the fabrication of solder alloy. In fabricating a solder alloy, for instance, the casting process of Sn–Cu alloy with Cr. Fig. 4 The basic process involved in the casting process

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The Sn–Cu alloy, pure Sn and Cu were first dissolved at 500 °C in a vacuum furnace filled with argon gas and further cooled in air. The Sn–Cu alloy and Cr were then dissolved together. The heating process was carried out at 1100 °C, and the Sn– Cu alloy and Cr were mixed thoroughly for 20 min. After that, the mixed alloy then cooled down to 600 °C and then subsequently air-cooled to 50 °C. After the alloying stage, the molten solder in the crucible was homogenised at 600 °C for 1 h (Bang et al., 2019). Meanwhile, in fabricating a composite solder, for example, Jung et al. had studied the influence of dual ceramic nanomaterials on the solderability and interfacial reactions between lead-free Sn–3.0Ag–0.5Cu (SAC305) and a Cu conductor. The fabrication of SAC305 added with dual ceramic nanomaterials (TiO2 and graphene) was also carried out via the casting process. The nano-composite solder was cast in an alumina crucible and cooled down to room temperature (Jung et al., 2018). However, for composite solder, the TiO2 or graphene particles were not melted and react with the solder matrix, as for Cr particles. Instead, they were distributed in the solder matrix and acted as the reinforcement to enhance the properties of the solder matrix. The solder composite produced via a casting process will proceed to the next process depending on the testing that needs to be carried out. For example, molten metal is poured into the stainless-steel sheet. Then, it is followed by cold roll milling to produce the sheet metal rolled, then punching the sheet metal and reflowed soldering to produce a solder ball.

3.2 Powder Metallurgy Powder metallurgy is the most common method used in fabricating composite solder. It includes three steps: blending/mixing, compaction and sintering. The powder metallurgy process offers very good potential in improving the mechanical, physical and chemical properties of composites materials (Idris & Kabir, 2001). Thus, the application of the powder metallurgy method in composite solder fabrication was advantageous to produce the better performance of composite solder. The first step in the powder metallurgy was the mixing process. The beginning process in the mixing involves precisely weighing the solder alloy and reinforcement particles. Next, the solder alloy and reinforcement materials were mixed in an air-tight container to produce a homogenous mixture. Several variables need to be considered in the mixing process, such as the particle size, ratio of powder volume, rotation speed, and time taken for the mixing process (Mohd Salleh et al., 2013). These variables were very important in producing a homogenous mixture and avoiding contamination with the formation of oxides on the material’s surface. The second step in powder metallurgy is the compaction process. First, the homogenous mixture powder was compacted in a die cavity with an application of high pressure to produce the desired shape. Then, a certain amount of load was applied by a movable punch to press the mixture powder together, producing a green (unsintered) compact pellet at room temperature. The densification of mixture powder occurred under uniaxial pressure from both the top and bottom planes of the die. In

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a compaction process, several variables, which are the sizes and shapes of particles, need to be considered as they will affect the success of the process. For example, in terms of powder shape, the spherical shape was harder to compact than the spongy shape (Mazur & Hebda, 2018). The final step in powder metallurgy involves the sintering process. Sintering is a process in which a green compacted pellet was bonded/sintered at a temperature below the melting point of the major constituents (Reardon, 2010). The sintering temperature is usually at 70–80% of melting temperature to produce a solid-state bonding (Black & Kohser, 2008). The bonding between the mixture powder particles and the shrinkage of pores during the sintering process provides more densification to the compact green pellet. Figure 5 shows the mechanism of particle bonding during the sintering process. At the early stage of the sintering process, particles will make contact, followed by the neck formation in between the particles. Prolonged sintering time, the neck will eventually grow larger. At the final stage, the particles were fully merged and forming larger particles with a diameter approximately 1.26 bigger than the diameter of the initial particles. A common sintering process is conventional sintering (e.g., resistance heating). The conventional sintering took place in a normal furnace and consumed higher energy (Oghbaei & Mirzaee, 2010). In conventional sintering, the heat was generated through the heating of filaments and was transferred to the samples via convection, conduction and radiation. Then in terms of the heating direction in conventional sintering, it occurred from outside to the inside part of the green compacted pellets, which could result in poor microstructure formation to the core of the pellets. In addition, factors such as sintering time and temperature are the most vital things that need to be considered (Reardon, 2010). In the conventional sintering, a longer sintering time was needed to assure the homogenisation of temperature, which then exposed the compact green pellets to oxidation. Due to this reason, microwave sintering was introduced. Microwave sintering provides special heating characteristics such as high energy efficiency and rapid heating. The heating direction in the microwave sintering occurred from the inside to the outside part of the green compacted pellets. Using microwave sintering offers many advantages compared to conventional sintering, such as reducing energy consumption, using much lower sintering temperature with shorter sintering time, improved diffusion process, simplicity and rapid heating rate (Oghbaei & Mirzaee, 2010). Two methods exist in microwave heating which are direct microwave heating and microwave hybrid technique. The green compacted pellet was placed in a microwave indirect microwave, and the pellets were exposed to microwave energy. However, the direct microwave heating caused thermal instability during the process, leading to overheating of samples catastrophically. Therefore, a microwave hybrid sintering technique was introduced to improve the heat distribution to the compact green pellet. The term “hybrid” referred to the heating mechanism in which a two-directional sintering technique was employed. The schematic diagram of the hybrid microwave assisted sintering is shown in Fig. 6. The microwave’s energy help to sinter a green compact pellet from inside to the outside direction. Meanwhile, a susceptor material helps to sinter from the inside to the outside direction. The function of the susceptor material was to prevent heat loss during the sintering process.

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Fig. 5 Mechanism of particle bonding during the sintering process

The susceptor material which is having high dielectric loss at room temperature will assist the sintering process. In addition, the hybrid microwave sintering method was successfully developed by Gupta and Wong and using a commercial microwave oven and susceptor material in the form of SiC powder. The study reported that the utilisation of microwave hybrid sintering techniques could provide superior tensile properties (Gupta & Wong, 2005). Therefore, the utilisation of hybrid microwave sintering technique was very recommended in fabricates composite lead-free solder.

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Fig. 6 Schematic diagram of hybrid microwave-assisted sintering

4 Characterisation Several characterisation techniques can be carried out to determine the properties of ceramic-reinforced lead-free solder. Many researchers have investigated and developed a ceramic composite solder that could meet the requirement of modern solder. Therefore, the composite solder characteristics such as microstructure evolution, electrical performance and mechanical performance will be discussed in this subchapter.

4.1 Microstructure and Properties of Ceramic Composite for Electronics The characterisation of the microstructure observation was mainly using the optical microscope (OM) and Scanning electron microscope (SEM) that is assisted with an Energy Dispersive X-Ray Spectroscopy (EDX) system. Recently, numerous studies have revealed that adding certain particles into the solder matrix to form a composite solder could improve the mechanical properties (Tsao et al., 2012). Therefore, over the last few years, lead-free solders have led the industry to focus on the Tin– Silver (Sn–Ag), Tin–Bismuth (Sn–Bi), Tin–Zinc (Sn–Zn), Tin–Copper (Sn–Cu) and Tin–Silver–Copper (Sn–Ag–Cu) alloys as the primary candidates for replacement. Recently, Sn–Ag–Cu, Sn–Cu and Sn–Ag solders are most commonly used lead-free

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solders in place of Sn–Pb solders. However, with the increasing cost of silver (Ag) in the international market, solder manufacturers are actively looking into lead-free solders that do not contain silver (Ag), making the Sn–Cu system one of the targeted choices (Zhong & Gupta, 2008). In addition, several reinforcing materials have been used as reinforcement material in producing robust composites, such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) (Nai et al., 2008; Ismail et al., 2021), ceramic material that consists of oxides, nitrides or carbides such as ZrO2 (Shen et al., 2006; Gain et al., 2011), SiC (Liu et al., 2014; Fathian et al., 2017; Wang et al., 2009), Al2 O3 (Zhong & Gupta, 2008), TiO2 (Chang et al., 2011; Mohd Salleh et al., 2015). Firstly, the most important microstructure analysis should be conducted to form intermetallic compounds (IMC). The intermetallic compound layers (Fig. 7) are formed during soldering due to an interfacial reaction between solder alloy and metal substrate. Cu substrate is commonly used in electronic solders due to the excellent conductivity of Cu. It is well known that the intermetallic compound is necessary for every solder joint because it determines the strength and wettability of a solder. However, thicker intermetallic is avoided due to brittle nature and poor interfacial bonding. Therefore, a thin, continuous and uniform intermetallic compound (IMC) layer between solder and the substrate material is essential for good bonding (Shen & Chan, 2009). The chemical compound of the IMCs depends on the type of lead-free solder being used. For instance, the two most common types of IMCs that are seen in Sn–Cu/Ni lead-free solder are tin/copper (SnCu) and tin/nickel (SnNi). Tin/copper intermetallic will occur in two phases. The first phase of CuSn intermetallic is formed nearest the copper interface and is designated as Cu3 Sn intermetallic. Then, a layer of Cu6 Sn5 will form on top of the IMC layer. Due to the unevenness of the IMC layer along with the interface, an average value of IMC thickness (t) was determined from the following equations:

Fig. 7 Intermetallic compound (IMC) layer formed at interface between solder and substrate

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t=

A L

15

(4.1)

where A is the area of the IMC layer, and L is the length of IMC along with the interface, as shown in Fig. 7. Many researchers also study the effect of ceramic on interfacial IMC. Mainly, the formation of the IMC layer is due to the diffusion of atoms from the substrate into the solder matrix. Diffusion rates control the IMC growth and solder/IMC interface energies control the coarsening of the IMC grain and nucleation kinetics (Lee & Mohamad, 2013). The presence of reinforcement in the solder matrix could alter the diffusion rate at the interfacial layer (Shen & Chan, 2009). Reinforcement particles are believed to act as a barrier that blocks the diffusion of Cu and Sn from reacting to each other. This phenomenon has suggested that the reinforcement addition could suppress the IMC layer growth. Leong and Haseeb (2016) found that the addition of TiO2 particles has reduced the Sn3.5Ag0.5Cu IMC layer thickness. However, the thickness of the IMC layer had increased when the addition of TiO2 was 1.25 wt%. This may be attributable to the agglomeration and segregation of TiO2 particles in the bulk solder due to the van der Waals forces, which caused TiO2 particles to get entangled with each other as they approach 1.25 wt%. Li et al. (2009) also summarised that the addition of rare earth elements suppresses the growth of the thickness of intermetallic compounds layer Sn3.8Ag0.7Cu solder alloy. Other than at the interface, the researcher also studied the formation of IMC at the bulk area of the composite solder. The ceramic reinforcement particles were mainly distributed along the grain boundary, as shown in Fig. 8. EDX analysis was also used to investigate the eutectic area, reinforcement area and β-Sn area in composite solder. Figure 9 shows the TiO2 particle located along the grain boundary in Sn– 0.7Cu–0.05Ni solder. Figure 10 shows the area mapping of Sn–Cu–Ni reinforced with Si3 N4 particles. It shows that the Si3 N4 particles are located along the grain boundary while the Cu and Sn elements are distributed throughout the grain.

4.2 Electrical Properties Solder joints function as an electrical and mechanical connection in electronics packaging. It connects the substrate and the electronics component. Therefore, the performance of the solder joint is vital to the whole electronic package. Many researchers have proven that the addition of ceramics into lead-free solder improved the mechanical properties, wettability and thermal stability. However, there are inconsistent findings regarding the electrical resistivity or electrical conductivity. Park et al. have reported that the addition of 0.3 wt% Ag-multi-walled carbon nanotubes (AgMWCNT) has decreased the electrical resistivity of Sn–Bi solder (Park et al., 2020). They concluded that the Ag-MWCNT had formed the net structure, which had functioning as an electrical path in that composite solder. It is on the contrary with the

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Fig. 8 SEM micrograph with addition of TiO2 reinforcement a 0.25 wt%, b 0.50 wt%, c 0.75 wt% and d 1.0 wt% Fig. 9 Energy dispersive X-ray spectroscopy (EDX) point analysis of Sn–Cu–Ni + TiO2 solder

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20 µm

BES

20 µm

Si K

20 µm

Sn L

20 µm

Cu K

Fig. 10 Area mapping of Sn–Cu–Ni reinforced with Si3 N4

report claimed by Skwarek et al. They claimed that the value of electrical resistivity of ceramics composite solder has increased. The authors also reported that by adding 1 wt% TiO2 into SnCu solder, the electrical resistivity has increased, causing the decrease in electrical conductivity (Skwarek et al., 2020). However, Nai et al. concluded that the addition of carbon nanotubes into SAC solder does not degrade monolithic solder’s electrical resistivity. They found that the small amount of ceramic addition will not affect the composite solder’s electrical resistivity (Nai et al., 2008). A similar conclusion was reported by Ramli et al. They found that adding 0.25–1 wt% of TiO2 into SnCu-based solder has slightly altered the electrical resistivity and can be negligible. Therefore, it can be concluded that the minor addition of ceramic reinforcement, less than 1 wt%, into lead-free solder will cause an insignificant effect on the electrical properties of the composite solder.

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4.3 Mechanical Performance Lead-free solders are mostly made from Sn-based alloys with zero solubility with ceramic materials. Thus, the bond between the ceramic/solder boundary is simply of adhesive manner because no diffusion can occur between them, leading to the formation of a new phase. Although ceramic reinforcement serves no chemical reaction benefit, the ceramic particles could improve the mechanical performance of the solder alloys via grain refinement and the modified grain boundary/interfacial characteristics (Zhang et al., 2014). The mechanical performance in recently reported studies on ceramic-reinforced lead-free solders is mainly evaluated using two measurements: shear strength and microhardness test (Mohd Salleh et al., 2015; Park et al., 2020; Skwarek et al., 2021). A bonding strength of a solder joint can be evaluated based on a shear strength which is the ability to resist forces that cause the material’s internal structure to slide against its selves. The shear strength of a ball grid array (BGA) solder joint can be performed using a global bond tester called DAGE400 at various shear speeds (Mohd Salleh et al., 2015; Park et al., 2020; Somidin et al., 2020). In general, at higher shear speeds, it is expected to shear a BGA solder joint completely through the solder/substrate interface. The result provides a measure of the strength of the interfacial intermetallic (Tsukamoto et al., 2010, 2011). While lower shear speeds can be used to ensure the crack propagates through the ball, the result provides a measure of the properties of the bulk solder (Park et al. 2020; Mohd Salleh et al., 2015) used a low-speed shear test of 200 μm/s to evaluate the bonding strength of Sn-58Bi/AgMWCNTs composite solder after various aging conditions. The result showed the bonding strength of the Sn-58Bi solder joints decreased with increasing ageing time due to the growth of the intermetallic compound layer at the solder/substrate interface and the coarsening of the Sn and Bi grains during ageing. However, the addition of 0.05 weight per cent of Ag-MWCNTs in the solder joints showed higher strength than the non-reinforced Sn-58Bi solder joint at all ageing conditions. A lap shear solder joint test can also be performed to evaluate the bonding strength of a solder joint (Mohamad Zaimi et al., 2020). Mohamad Zaimi et al. performed a single-lap shear test to evaluate the strength of the Sn–3.0Ag–0.5Cu/kaolin composite solder joints, which were bonded to the Cu substrate (PCB FR4-type). The highest average shear strength was found in adding 1 weight % of kaolin into the Sn–3.0Ag–0.5Cu solder matrix. Microhardness values of the composite solder samples can be obtained using a Vickers microhardness tester machine (Mohd Salleh et al., 2015; Tikale et al., 2020) and a nano-indenter instrument (Wang et al., 2020). A hardness test is used to determine a material’s resistance to deformation. The microhardness value linearly correlates with the tensile strength in metals. Tikale and Prabhu explored the effect of the Al2 O3 nanoparticles in the range of 0.01–0.5 weight per cent on the hardness of Sn–0.3Ag–0.7Cu solder using micro-Vickers hardness tester; Shimadzu HMV G20ST (Tikale and Prabhu., 2020a, 2020b). The results indicated that the rate of increase in microhardness appears high with the presence of the Al2 O3 nanoparticles

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Table 2 Mechanical performance of Sn–Ag–Cu solders reinforced with ceramic particles during thermal ageing References

Materials (in wt%)

Average shear strength after thermal ageing at 150 °C (MPa) Reflowed on Au/Ni-plated Cu (960 h)

Reflowed on uncoated Cu (240 h)

Reflowed on Ni–P coating (240 h)

28.80 35.60

–

–

Gain and Zhang (2018)

Sn–3Ag–0.5Cu (SAC305) SAC305 + 1.0TiO2

Tikale and Prabhu (2020a)

Sn–0.3Ag–0.7Cu (SAC0307) SAC0307 + 0.01Al2 O3 SAC0307 + 0.05Al2 O3

–

20.27 20.00 25.80

25.44 27.89 29.85

Tikale and Prabhu (2020b)

Sn–3Ag–0.5Cu–0.06Ni–0.01Ge (SACNiGe) SACNiGe + 0.01Al2 O3 SACNiGe + 0.05Al2 O3

–

26.74 28.43 32.20

28.26 30.10 35.95

up to 0.1 weight per cent but declined on further addition. Tikale and Prabhu (2020a, 2020b), also reported the increment in microhardness on the addition of 0.01 and 0.05 weight % Al2 O3 nanoparticles in Sn–3Ag–0.5Cu–0.06Ni–0.01Ge (SACNiGe) solder alloy. Pal et al. (2019) reported that the addition of 1.5 wt% and 2 wt% SiC addition into Sn–3.0Ag–0.5Cu solder materials had increased the compressive strengths and microhardness up to 38 and 68% compared to those of the monolithic sample. The widespread use of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder alloys was due to the endorsement of this alloy by the IPC, the Association Connecting Electronics Industries (Cheng et al., 2017). Thus, various attempts were made to upgrade Sn–Ag–Cu solder alloys using ceramic powders in recent years. However, most of the typical commercially available high-temperature electronics are designed to withstand operation at an ambient temperature above 150 °C. Table 2 shows the average shear strength of SAC-based solder alloys reinforced with ceramic particles after thermal ageing. It can be seen that the joint strength of the ceramic composite solders was higher than the monolithic solder alloys after thermal ageing at 150 °C. Furthermore, it shows that minor ceramic reinforcement particles can substantially improve the SAC solder joint reliability.

5 Advantages and Limitation of Ceramics Composite Solder There were abundant reported studies on the incorporation of ceramic particles into lead-free solders (Yakymovych et al., 2017; Vaidya & Pathak, 2019). The effects on the mechanical, physical, electrical properties and microstructure evolution have

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been analysed and discussed. Now, we are going to look at the positive and negative effects of ceramics reinforcement particles on the properties of the composite solders. Ceramic particles, including metal oxide, nitride and carbide particles, have gained researchers’ attention in finding the alternative for lead solders. Usually, the method to produce ceramic composite solder is via powder metallurgy method since ceramic possesses a high melting temperature characteristic and would be caused a limitation to casting method as discussed in fabrication techniques topic. TiO2 is one of the most studied ceramic particles (Skwarek et al., 2020; Wen et al., 2017; Siti Shareeda Mohd Nasir et al., 2019; Ramli et al., 2016). They have reported that the addition of TiO2 up to 1 wt% could improve the wettability and showed grain refinement effects. Grain refinement effect to the IMCs formation is the major reason for the selection of TiO2 ceramic particles. The grain refinement effect has improved the toughness, physical and mechanical properties of the composite solder. Although there was a slight decrease in electrical properties, it can still be negligible. However, if the reinforcement amount is high, more than 1 wt%, negative effect trends for physical, electrical and mechanical properties were reported (Ramli et al., 2016; Pal et al., 2021). Carbon nanotube (CNT) and multi-wall carbon nanotube (MWCNT) have also gained researchers’ attention. Interestingly, recent studies showed that some researchers had made an improvement regarding the usage of CNT and MWCNT (Lee et al., 2020; Wang et al., 2020). Nickel particles were used to treat the surface of CNT and MWCNT. As a result, the Ni-CNT/MWCNT has restricted the growth of the IMC particles, which subsequently increases the shear strength of the composite lead-free solder. Apart from that, the coarsening process of the microstructure during the ageing test was also slower. It shows that the activation energy possessed by the composite solder has increased with the addition of treated CNT/MWCNT. The higher the activation energy, the slower the growth rate of the coarsening process and the thickening process of the IMCs at the interface. However, still, there is a major limitation for ceramics composite solders. The close contact parameter, or rather the bonding between ceramics particles and the solder matrix, was not excellent (Gyökéra et al., 2019). The poor adhesion could cause a crack at that interface area. Since a good composite solder should have good bonding and wetting properties between particles for the solder connection to be reliable, the ceramic particles need to be treated. One of the treatments is by coating the ceramics particles with a very thin IMC, where the outer layer will have ionic behaviour. For instance, Ni-coated SiC and Ni-coated MWCNT. However, most surface treatment methods applied, including electroless coating or laser surface texturing, have caused additional time and cost to the composite solder fabrication process (Alessandro De Zanet et al., 2021; Wang et al., 2020). Nevertheless, it is undeniable that most of the experimental data have shown that ceramic particles have acted as good reinforcement particles for lead-free solder. Apart from TiO2 and CNT, there are other promising ceramics particles such as Si3 N4 , SiC and Al2 O3 . Their thermal stability, mechanical properties and physical properties were excellent (Liu et al., 2018; Ramli et al., 2015; Pal et al., 2020). On top

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of that, there is no need for any changes in soldering parameters since the addition of ceramic particles did not affect the melting temperature of the composite solder.

6 Conclusion In summary, this chapter has discussed the development of ceramic composite solder starting from the type of reinforcement selection, considering the factors such as physical properties, thermal properties and the most important thing, the cost. A suitable processing technique is required to produce a ceramic composite solder with optimum properties. The inappropriate technique could jeopardise the properties of the composite solder, and subsequently, the whole electronics connections. For instance, to incorporate micron size ceramic particles into lead-free solder, a powder metallurgy technique is the most suitable method instead of the casting technique. It is anticipated that the characteristics of lead-free ceramic composite solder improved compared to the monolithic solder, especially with its good thermal stability and mechanical properties. Those promising properties are necessary for high-temperature applications. Nevertheless, the study on ceramic composite solder is still at the academic level and not yet applied in the electronics industry since there are still few limitations. Due to that, mass production and application still cannot be achieved. An established experimental data is needed to implement ceramic composite solder in the industry successfully. Acknowledgements The authors acknowledge the support of Electronics Packaging Materials Research Group, Centre of Excellence Geopolymer and Green Technology (CEGeoGTech), Universiti Malaysia Perlis.

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Somidin, F., McDonald, S., Ye, X., Qu, D., Sweatman, K., Aikawa, T., Nishimura, T., & Nogita, K. (2020). Reducing cracking in solder joint interfacial Cu6 Sn5 with modified reflow profile. Transactions of the Japan Institute of Electronics Packaging, 13, E19-004-1–E19-004-011. Tikale, S., & Prabhu, K. N. (2020a). Development of low-silver content SAC0307 solder alloy with Al2 O3 nanoparticles. Materials Science and Engineering: A, 787, 139439. Tikale, S., & Prabhu, K. N. (2020b). Performance and reliability of Al2 O3 nanoparticles doped multicomponent Sn-3.0Ag-0.5Cu-Ni-Ge solder alloy. Microelectronics Reliability, 113, 113933. Tsao, L. C., Huang, C-H., Chung, C. H., & Chen, R. S. (2012). Influence of TiO2 nanoparticles addition on the microstructural and mechanical properties of Sn0.7Cu nano-composite solder. Materials Science and Engineering A, 194–200. Tsukamoto, H., Nishimura, T., Suenaga, S., McDonald, S. D., Sweatman, K. W., & Nogita, K. (2011). The influence of solder composition on the impact strength of lead-free solder ball grid array joints. Microelectronics Reliability, 51(3), 657–667. Tsukamoto, H., Nishimura, T., Suenaga, S., & Nogita, K. (2010). Shear and tensile impact strength of lead-free solder ball grid arrays placed on Ni (P)/Au surface-finished substrates. Materials Science and Engineering: B, 171(1–3), 162–171. Vaidya, A., & Pathak, K. (2019). Application of nanocomposite materials in dentistry. Woodhead Publishing Series in Biomaterials. Wang, H., Hu, X., & Jiang, S. (2020). Effects of Ni modified MWCNTs on the microstructural evolution and shear strength of Sn-3.0Ag-0.5Cu composite solder joints. Materials Characterizations, 163, 110287. Wang, X., Liu, Y. C., Wei, C., Gao, H. X., Jiang, P., & Yu, L. M. (2009). Strengthening mechanism of SiC-particulate reinforced Sn–3.7Ag–0.9Zn lead-free solder. Journal of Alloys and Compounds, 480(2), 662–665. Wen, Y., Zhao, X., Chen, Z., Gu, Y., Wang, Y., Chen, Z., & Wang, X. (2017). Reliability enhancement of Sn-1.0Ag-0.5Cu nano-composite solders by adding multiple sizes of TiO2 nanoparticles. Journal of Alloys and Compounds, 696, 799–807. Yakymovych, A., Pleavachuk, Y., Svec, P., Janickovic, D., Sebo, P., Beronska, N., Roshanghias, A., & Ipser, H. (2016). Morphology and shear strength of lead-free solder joints with Sn3.0Ag0.5Cu solder paste reinforced with ceramic nanoparticles. Journal of Electronic Materials, 45(12), 6143–6149. Zhang, L., & Tu, K. N. (2014). Structure and properties of lead-free solders bearing micro and nano particles. Materials Science and Engineering: r: Reports, 82, 1–32. Zhang, L., Sun, L., Guo, Y. H., & He, C. W. (2014). Reliability of lead-free solder joints in CSP device under thermal cycling. Journal of Materials Science: Materials in Electronics, 25(3), 1209–1213. https://doi.org/10.1007/s10854-014-1711-y. Zhong, X.L., & Gupta, M. (2008). Development of lead-free Sn-0.7Cu/Al2 O3 nanocomposite solders with superior strength. Journal of Physics D: Applied Physics, 41(9), 095403

Development of Geopolymer Ceramic-Reinforced Solder Mohd Mustafa Al Bakri Abdullah, Mohd Arif Anuar Mohd Salleh, Nur Nadiah Izzati Zulkifli, Nur Syahirah Mohamad Zaimi, Romisuhani Ahmad, Noorina Hidayu Jamil, Ikmal Hakem Aziz, and Mohd Izrul Izwan Ramli

Abstract This chapter aims to provide a general understanding of the geopolymer usage as a ceramic reinforcement material in the solder alloy. The metal matrix composite (MMC) technique has been introduced to mix the reinforcement and matrix into composite solders. In MMC materials, the matrix used came from a metal group, and the reinforcement particles are either metallic or non-metallic. These reinforcement particles in lead-free solder could enhance the properties of the existing lead-free solder alloy. Moreover, the geopolymer ceramic as a reinforcement material could distribute homogeneously along the grain boundaries of solder alloys. The potential of geopolymer ceramic reinforced solders was fabricated via powder metallurgy assisted by a hybrid microwave sintering. The acceptable value of electrical conductivity obtained by geopolymer ceramic influenced the fast ionic conduction, which is used in applications such as solid-state batteries and electrochemical sensors. The chapter encapsulates the properties of geopolymer ceramics as a reinforcement material in solder application. It also includes a discussion and examples of the geopolymer ceramic synthesis and fabrication. The advantages of geopolymerisation as a self-fluxing agent have been discussed. This topic covers several types of geopolymer ceramic. Preparing the ceramic at lower sintering temperature within the same conventional sintering technique will give a significant achievement. The initial mechanism that involves geopolymerisation shows significant potential to reduce the sintering temperature of the geopolymer ceramic. The composition of sodium oxide (Na2 O), which is contributed by the alkali activator, might act as a fluxing agent and help lower the sintering temperature of geopolymer ceramic. Hence, a lowtemperature sintered geopolymer ceramic can be produced. To date, no research M. M. A. B. Abdullah (B) · M. A. A. M. Salleh · N. N. I. Zulkifli · N. S. M. Zaimi · R. Ahmad · N. H. Jamil · I. H. Aziz · M. I. I. Ramli Center of Excellence Geopolymer and Green Technology (CEGeoGTech), Universiti Malaysia Perlis, 01000 Kangar, Perlis, Malaysia e-mail: [email protected] Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Arau, Perlis, Malaysia Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Arau, Perlis, Malaysia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 M. A. A. M. Salleh et al. (eds.), Recent Progress in Lead-Free Solder Technology, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-3-030-93441-5_2

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available reported on the usage of kaolin geopolymer ceramic on the lead-free solder alloy. Therefore, in this study, research on the usage of kaolin geopolymer ceramic as reinforcement materials to SAC305 lead-free solder alloy will be conducted as a fundamental to investigate the reaction of geopolymer ceramic toward SAC305 lead-free solder alloy. Keywords Geopolymer · Ceramic · Reinforcement material · Solder

1 Introduction Computers and cell phones, for example, have become thinner and smaller, with more intricate functions. Due to the miniaturisation of these electrical devices, better solder-joint reliability is required. Composite solder, which comprises a solder matrix plus reinforcements, is one way to enhance reliability. Due to the reinforcing particles limiting grain-boundary slippage, the development of the intermetallic compound, and grain growth, as well as redistribute stress uniformly, composite solder is noted for its high reliability (Hwang et al., 2002). In addition, intermetallics, metallic powders, carbon fibres, and fine oxide particles can be applied as reinforcements in composite solder. However, agglomeration, segregation, gas entrapment, and coarsening of the particles are common difficulties in traditional composite solder ball manufacturing techniques, resulting in non-uniform reinforcing. A novel manufacturing method for composite solder production has been introduced. Metal matrix composites are made up of two or more different materials, at least one of which is metal and another such as ceramics. Hybrid composites are defined as a composite with at least two or more reinforcements (Jamwal et al., 2019). As consisting of diversified qualities, aluminium matrix composites have grown in popularity. Graphite (Guo & Tsao, 2000), TiC (Albiter et al., 2006), SiC (Kumar et al., 2020), carbon nanotube (Bradbury et al., 2014), and Al2 O3 (Shorowordi et al., 2003) is the most commonly utilised reinforcement in aluminium metal matrix composites. The amount of reinforcement (wt%), particle size, shape, and processing techniques have all been reported in previous studies to affect the character of aluminium matric composites. The aluminium matrix composite can be fabricated by stir casting (Surappa, 2003), squeeze casting (Ghomashchi & Vikhrov, 2000), and powder metallurgy (Saxena et al., 2017). Geopolymer is classified as an inorganic material manufactured by an amorphous three-dimensional (3D) aluminosilicates system activated by an alkali solution at ambient or slightly higher temperatures. Also, geopolymer material provided the ceramic characteristic with the crystalline phase existence up to 1000 °C (Davidovits, 2013). The phase crystallisation is influenced by several factors, such as the mineralogical phase composition of the raw materials used to design ceramic mixtures and the level of exposed temperature in which the ceramic edifice is fired or sintered (Ganapathe et al., 2020). Standard methods for sintering glass–ceramic commonly

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include two steps: vitrification of raw materials at 1300–1500 °C, followed by nucleation and crystal growth (Rincón et al., 2018). Ironically, geopolymer ceramics can be fabricated using geopolymer powder via a slightly lower sintering temperature, yielding acceptable mechanical performance than conventional ceramics (Liew et al., 2017). Furthermore, geopolymer ceramic incorporates enormous chemical elements, which play an additional nucleation site to enhance the solder alloy properties. However, limited research is reported on the usage of geopolymer ceramic as the reinforcement material in existing solder alloys. Hence, the potential of geopolymer ceramic materials reinforced solder was presented in this chapter.

2 Geopolymer Materials The dissolution, hydrolysis, and condensation reaction occurred during the geopolymerisation reaction. The condensation polymerisation occurs at ambient temperature, which is generically considered cementitious materials. Meanwhile, at high heating temperature exposure, the crystallisation phase via sintering reactions resulted from the formation of ceramic products.

2.1 Fly Ash Fly ash, also referred to as coal combustion residuals (CCRs), is a well-known material in geopolymer industries. Fly ash is in a group of coal ash, with bottom ash, boiler slag, and flue gas desulfurisation material. The difference between fly ash and the others is fine and powdery particles that are mostly made of silica. Generally, most fly ash particles are solid spheres with hollow shapes, also known as cenospheres, as shown in Fig. 1. It is usually used as an alternative source in geopolymer concrete production since it is known as a material with low-cost consumption. It came from the waste disposal of mineral extraction from power plant industries (Albitar et al., 2017; Salwa et al., 2013). Typically, different power plants contribute to different chemical compositions of fly ash due to the different types and amounts of matter in the coal (such as bituminous and lignite) (Abdulkareem et al., 2014). The mineral substances of fly ash are comprised of fine oxide particles and compounds such as quartz, hematite, feldspar, shale, mullite, and some amorphous particles. Based on American Standard for Testing and Materials (ASTM 618), fly ash can be categorised into three classes such as fly ash class C, fly ash class F, and fly ash class N. In geopolymer industries, fly ash class C and fly ash class F are commonly used since it has pozzolanic characteristics (Wardhono, 2018). Class C fly ash consists of at least 20% of Ca percentage compared to class F fly ash. The class C fly ash is typically a product of low-rank coals of sub-bituminous and lignite, which contain at least 50% of SiO2 , Al2 O3 , and Fe2 O3 . Meanwhile, class F fly ash is a high-rank

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Fig. 1 SEM micrographs of fly ash particles at 1000X

coal product of sub-bituminous and lignite that contains more than 50% or at least 70% of the weight percentage of SiO2 , Al2 O3 , and Fe2 O3 (Bouaissi et al., 2020). A study by Temuujin et al. (2009) determined that the application of high calcium fly ash produced an increment of mechanical properties of fly ash geopolymer paste. For example, the fly ash with high calcium content achieved high compressive strength up to 86 MPa at 28 days (Temuujin et al., 2009). Besides, the increase of paper mill sludge ash (PMSA), which is known as high CaO ashes, into a fly ash class C is believed to activate the cations and thus improve the setting time of the geopolymer (Rahman et al., 2018). During geopolymerisation, the reaction that occurs is diffusion and reorganisation of dissolved ions. Therefore, the alteration of cations with the presence of high calcium elements in fly ash plays a crucial role in accelerating the geopolymer reaction (Rahman et al., 2018). The electrical resistivity of a class F fly ash geopolymer has higher resistivity (98 m) compared to metakaolin geopolymer (19.5 m) (Cai et al., 2020). Theoretically, electrical resistivity is defined as the inverse of conductivity, which means high resistivity is the same as low conductivity, and resistivity is the same as high conductivity (Heaney, 2017). This occurrence can be due to the higher specific surface area and denser structure due to the high thermal treatment of the metakaolin material. In addition, the usage of low calcium fly ash material can also reduce the electrical conductivity due to their pozzolanic features. However, there is a limited study on electrical properties using class C fly ash.

2.2 Ground Granulated Blast Furnace Slag Slag is a waste material product formed from the smelting process of hot metal from its raw ore. It is generated from the combustion residue of coke, pig iron, or fluxes during the combustion of the blast furnace. The chemical element of slag depends on

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Fig. 2 SEM micrographs of slag particles at 1000X

where it came from Cheng and Chiu (2003). It mostly consists of oxides from silicon, aluminium, sulfur, phosphorus, and limestone elements. Slag can be advantageous because of its chemical and physical properties. It is usually applied in concrete and cement industries due to its cementitious characteristic from high Ca content. Most of the slag material has an irregular shape with a rough surface, as shown in Fig. 2. Recently, slag usage in geopolymer industries has been widely investigated due to the high number of silicon–aluminium components. Using slag in geopolymer production can give many potential benefits such as high compressive strength, fireresistant materials, and fast-setting time. The high calcium content of slag was also used as a filler in low calcium geopolymer since it improves the setting time and hardening. The dissolution of slag to dissolve silicon and aluminium ions is said to be higher than fly ash (Cheng & Chiu, 2003). Slag-based geopolymer used (Si + Ca)) system since it has a high composition of Si and Ca element in the composition (Aziz et al., 2017). The (Si + Ca) system might be attributed to the formation of C-A-S-H gel, which is primarily driven by the alkaline reaction of the activated alkali slag (Aziz et al., 2019). The addition of slag during geopolymer processing has been used to improve the structure of geopolymer behaviour since it is known as an activation filter. Based on previous research, the addition of slag in the fly as-based geopolymer mixture developed higher compressive and flexural strength compared to the 100% of fly ash itself (Hadi et al., 2019; Yazdi et al., 2018). The improvements of such properties are based on the formation of the C-A-S-H gel phase during the geopolymerisation process. Material with high Ca content was observed to exhibit high electrical conductivity compared to low Ca content. This occurrence is because more + + ions such as Ca+ , OH− , SO2− 4 , Na , and K in the geopolymer structure can allow a more current flow rate (Tomlinson et al., 2017).

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2.3 Clay Kaolin is a family of kaolinitic clays, which is in white or near-white colour. It is also known as China’s clay; it is derived from the Chinese word (Gao-Ling), meaning high ridge. The Chinese were the first to use kaolin in the seventh and eighth centuries to make porcelain for their daily lives. Kaolinite is a mineral composed of aluminium silicates that are produced after chemical weathering changes the structure of rocks and soil. It consists of the 1:1-type uncharged dioctahedral layer structure of clay minerals with one octahedral sheet and one tetrahedral sheet depending on their geological condition (Abdullah et al., 2018). The microstructure of kaolinite is usually in plate-like or needle-like structure, as shown in Fig. 3, and is not easily separated due to its tightly-packed structure. Chemically, kaolinite is composed of SiO2 and Al2 O3 , and the percentage of these components determines kaolin’s efficiency. Using kaolin as geopolymer sources required a certain period of time to complete the geopolymerisation process since it has low reactivity and needs sufficient time to undergo the geopolymerisation process due to its weak structure. In addition, the particles of kaolin itself have a small surface area, causing minimal dissolution of Si and Al during the geopolymerisation process (Aziz et al., 2015). Another way to increase the geopolymerisation reactivity is by thermal treatment. The kaolin material was usually introduced to high sintering temperatures ranging from or between 500 and 900 °C to convert kaolin into metakaolin products. In addition, the pozzolan characteristic of metakaolin that has little or no cementitious element will enhance the chemical reactivity of calcium hydroxide (Ca(OH)2 ) in the water to form a compound (Zhang et al., 2016). Fig. 3 SEM micrographs of kaolin particles at 5000X (Jamil et al., 2020)1

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1 “Reprinted from Materials Research and Technology, 9, N. H. Jamil, M. M. Al Bakri Abdullah, F. C. Pa, H. Mohamad, W. M. A. W. Ibrahim, & J. Chaiprapa, Influences of SiO2 , Al2 O3 , CaO, and MgO in phase transformation of sintered kaolin-ground granulated blast furnace slag geopolymer, 14922–14932, Copyright (2020), with permission from Elsevier.”

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Normally, kaolin can be categorised as a low calcium system due to its high silica and aluminium content compared to high calcium systems. When the alkali solution activates the kaolin, the primary reaction of sodium aluminosilicate hydrate (N-A-S-H) will occur.

2.4 Addition Mixture The addition of organic polymer as a binder on kaolin-based geopolymer ceramics to be potentially used as reinforcement in solder alloy will be elaborated and discussed in this section. Binder is frequently used in ceramics production due to the extreme hardness of powder particles and low plastic deformation properties that pose difficulty in compacting the powder. Additionally, the addition of binder also promises to provide the necessary green strength by reducing density and increasing polymer to polymer and ceramics to polymer interaction. A huge number of organic substances can be used as binders, some of which are soluble in water. In contrast, others are soluble in an organic liquid, including vinyl, acrylics, and ethylene oxides (glycols) (Rahaman, 2017). The polymeric molecules provided a binding action and coagulated colloidal particles that are adsorbed and acting as a bridge between the particles. However, heating above their specific decomposition temperature causes the polymer to disintegrate into a volatile degradation product. Hence, the removal of binder plays a crucial part in ceramic processing that necessitates a longer heating time, lowering the production rates, and releasing hazardous gases that will contribute to air pollution and global warming. Ultra-High Molecular Weight Polyethylene (UHMWPE) is a thermoplastic polymer with a wide range of characteristics, including great impact strength, low embrittlement temperatures, and high resistance to stress cracking and wear (Diop et al., 2014). Due to their superior properties, many applications in mining, transportation, foundries, and medical have been found in UHMWPE. The molecular chain of UHMWPE can be compared to an interwind mass of spaghetti that becomes mobile at higher temperatures. Still, the chains fold and rotate to create the crystalline region at a temperature below the melting temperature (Musib, 2011). The melting point of UHMWPE is about 134 °C. A certain amount of polymer remains in the ceramic body in the form of carbon residue during the decomposition process (Rajeswari et al., 2015). Thus, the properties shown by UHMWPE could be employed as binders to improve the performance of lightweight geopolymer ceramics. In conclusion, to produce geopolymer reinforcement for solder composite, one of the most important criteria is to study the electrical behaviour of the reinforcement. Some factors that led to improved electrical conductivity are the type of natural sources, curing and sintering temperature, the concentration of alkaline activator, solid-to-liquid ratio, and the mix design of the geopolymer based. The selection of natural sources material for geopolymer products should be emphasised since different material contributes to different chemical composition. As stated before, high Ca content commonly gives higher conductivity due to the presence of Ca+

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ions. Besides, the high curing and sintering temperature application are believed to accelerate the geopolymerisation process by reducing the pore distribution and producing a stable electrically conductive path. The development of conductivity at high temperatures is normal for ceramic material. The shrinkage process occurred when the high temperature was introduced to the geopolymer samples, reducing the bulk samples’ thickness. The shrinkage of the samples or the reduction in the size of the sample can be related to the porosity and heterogeneity of materials which is inherent to ceramics materials (Rice, 2017). Typically, the preferred mix design for each geopolymer material is different depending on its chemical composition. For example, a highly concentrated alkaline activator solution developed high electrical conductivity compared to a less concentrated solution (Ghosh & Ghosh, 2018). The highly concentrated alkaline solution introduces more conductive OH− , Na+ , and K+ ions, increasing the geopolymerisation system (Cai et al., 2020). In addition, the S/L ratio also influenced the electrical conductivity since the solid precursors are less, and contributes to a high alkaline solution.

3 Synthesis of Geopolymer Ceramics The word “Ceramic” came from the Greek word “Keramicos,” which means the burned material with an amorphous state (such as glass), crystalline state, or partially crystalline state. Geopolymer ceramic can be determined as an alkali-activated aluminosilicate source such as fly ash, kaolin, or slag that undergoes thermal treatment process to improve the properties of geopolymer product such as microstructure refinement, thermal and chemical resistance, increase hardness, and improve compressive strength. Currently, geopolymer material is capable of withstanding high thermal temperatures ranging from 950 to 1350 °C (Rovnaník & Šafránková, 2016). The range of the thermal temperature depends on the chemical composition of the raw sources of the geopolymer-based since the natural source came from different environments. For example, the thermal treatment of fly ash-based geopolymer can withstand at least up to 1100 °C because it was completely damaged at 1200 °C since it already achieved its melting point. However, the kaolin-based geopolymer was subjected to the same temperature, but it produces a smooth and glassy texture with high flexural strength compared to other temperatures. The kaolin-based geopolymer was found to cause softening and melting when subjected to 1300 °C as it is high enough to fuse the geopolymer matrix (Liew et al., 2017).

3.1 Powder Metallurgy Numerous possible methods can be used in manufacturing kaolin-based geopolymer ceramic materials; however, the selected method is one of the key factors since it can

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influence the properties of the end-products. A few sequences of steps are shaped for ceramic materials, starting with the raw material and progressing through batch preparation, forming, and firing. The powder metallurgy (PM) technique can produce the desired final shape and dimension of ceramics parts. Consequently, most ceramic materials are prepared using the powder metallurgy method, which involves exerting stresses on the powder’s precursor (Zhang et al., 2020). Furthermore, this method is preferred due to a uniform distribution of the particles that can be produced. The lower temperature used can also reduce the degradation of the particles (Reddy et al., 2007). Overall, the PM method involves producing powders and converting these powders into engineered structures. The expanding usage of the PM method can develop an endless variety of materials and products, emphasising the proper application of experimental techniques. Generally, the PM method consists of three key steps: creating a regulated blend of powders, pressing the powders in suitable dies, and finally heating (sintering) the compacted powder in a controlled atmosphere and temperature to achieve the necessary density and strength (Al-Qureshi et al., 2005). From the steps mentioned above, compaction and sintering have the most pronounced effect on product quality. The liquid-like character of the powder allows it to flow filling out the desired shape, making the PM method similar to the casting process. The difference is that in the PM method, the deformation densifies powder as it flows into a die cavity at room temperature due to the weak friction bond deformation under pressure (Yong et al., 2017). The structure is then heated to a temperature at which the particles sinter and create a strong connection, improving the strength. This method can then be practical where the main concerns are strength, stiffness, cost, and toughness. However, due to the possible residual porosity, most applications attempt to avoid oxidation, corrosion, and wear situations and focus more on applications where shape complexity, surface finish, and mechanical or electrical or magnetic properties are of concern at low manufacturing cost.

3.2 Direct Sintering The contribution of the oxides from raw materials such as SiO2 , Al2 O3 , CaO, and MgO in reducing the sintering temperature of geopolymer would be an exciting discussion in the geopolymer field. Hence, the technique to sinter geopolymeric materials at lower sintering temperatures remains a challenge. Therefore, the simplified method is by directly using the moulded sample after curing to the sintering process. Kaolin is a pozzolan type that reacts with alkali solution but requires a higher temperature than metakaolin to be a hardened geopolymer. This occurrence is due to the low reactivity of kaolin that needs sufficient time for the geopolymerisation process to complete (Tiffo et al., 2020). The use of kaolin as a raw material was reported in few studies, but there is more information on the utilisation of metakaolin to make geopolymer materials (Naghsh & Shams, 2017). Metakaolin was obtained

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from the pre-treatment of kaolin, such as calcination at a temperature range of 700 to 850 °C or mechanical activation by milling process (Wan et al., 2017). Meanwhile, ground granulated blast furnace slag (GGBS) seems to be the candidate that can accelerate the hardening of the kaolin geopolymer. The hygroscopic properties of CaO from GGBS will help attract water from its surroundings and will improve the dihydroxylation reaction. The hardened kaolin-GGBS geopolymer remained in cubical shape from geopolymerisation to the sintering process. It is common for the hardening process of geopolymer made from raw kaolin thus will suffer from the cracking issue due to the excessive water that remains in the geopolymer. The excessive water normally coming from the unreacted H2 O during the dissolution stage in geopolymerisation does not remove. The curing condition during geopolymerisation will influence the dihydroxylation reaction, thus preventing the initiation of crack. Apart from the geopolymerisation component, sintering played a vital role in producing geopolymer ceramic. However, direct sintering of geopolymer material at high temperatures causes severe shrinkage and cracking of the geopolymer (Zhang et al., 2016). The current method for producing sintered geopolymer is using the powder metallurgical method, where the hardened geopolymer paste needs to be in powder form. However, the paste must undergo a milling process, which requires another step of energy consumption (Zhang et al., 2016). Conventional approaches of sintering glass–ceramics usually required high temperatures ranged from 1300 to 1500 °C (Rincón et al., 2018). K-feldspar, Cafeldspar rock, Na-feldspar rock, and bone ash have been added as fluxing agent in the mixture with kaolin used and achieved to lower the sintering temperature about 1200 °C (Sokoláˇr et al., 2017). Kaolin-GGBS geopolymer was prepared by mixing the raw materials with 8 M of NaOH and sodium silicate. Samples were then allowed to cure at room temperature for 24 h, followed by curing at 60 °C in the oven for the following 14 days. The sintered cured sample followed the heating profile given in Fig. 4. Step 1 was sintered to a lower temperature, 500 °C, with a heating rate of 2 °C/min. Meanwhile, higher temperature settings were applied in step 2, with temperatures ranging from 800, 900, 1000, and 1100 °C with a heating rate of 4 °C/min. The soaking time was set to 1 h for each step. Two sintering steps were applied in this method purposely to control the major cracking effect on the sintered samples. The first stage (T1) of the two-step sintering profile was done at a relatively low temperature, followed by a higher temperature step (T2) and subsequent cooling (Lóh et al., 2017). The process enables microstructure refinement and, as a result, material characteristic enhancement. The two-step sintering process worked well for systems including ordinary powders with a wide particle size distribution, and it merely generated homogenous ceramic bodies. Controlling the heating curve to change the microstructure during sintering is a method being researched because of its simplicity and cost-effectiveness. It is important to obtain relatively high temperatures in solid-state sintering in order for diffusion to occur and, through various paths, promote the material’s densification. On the other hand, diffusion is a matter transport mechanism that encourages both grain expansion and densification. As a result, microstructural refinement can be achieved using sintering temperatures that

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Fig. 4 Two-steps of sintering profile for kaolin geopolymer ceramic (Jamil et al., 2021)2

allow for densification without considerable grain growth. The ability to produce exceptionally dense ceramics is due to the separation of densification and grain growth.

3.3 Self-fluxing Agent of Low-Sintered Kaolin Geopolymer In recent years, many researchers have been working on developing low-sintered ceramic in wider applications without compromising its performance. Microwave sintering, spark plasma sintering, and pressure sintering is the other techniques to produce a low-sintered ceramic. But these techniques demand sophisticated equipment that leads to higher manufacturing costs than conventional sintering processes. Therefore, the preparation of low-sintered ceramics by maintaining conventional techniques will significantly improve ceramic industries. The sintering temperature of kaolin-based ceramic commonly is within 1200– 1300 °C by using the conventional sintering process. Hence, an initial material treatment that involves geopolymerisation before sintering potentially reduces the sintering temperature to 900 °C of the sintered ceramic. There appears to be a direct qualitative correlation between the reactions of kaolin-GGBS geopolymer during

2 “Reprinted from Materials, 14(6), Jamil, N.H et al. (2021). Self-Fluxing Mechanism in Geopolymerisation for Low-Sintering Temperature of Ceramic, 1325, Copyright (2021), with permission from MDPI AG.”

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Fig. 5 Microstructure evolution of kaolin-GGBS geopolymer ceramic (Jamil et al., 2021)3

sintering at a different temperature, as revealed by SEM. The microstructure evolution of kaolin-GGBS geopolymer ceramic compared with the microstructure of ascured geopolymer is illustrated in Fig. 5. A similar microstructure was observed in Fig. 5a as-cured and Fig. 5b sintered at 800 °C. Apart from that, the microstructure of 800 °C, Fig. 5b shows the formation of elongated and irregular pores. The specimen sintered at 800 °C displays irregular pores and granular grain microstructure. The layered structure of kaolin still occurred, which indicates that low sintering temperature resulted in an incomplete reaction of kaolin-GGBS. Furthermore, it was suggested an initial stage of densification for the 800 °C sintered kaolin-GGBS geopolymer. It was also clear that the amount, width, and length of pores increased at 1000 and 1100 °C. Sintering at 1000 and 1100 °C shows incomplete densification with high porosity in the sintered samples. The pores became primarily network-like or interconnected, which further increases porosity. The formation of interconnected pores can be correlated with the drop of compressive strength at these two sintered samples. There was a spot of granular grain structure at 1100 °C sintered samples (Jamil et al., 2021). More packing density was observed at 900 °C, but open pores with an irregular shape still existed. The densified area can be clearly seen in 900 °C geopolymer ceramic. There was a formation of open pores with irregular shapes obtained from the sintering process. Pore formation was believed to be contributed by reactive MgO, which can considerably increase the geopolymer paste’s early and late densification (Jamil et al., 2021). This occurrence was verified by the akermanite 3

“Reprinted from Materials, 14(6), Jamil, N.H et al. (2021). Self-Fluxing Mechanism in Geopolymerisation for Low-Sintering Temperature of Ceramic, 1325, Copyright (2021), with permission from MDPI AG.”

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Fig. 6 The compressive strength of cured and sintered kaolin-GGBS geopolymer affected by the curing temperature (Jamil et al., 2021)4

(Ca2 Mg(Si2 O7 ) phase, which existed at 900 °C from the X-ray diffraction result. Besides, the addition of reactive MgO into the geopolymer paste would refine the pore size, thus increasing the compressive strength (Li et al., 2019). The existence of nepheline and albite, which consists of Na2 O, acts as a self-fluxing agent to complete the sintered geopolymerisation with densified properties at low temperatures. The major influencing factor is the pore distribution of sintered kaolin-GGBS geopolymer. It implies that a higher temperature has drastically changed the microstructure. The crystallinity, which determines the strength of glass–ceramic, is the second contributing factor. To attain high compressive strength, a high volume fraction of crystallinity must be obtained. By referring to Fig. 6, the compressive strength first increased and then decreased with the increase of sintering temperature, reaching a maximal value of 9 MPa at 900 °C. However, the compressive strength was reduced at a high temperature of sintering (1000 and 1100 °C). The phase transformation of kaolin-GGBS geopolymer ceramic with different sintering temperatures is presented in Fig. 7. Sintering at 800 °C also resulted in the formation of a semi-crystalline phase with an amorphous phase of magnetite, which exists at a lower degree related to the deformation of the fibroferrite phase in kaolin during sintering. In addition, the degree of crystallinity of quartz was increased during sintering at 800 and 900 °C. The composition of quartz was 77% at 800 °C sintered kaolin-GGBS geopolymer had contributed to the high compressive strength. Quartz (SiO2 ) is unreactive at low temperatures, transforming into a highly viscous liquid (Amar et al., 2018). However, the composition of quartz was decreased to 4% at 900 °C sintered sample though the compressive strength obtained was the highest at this temperature. It is due to the formation of 10% akermanite and 13% of nepheline. Akermanite belongs to the melilite group, which has excellent wear and corrosion resistance (Aziz et al., 2020b; Liu et al., 2009). The intensity of akermanite peak decreased is compared between sintering temperature of 900 and 1000 °C. However, the albite phase had 4 “Reprinted from Materials, 14(6), Jamil, N.H et al. (2021). Self-Fluxing Mechanism in Geopolymerisation for Low-Sintering Temperature of Ceramic, 1325, Copyright (2021), with permission from MDPI AG.”

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Fig. 7 Phase analysis of kaolin-GGBS geopolymer ceramic at different sintering temperatures. Q—Quartz, M—Mullite, N—Nephaline, AL—Albite, A—Akermanite and AN—Anorthite (Jamil et al., 2021)5

become the major crystalline in 1000 °C sintered kaolin-GGBS geopolymer. Albite occurred due to the transformation of nepheline when there were excessive Al2 O3 and Na2 O during sintering at 1000 °C. The importance of viscosity during sintering and crystallisation cannot be overstated. When the viscosity is too low, the crystallisation process is accelerated, preventing sintering and resulting in many porosities. If the viscosity is too high, however, crystallisation will be difficult. The presence of anorthite phase at 1000 °C sintered samples shows that the primary calcite phase from GGBS had decomposed during sintering due to heating above the decarbonisation temperature. CaO that originates from calcite and akermanite has aided the formation of anorthite phase with inadequate time and temperature for the reactions to form Ca-silicates, which is anorthite. At 1000 °C, the peak of quartz phase (SiO2 ) and akermanite (Ca2 Mg(Si2 O7 ) had decreased. The permitted SiO2 and CaO from these phases will initiate the formation of an anorthite phase. Because 5

“Reprinted from Materials, 14(6), Jamil, N.H et al. (2021). Self-Fluxing Mechanism in Geopolymerisation for Low-Sintering Temperature of Ceramic, 1325, Copyright (2021), with permission from MDPI AG.”

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kaolinite gives the ceramic mixture plasticity, quartz (SiO2 ) keeps the shape of the produced product during sintering and Na2 O from the alkali activator, which serves as a self-fluxing.

4 Properties of Geopolymer Ceramic Particles for Solder Reinforcement The addition of geopolymer ceramic as reinforcement material in a metal matrix composite (MMCs) developed economic ceramics with less high energy consumption. Furthermore, geopolymer ceramics demonstrate enhanced strength, thermal expansion, stiffness, and durability compared to unreinforced materials.

4.1 Geopolymer Ceramic During sintering, the high sintering temperature will induce the properties of geopolymer structure. Before sintering, the geopolymer grains are loosely associated with each other, leaving a large vacancy and causing large pores inside the geopolymer structure. However, after being exposed to high thermal treatment, the expansion and shrinkage of the geopolymer matrix occurred due to dehydration of the structure, dehydroxylation, phase transformation of quartz, and viscous sintering (Aziz et al., 2020a; Bernal et al., 2011). As a result, the microstructure is refined, becoming a more compact and denser structure. As a result, the geopolymer structure becomes strengthened and therefore improves the compressive strength of the geopolymer. The strengthen of geopolymer can also be related to the phase changes at specific sintering temperatures. Different sintering temperatures can cause different properties depending on how the geopolymer-based elements react at high temperatures. Sintering caused the transformation of the phase from the amorphous state to the crystalline state. For example, the exposure of fly ash-based geopolymer to heat treatment between 370 and 600 °C emanated the crystallisation of hematite and magnetite into the maghemite phase (Cheng-Yong et al., 2017). The nepheline phase can be detected when the fly ash-based geopolymer is subjected to 800 °C, and the diffraction peak of the nepheline increases when subjected to 1000 °C (Rickard et al., 2015). At high sintering temperatures, the appearance of akermanite and gehlenite phases were the dominant crystalline compound in slag-based geopolymer. The formation of gehlenite appeared based on the transformation of free calcium oxides that reacts with silica and aluminium oxide from the geopolymer matrix (Cheng & Chiu, 2003). At 800 °C, the presence of the calcite phase begins reducing intensity compared to sintering at 600 °C. Some noted that increasing the sintering temperature of

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slag-based geopolymer above 900 °C creates wollastonite, merwinite, and larnite formation. The kaolin-based geopolymer is composed of kaolinite and quartz due to its high Si content. However, previous researchers elucidated the appearance of kaolinite begins to diminish, transforming into the zeolite phase after the alkaline activation reaction process (Ahmad et al., 2017; Heah et al., 2012). The presence of nepheline also is detected in kaolin-based geopolymer at a sintering temperature of 1200 °C. The appearance of the nepheline phase is related to high strength structure. Several researchers have also found the appearance of the mullite phase at sintering temperatures ranging from 1050 to 1230 °C. Ceramic products are famous for their high electrical resistivity and have always been an attractive subject due to their good chemical, fire, and thermal resistance (Topçu et al., 2012). However, there are limited studies about the electrical conductivity of geopolymer ceramic. Electrical conductivity is defined as the vice versa of the electrical resistivity where high resistivity is equal to low conductivity (Heaney, 2003). The electrical property of geopolymer ceramic material depends on the type of solid precursor used in geopolymer production, the concentration of alkaline activator, and the curing temperature. Based on Cai et al. (2020), the fly ash-based geopolymer has higher resistivity compared to metakaolin-based geopolymer, which means the fly ash-based geopolymer exhibit lower conductivity than metakaolinbased geopolymer. This might be related to the specific surface area of metakaolin, which is higher than the fly ash (Cai et al., 2020). The influence of the sintering temperature on the development of electrical properties is normal for ceramic materials. After sintering, the microstructure of geopolymer ceramic is refined and reduced in size due to the shrinkage process by dehydration of water molecules in the geopolymer structure. Thus, the limitations of porosity encouraged the electrical ion mobility to move to their neighbouring ion. Besides, the material with high Ca concentration, such as slag, can also improve the electrical conductivity. The element Ca contributes to fast-setting time in accelerating the geopolymerisation reaction process. Aluminosilicate sources with high Ca content + tend to consume more conductive ions due to the ions such as Ca+ , OH− , SO2− 4 , Na , + and K can allow the current flow rate (Liu et al., 2015).

4.2 Geopolymer Ceramic with Polymer Addition Table 1 summarises the physical parameters of kaolin ceramics, kaolin-based geopolymer, and kaolin-based geopolymer ceramics with UHMWPE as a binder, including flexural strength, density, porosity, volumetric shrinkage, and water absorption. The production of the smooth matrix due to the establishment of a broad sintered are afforded by the good diffusion of the sample particles resulted in the greatest flexural strength of 92 MPa for kaolin-based geopolymer with the addition of UHMWPE (Bernard-Granger et al., 2007). The degradation of UHMWPE increases the carbon content in the geopolymer system, which increases flexural strength. Because of the

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Table 1 Kaolin ceramics, kaolin-based geopolymer ceramics, and kaolin-based geopolymer ceramics with UHMWPE as a binder: physical and mechanical properties (Romisuhani Ahmad et al., 2021)

Flexural strength (MPa)

Kaolin-based geopolymer ceramics

Kaolin-based geopolymer ceramics/UHMWPE

85.83

92

Theoretical density (g/cm3 )

2.13

1.88

Total porosity (%)

8.42

34.01

Volumetric shrinkage (%) Water absorption (%)

14.1

18.32

1.2

11.31

liquid sintering and decomposition of UHMWPE, which leads to the closure of accessible pores and pore channels, the theoretical density of the samples with UHMWPE reduced compared to the sample without UHMWPE (Olevsky & Molinari, 2000). The porosity of ceramic materials determines how much water may be absorbed. Because of the formation of pores caused by the degradation of the polymer during sintering, the percentage increased. Furthermore, the sintering of kaolin geopolymer causes pores to form throughout the samples. The existing pores permit more water absorption, increasing the water absorption value (Romisuhani Ahmad et al., 2021). The densification process was credited for decreasing the percentage of water absorption, which was then increased due to the creation of micropores when the binder was dehydroxylated. In conclusion, the addition of binder influences kaolinbased geopolymer ceramics’ physical and mechanical properties; hence, it could be potentially used as reinforcement material in the solder alloy.

5 Application of Geopolymer Ceramic as a Reinforcement Particle in Lead-Free Solder As mentioned in the previous subtopics, geopolymer ceramic with good mechanical strength and electrical conductivity might function as reinforcement particles in the lead-free solder alloy, forming a composite solder. Composite solder was a termed referring to the incorporation of reinforcement particles in the lead-free solder alloy. There are two types of reinforcements: intrinsic and extrinsic reinforcement particles (Guo, 2007). Intrinsic reinforcements refer to the addition of preformed intermetallic particles to the solder matrix or the intermetallic phase particles formed in the process of soldering or annealing. Meanwhile, extrinsic reinforcements were termed, describing the reinforcement particle that was added extrinsically. Since geopolymer ceramic particles were added extrinsically in lead-free solder alloy, it can be classified as one example of extrinsic reinforcement. The incorporation of geopolymer ceramic in the solder alloy used a similar concept with metal matrix composites (MMC) in materials. In MMC materials, the matrix comes from metals,

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and the reinforcement can be either metallic or non-metallic particles. The incorporation of reinforcement particles to lead-free solder aimed to improve the existing lead-free solder alloy properties. To date, there is still no research has been published on the incorporation of the geopolymer ceramic particles in the lead-free solder alloy. Therefore, kaolin geopolymer ceramic material with fair electrical conductivity and good strength was chosen to be incorporated into the lead-free solder alloy. Moreover, the kaolin geopolymer ceramic elements such as Si, Al and Fe may provide additional nucleation sites in lead-free solder alloy, thus improving the existing leadfree solder alloy. Research on the incorporation of kaolin geopolymer ceramic will be fundamental in elucidating the performance of geopolymer ceramics toward leadfree solder alloy. Therefore, this subtopic will cover and discuss the method used in fabricating the composite solder and the properties exhibited by composite solder with kaolin geopolymer ceramic as the reinforcement particles.

5.1 Composite Solder Fabrication To fabricate composite solder, various methods are reported: through the in-situ method and mechanical-mixing method (Shen & Chan, 2009; Nai et al., 2006, 2008). The in-situ method involving adding the reinforcement particles intrinsically. Meanwhile, in the mechanical-mixing method, the reinforcement particles were added extrinsically to solder alloy. There exist various methods in mechanical mixing that are (i) mixing molten solder alloy with reinforcements, (ii) mixing solder pastes with reinforcements and (iii) mixing solder alloy powder with reinforcements particles (Guo, 2007). Mixing solder alloy powder and reinforcement particles could be conducted using the powder metallurgy method. Thus, this research on the addition of kaolin geopolymer ceramic into lead-free solder alloy will be conducted using the powder metallurgy route. Solder-reinforced kaolin geopolymer ceramics were fabricated using a powder metallurgy method using hybrid microwave sintering (Mohamad Zaimi et al., 2020). In a powder metallurgy method, there are three important steps. Firstly, a solder material was mixed with 1 wt% kaolin geopolymer ceramics until the mixture was homogenous in an airtight container. The mixing process occurred for an hour by using a planetary mill. As the mixture was homogeneously mixed, the mixture underwent a compaction process. The compaction process was carried at room temperature. The mixture was placed in a stainless-steel mould cavity, and a load of 4.5 tons was applied to hold the particles of the mixture together to produce a compact green pellet. To sinter the compact green pellet, a hybrid microwave sintering technique was performed. The green compacted pellet was sintered at 185 °C under ambient conditions using a 50 Hz microwave oven for about 3 min. A microwave susceptor material, silicon carbide (SiC), was used to prevent heat loss during the sintering process. Finally, the sintered pellet was rolled into a thin sheet at room temperature before the reflow-soldering process occurred.

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5.2 Properties of Solder Reinforced Geopolymer In this subtopic, the characterisation of the composite solders specifically by using Sn3.0Ag–0.5Cu (SAC305) reinforced with 1 wt% kaolin geopolymer ceramics, including physical, mechanical, and thermal properties, had been performed. As depicted in Fig. 8, kaolin geopolymer ceramics were well distributed along the grain boundaries of the SAC305 solder alloy. The distribution of kaolin geopolymer ceramics along the grain boundaries was advantageous in improving the strength of solder alloy. This occurrence was due to the fact that the kaolin geopolymer ceramics hold the grains to reduce the occurrence of grain dislocations and retard the growth of grains. To understand the effects of kaolin geopolymer ceramics on SAC305 solder alloy, the microstructure of solder joints in the solder bulk and the interfacial layer was analysed. Figure 9 shows the microstructure of SAC305 reinforced kaolin geopolymer ceramics at the bulk solder and interfacial layer. Based on Fig. 9a, c, there were two different phase presence in SAC305 solder alloys that are β-Sn and eutectic phases before the reaction occurs in solder alloy during the reflow-soldering process. In eutectic phases, there was the formation of two intermetallic particles (Cu6 Sn5 and Ag3 Sn). The addition of 1 wt% kaolin geopolymer ceramics to SAC305 solder refined the β-Sn area by about 11%. Meanwhile, for the eutectic phase, the addition of kaolin geopolymer ceramics increased the area of eutectic by 52% with the smaller size of primary IMCs (Cu6 Sn5 and Ag3 Sn). The enhancement in the microstructure at the bulk solder of SAC305 is attributed to the theory of heterogeneous nucleation. Furthermore, the formation of finer microstructure with the addition of silicon dioxide (SiO2 ) in solder alloy was also has been reported (Wang et al., 2015). On top of that, the intermetallic compound (IMC) layer, resulting from the reaction of molten solder alloy with the copper substrate during reflow-soldering, is crucial in determining the solder joint’s reliability. The morphology of the IMC layer in SAC305 without the addition of kaolin geopolymer ceramic shows an elongated

Fig. 8 Illustration on the distribution of kaolin geopolymer ceramics in solder matrix

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Fig. 9 Microstructure of SAC305 without the addition of kaolin geopolymer ceramics a at bulk solder and b interfacial IMC. Microstructure of SAC305 with the addition of 1 wt% kaolin geopolymer ceramics c at bulk solder and d interfacial IMC

and small scallop, as depicted in Fig. 9b. Meanwhile, in Fig. 9d, the morphology of SAC305 with the addition of kaolin geopolymer ceramics showed a combination of small and shallow scallops. The formation of an elongated scallop can affect the solder joints’ strength because the elongated scallop leads to brittle fractures. Thus, an elongated scallop of IMC morphology was undesired for a good solder joint. Additionally, the formation of the IMC layer was crucial to indicate the metallurgical bonding in the solder joints. Excessive formation of the IMC layer degrades the solder performance since IMC was brittle. Thus, proper controlling in the thickness of the IMC layer as desired. The addition of kaolin geopolymer ceramics reduced the average thickness of the IMC layer from 5.92 to 4.14 μm. The improvement to the thickness of the IMC layer was governed by the existence of kaolin geopolymer ceramics particles along with the IMC layer, as proved by line analysis of energydispersive X-ray presented in Table 2. The existence of Si and Fe elements from kaolin geopolymer ceramics act as a barrier for the diffusion of copper from the substrate with the molten solder alloy. The formation of reliable metallurgical bonding between a solder joint and substrate is one of the important properties of electronic packaging. Solderability testing is one of the important tests to evaluate the quality of the bonding. Solderability can be assessed by measuring the contact angle (wettability) between solder molten and substrate and through the areas of solder spreading (spreadability) on

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Table 2 EDX analysis at the IMC layer (Mohamad Zaimi et al., 2020)6 Elements

Mass (%)

Atom (%)

Sn

62.40

47.66

Ag

3.46

2.90

Cu

33.50

47.80

Si

0.37

1.18

Fe

0.28

0.45

Table 3 Contact angle and spreading areas of SAC305 and SAC305 with the addition of 1 wt% kaolin geopolymer ceramic (Mohamad Zaimi et al., 2020; Syahirah Mohamad Zaimi et al., 2019) Solder type

Contact angle (°)

Spreading areas (mm2 )

SAC305

24.00

74.00

SAC305 with 1% kaolin geopolymer ceramic

20.80

91.45

the substrate. Smaller contact angles and bigger spreading areas can offer good solderability (Chen et al., 2016). Table 3 depicts the values of the contact angle and spreading areas for SAC305 and SAC305 with the addition of 1 wt% kaolin geopolymer ceramics. According to the results, adding 1 wt% kaolin geopolymer ceramic can reduce the contact angle to 20.80° with higher spreading areas of 91–45 mm2 . The SAC305 wettability and spreadability enhancements might be attributed to the accumulation of kaolin geopolymer ceramics at the interface between molten solder and flux, which lowers the interfacial surface energy and reduces the interfacial surface energy the surface tension. However, excessive addition of reinforcement particles worsens the solderability properties due to the increased viscosity of the solder itself (Sharma et al., 2015). Increasing the viscosity of molten solder hinders the molten solder from spreading further, and as a result, molten solder is not properly wet on the substrate. The performance of SAC305 with the addition of kaolin geopolymer ceramics can be evaluated through thermal properties. In thermal properties, important components, which are melting point and undercooling values, can be evaluated. The melting point of SAC305 with the addition of 1 wt% kaolin geopolymer ceramic was 220.56 °C. Meanwhile, in SAC305, without the addition of kaolin, geopolymer ceramics was 220.86 °C. A slight difference in the melting point proved that SAC305 solder with the addition of kaolin geopolymer ceramic could be integrated and used in the existing soldering process. For undercooling, the addition of kaolin geopolymer ceramics reduced the undercooling value by about 24% compared to SAC305 without the addition of kaolin geopolymer ceramics. Undercooling is the difference between Tonset during heating and Tonset during cooling. The reduction in the undercooling 6

“Reprinted from Materials Today Communications, 14, N.S. Mohamad Zaimi, M.A.A. Mohd Salleh, M.M.A.B. Abdullah, R. Ahmad, M. Mostapha, S. Yoriya, J. Chaiprapa, G. Zhang, D.M. Harvey, Effect of kaolin geopolymer ceramic addition on the properties of Sn-3.0Ag-0.5Cu solder joint, 776, Copyright (2021), with permission from Elsevier.”

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Fig. 10 Fractured surface of SAC305 a without the addition of kaolin geopolymer ceramics and b with the addition of kaolin geopolymer ceramics (Mohamad Zaimi et al., 2020)7

value was beneficial to solder alloy since it led to finer microstructure formation. As discussed earlier, the area fraction of β-Sn was reduced with the formation of smaller IMC particles dispersed in the eutectic area. This was due to the effect possess by reinforcing particles in the solder. The decrease in the value of undercooling with the addition of ceramic particles in solder matrix was due to the reduction in the undercooling value of β-Sn that then inhibits the formation of IMCs in solder alloy (El-Daly et al., 2013). Moreover, reinforcing particles in the solder alloy can act as additional nucleation sites. By promoting higher nucleation, the nucleation rate of β-Sn will be faster and lessen the time for IMCs to grow further. Therefore, a finer microstructure will be formed. Mechanical testing is important to evaluate the mechanical performance of new solder alloys. Since it deals with the material’s response under the applied force, in this study, the mechanical properties of SAC305 solder were analysed based on the shear strength and fracture behaviour after the lap shear testing. The average shear strength of SAC305 with the addition of kaolin geopolymer ceramic was 13.01 MPa. Meanwhile, the average shear strength for SAC305 without the addition of kaolin geopolymer ceramic was 9.95 MPa. The increase of 31% in the average shear strength was attributed to dispersion strengthening theory. The existence of IMC particles, Cu6 Sn5 and Ag3 Sn, in the solder matrix strengthens the solder alloys. Moreover, the average shear strength enhancement was related to forming a thinner IMC layer at the solder joints. On the other hand, the solder joint’s strength is weakened by the thicker IMC layer in the solder joints, as the thicker IMC layer is more susceptible to brittle failure. Besides, the failure behaviour of SAC305 solder alloy was also evaluated through post-shear-testing samples under a scanning electron microscope (SEM). Figure 10 shows the fractured surface for SAC305 solder alloy. Based on Fig. 10a, SAC305 without the addition of kaolin geopolymer ceramic shows a combination of failure 7

“Reprinted from Materials Today Communications, 14, N.S. Mohamad Zaimi, M.A.A. Mohd Salleh, M.M.A.B. Abdullah, R. Ahmad, M. Mostapha, S. Yoriya, J. Chaiprapa, G. Zhang, D.M. Harvey, Effect of kaolin geopolymer ceramic addition on the properties of Sn-3.0Ag-0.5Cu solder joint, 776, Copyright (2021), with permission from Elsevier.”

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mode, ductile, and brittle mode. Meanwhile, SAC305, with the addition of kaolin geopolymer ceramic, exhibits ductile fracture mode with the formation of dimples, showing a better plastic property in the ductile materials. As a result, it can be inferred that adding kaolin geopolymer ceramic to SAC305 solder alloy could be a viable option for improving the qualities of an existing solder alloy. However, research on the performance and the reaction of solder alloy with the addition of kaolin geopolymer ceramic under multiple reflow soldering and the isothermal ageing process should be further investigated. Moreover, the incorporation of fly ash and slag-based geopolymer ceramic into lead-free solder alloy can also be further investigated.

6 Conclusion This research has shown the potential use of geopolymer ceramic reinforced solders using powder metallurgy technique assisted using a sintering method. The effect of direct sintering, the effect of double sintering (hybrid microwave sintering), the effect of UHMWPE, and the effect of self-fluxing have been discussed in this chapter. The types and properties of geopolymer ceramic, such as fly ash, ground granulated blast furnace slag, and clay, have been discussed in this chapter. Each geopolymer ceramic has been discussed in terms of synthetic materials and chemical composition. This chapter explains that the geopolymer ceramic reinforced solder has been manufactured using powder metallurgy technique and sintered using direct sintering and double sintering methods. The properties of geopolymer ceramic on electrical and mechanical properties were explained to give fundamental knowledge to this material for reinforced solder. This chapter discussed some important properties such as microstructure, porosity, electrical resistivity, and thermal resistance. This chapter also discussed the performance of geopolymer ceramic composite solder, including the mechanical properties. This chapter also mentioned some possible addition, which is polymer addition such as UHMWPE into geopolymer ceramic. Finally, the addition of geopolymer ceramic has been added to Sn–3.0Ag–0.5Cu (SAC305) solder alloy using the powder metallurgy method. The main properties such as mechanical properties, wettability performance, and thermal properties have been discussed in this chapter. It is crucial to note that more research still needs to be investigated to prove the suitability of this geopolymer ceramic as a reinforcement in composite solder. This study improved the industry for using lead-free solder with environmental-friendly material that is without Pb-content solder. This finding can also reduce the electronic waste due to less replacement of the reliable electronic product with longer lifetimes. Acknowledgements The authors acknowledge the support of Center of Excellence Geopolymer & Green Technology, Universiti Malaysia Perlis, and The Use of ISIS Neutron and Moun Source for the project Neutron Tomography Studies of Geopolymer Ceramic used for Reinforcement Materials

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in Solder Alloys for a Robust Electric/Electronic Solder Joint (JPT.S(BKPI)2000/016/018/019(42) (9022-00009) from the Ministry of Higher Education, Malaysia.

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Surface Modifications on Ceramic Reinforcement for Tin-Based Composite Solders Leong Wai Keong, Ahmad Azmin Mohamad, and Muhammad Firdaus Mohd Nazeri

Abstract This chapter aimed to provide a general overview of several surface modifications’ techniques of ceramic reinforcements on the properties of composite solders. It is commonly known that composite solders experienced significant aggregation and non-wetting issues between the ceramic reinforcements and solder matrix, which makes the addition of the reinforcement phase inefficient in improving the properties of lead-free solders. Surface modification techniques such as pyrolysis, chemical route surface modification and metal plating have been developed over the years to address this issue and have shown progressive impact in improving the interface of the reinforcements and solder matrix. The chapter highlights the fundamental understanding, main parameters and setup, and latest developments on the surfacemodified ceramic-reinforced composite lead-free solders. Finally, the chapter also summarised and discussed the advantages, current trends, and significant findings in this field. Keywords Lead-free solder · Composite · Surface modifications · Ceramic reinforcements

1 Introduction Solder alloys are used as an interconnection material in electronic packaging that provides physical, mechanical, and electrical connectivity. Through various levels of electronic packaging, the use of solder joints has become essential as the connection made was reliable at an affordable price. With a long history of application, the use of solder alloys has become indispensable. Nowadays, the demand for high performance and diverse functionalities into single mobile electronic devices keeps increasing. As a natural outcome, the dependability and performance of each solder joint created were vital, as the loss of a single solder joint can trigger a device component’s L. W. Keong · A. A. Mohamad · M. F. M. Nazeri (B) Center of Excellence Geopolymer & Green Technology (CEGeoGTech), Universiti Malaysia Perlis (UniMAP), 02600 Arau, Perlis, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 M. A. A. M. Salleh et al. (eds.), Recent Progress in Lead-Free Solder Technology, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-3-030-93441-5_3

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malfunction, consequently causing the whole electronic system to crash (Abtew & Selvaduray, 2000). The most frequently used solder alloys are tin–lead (Sn–Pb) alloys, particularly the near-eutectic Sn-40Pb and eutectic Sn-37Pb. The physical metallurgy, dependability, production methods, flux chemistries, and general characteristics of this alloy are well-established, with a record of application extending to over 2000 years ago (Tsao, 2012). In addition, Sn–Pb alloys’ melting eutectic temperature is also low, allowing them to be used on a wide range of substrate materials. Nevertheless, relying on the available information, the Pb toxicity matter has generated substantial questions and concerns about the metal’s presence and influence on the environment (Nazeri et al., 2019). As a consequence, legislation was enacted to restrict and prohibit the use of Pb-containing solder in the electronic industry (Abtew & Selvaduray, 2000; Frear, 2001; Tsao, 2012, Nazeri et al., 2019). As a result, a variety of Pb-free solder alloys have been suggested and produced. The majority of these alloys use Sn as the primary element, such as tin–bismuth (Sn–Bi), tin–copper (Sn–Cu), tin–zinc (Sn–Zn), tin–silver–copper (Sn–Ag–Cu), and tin–silver (Sn–Ag). In recent years, the miniaturisation trend in electronic devices has compelled electronic assembly to provide the highest density in a restricted area in the input/output (I/O) connections. To address this issue while preserving solder joint reliability, researchers have worked to produce new Pb-free solders with a greater creep resistance, enhanced microstructure properties, better mechanical properties, as well as reduced melting point using non-coarsening oxide dispersoids, non-reacting, and nano-sized to create composite solder (Tsao, 2012).

2 Issues in Ceramic-Reinforced Composite Solder Considerable type of ceramic nano-sized particles has been used as reinforcement phase to produce composite solder, including silicon carbide (El-Daly et al., 2013, 2015; Liu et al., 2008), carbon nanotubes (Dele-Afolabi et al., 2017; Ismail et al., 2020a, b), graphene (Ma et al., 2017a, b; Han et al., 2020), zirconia (ZrO2 ) (Amares et al., 2021; Dong et al., 2019a, b; Rajendran et al., 2020), titanium dioxide (TiO2 ) (Nasir et al., 2019; Skwarek et al., 2020; Yahaya et al., 2020), tin oxide (SnO2 ) (Wattanakornphaiboon et al., 2018), and cerium dioxide (CeO2 ) (Sharma et al., 2013). The choice of selection depends on various factors such as specific physical properties, special structure, and suitable synthesis techniques. For example, graphene nanosheets (GNSs) were chosen due to its unique optical and biomedical properties (Wang et al., 2019a, b), and tetra needle-like zinc oxide (T-ZnO) was used based on its special three-dimensional spatial structure (Huo et al., 2021a, b). However, several problems have been identified in hindering the full potential expected with the additions of reinforcement phase in Pb-free solder, namely, phase segregation and non-wetting and non-uniform distribution properties of reinforcement particle.

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2.1 Phase Segregation Particle aggregation or agglomeration is a process of particles spread in the liquid phase attached to each other. This process starts when the particles are induced after colliding with each other to form larger clusters after the repulsive forces weaken (Yu et al., 2018). At this state, the Van der Waals forces become dominant and trigger more aggregation. The reinforced particle agglomerates tend to reduce the interfacial interaction between matrix and filler and serve as a route for stress transfer, leading to the diminishment of properties. Gyökér et al. have demonstrated that the SiC particles were seen aggregated at the outer side of the solder joint (Gyökér et al., 2019). Poor distribution of reinforcement particles has insufficient wetting behaviour and non-uniform properties of the composite solder. The reinforcement coarsening and poor distribution also directly affect the microstructure and other solder joint properties.

2.2 Non-wetting and Non-uniform Distribution Properties of Reinforcement Particle It is well known that the wettability in the interface of ceramic–metal is essential in producing fully functional ceramic-reinforced metal matrix composite. According to Nogi (2010), many regulating elements influence wettability, including the metal and ceramic coefficients of thermal expansion (CTE), exterior factors, alloying constituents, thermodynamic equilibrium of liquids and solids, humidity, and environment. In most situations, metals have a significantly greater CTE than ceramics, resulting in significant incompatibility that impedes the wetting process. Coarse texture on ceramic-reinforced composite solder is one of the features noticed as a reaction of non-wettable reinforcement movement from the molten solder during the reflow process (Chen et al., 2016). This displacement issue leads to cracks and porous structures forming within the solidified composite solder, which directly reduces mechanical integrity (Pal et al., 2020a, b). Furthermore, significant variations in the primary physical characteristics, such as surface energy and density, can induce reinforcement displacement (Khodabakhshi et al., 2017; Mehrabi et al., 2016). For improvement, special approaches such as pressure-less activated infiltration and several other techniques have been developed for fabricating ceramic/metal composite (Qi et al., 2021).

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3 Type of Surface Modification Technique on the Reinforced Solder Recently, surface modification of the reinforcement phase has gained much attention with the specific aim to improve the reaction by removing voids in the interface region. The formation of interfacial voids indicates incompatibility between the reinforced particle and molten solder (Fig. 1a). Therefore, a good bonding network needs to be considered through void rigidification (Maghami et al., 2020), as shown in (Fig. 1b). The pyrolysis techniques, chemical route surface modification, and metal plating have been touted as some of the most promising strategies to create bridging and allow a strong connection between reinforcement with the molten solder matrix (Fig. 1c).

3.1 Pyrolysis Pyrolysis is a thermochemical process in which the sample is heated under an inert atmosphere, normally around 500 °C, to produce various states of the sample such as gas, liquid, and solid products. Pyrolysis is widely used as it is a cost-effective technology and environmentally friendly as it releases less harmful gas (Abdallah et al., 2020; Dong et al., 2019a, b). This straightforward technique is mostly utilised in the agriculture sector (Fodah et al., 2021; Roy & Dias, 2017). As for surface modification, pyrolysis is normally used to convert carbon-based materials such as polysilazane (Salameh et al., 2019; Wie & Kim, 2020) and cellulose aerogels (Gopinath et al., 2021; Sabzehmeidani et al., 2021; Torres et al., 2020) into ceramics or carbon aerogels. This aimed to improve interfacial adhesion between matrix and reinforcement materials. Various positive impacts have been reported based on the effect of pyrolysis surface modifications such as increase in thermal conductivity (Wie & Kim, 2020), electrical, flame retardant, and water proofing

Fig. 1 a Formation of voids at the interfacial region, b void rigidification to remove void c bridging effect take place

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(Wan & Li, 2016), and ease in tailoring optoelectronic properties (AlShammari et al., 2018). The polymer precursors were applied using different coating methods, for instance, dip coating or spraying, in order to use these polymer-derived ceramics as exterior modifiers. This is preceded by an initial heating step to enable the precursors’ crosslinking. Following the development of the infusible system, it will be transformed into a crystalline and amorphous ceramic by pyrolysis (Kim et al., 2017). Several main factors identified to play the most important effect on the quality of the surface modification in pyrolysis techniques, especially on the microstructure, composition, mechanical and chemical properties are (Bauer et al., 2005): i. ii. iii. iv.

Precursor system used The degree of crosslinking produced Pyrolysis temperature and atmosphere Environment humidity.

The nickel-modified graphene nanosheets (Ni-GNSs) were previously prepared using a segmented pyrolysis method (Wang et al., 2019a, b). To cover the Ni particle on the graphene oxide nanosheets (GONSs), the GONSs are diminished to produce reduced-graphene oxide sheets (r-GONSs) by refluxing and reducing the N2 H4 H2 O with the GONSs solution for 22–24 h at around 100 °C. After drying the r-GONSs sample under vacuum to achieve high purity, the r-GONSs and the source of Ni2+ (nickel acetate tetrahydrate) were homogenised using a ball milling technique. Then, the mixture was placed in corundum crucibles and underwent a segmented heating process. The fragmented pyrolysis process implicated three stages of heating the graphene oxide nanosheets-N2 H4 H2 O mixture, with the first phase resulted in crystal water’s loss, the second phase induces the formation of intermediate nickel oxide (NiO) nanoparticles in an inert atmosphere, and the final phase involving the final heating process at 380–450 °C to generate Ni-GNSs. A first-principle calculation was used to verify the efficacy of the pyrolysis process. When the adatoms were put on each side of the graphene’s H, B, and T, the highest durability was observed at the H site following complete relaxation (Wang et al., 2019a, b). After pyrolysis, a chemical adsorption process involving electron amplification between graphene and nickel atoms produces covalent bonds, resulting in high adsorption strength between GNSs and Ni nanoparticles. Additionally, the Ni atoms’ adsorption energy on faulty graphene structures was six times greater than that of flawless graphene. According to the first-principle computation, the doped Ni nanoparticles generated additional active sites on the GNSs’ surface without significantly raising the Ni-GNSs’ defect level, according to the first-principle computation. Copper is of the alternatives element available to be used as surface modification metals. This element is known to provide good advantages over other elements in terms of its price and availability while producing good electrical and thermal conductivity. Furthermore, as one of the transition metals, it is known that Cu atoms strongly bind to the defected graphene structure (Krasheninnikov et al., 2009). Therefore, with

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proper processing methods, surface modifications using Cu and GNSs could produce promising and reliable interfacial adhesion. There were three steps needed to synthesis Cu-coated Graphene nanosheets, which include ball milling, mixing, and heating processes (Zhang et al., 2019). According to Zhang et al. (2019), XQM-0.4L planetary ball mill was used to mill GNSs for 3 h. Following that, GNS and copper acetate (copper origin) were carefully infused before being heated for 3 h in an inert environment in a tube furnace at 500 °C. Cu-GNSs with a 13 nm’s mean size was obtained after cooling the sample at ambient temperature. Finally, the powder metallurgy method was used to prepare the composite solder (SAC + Cu-GNSs). High-energy milling treatment combined with pyrolysis allows the Cu-GNSs to change the adsorption type from physisorption to chemisorption. Significant reduction in average β-Sn grain size started from the additions of 0.03 wt.% Cu-GNSs, while further additions beyond this gave no obvious changes (Zhang et al., 2019). It is indeed worth noting that the existence of pyrolysed CuGNSs improves the eutectic phase dispersion from random distribution to eutectic network formation. Cu-GNSs were discovered at the interface of the alloy granules, hindering the dissemination of matrix Sn atoms and the bonding of alloy particles, and enveloped in metallurgically bonded alloy powder particles at the grain boundary, according to the microstructural analysis of composite solders at high magnification. This discovery revealed that pyrolysed Cu-GNSs are randomly distributed with reduced aggregation in the SAC-rare earth element (RE)/0.05Cu-GNS composite solder matrix. Consequently, the growth of the β-Sn grains was restricted, producing finer grain size. The pyrolysis technique has also shown good capacity in tailoring specific arrangements needed to modify the surface of the reinforcement of composite solders. In need of improving interfacial contact between the matrix and the reinforcement, mechanical interlocking is considered one of the beneficial approaches that can be made. Surface modification of tetra-needle zinc oxide (T-ZnO) by using nickel oxide (NiO) nanoparticles revealed that hemispherical-shaped NiO was successfully arranged in sawtooth-like configurations on the surface (Huo et al., 2021a, b). At the interface of the Sn1.0Ag0.5Cu solder and the NiO/T-ZnO, micromechanical interlocking was clearly visible after being examined, with no gaps or nanopores surviving in between the interface. Based on the transmission electron microscope image, it can be concluded that the atomic inter-diffusion was effectively conducted at the interface.

3.2 Chemical Route Surfactants are chemical compounds that can produce self-assembled molecular clusters known as micelles (Free, 2008; Nakama, 2017). This ability came from the amphiphilic nature of the chemical compound. Being amphiphilic means the molecules are made of two different functional groups that possess different polarities, hydrophobic and hydrophilic (Free, 2008; Nakama, 2017). The presence of two

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opposing structures allows surfactants to be soluble in water through the hydrophilic group, while to some extent, soluble in lipid systems through the hydrophobic group. This consequently reduces the surface tension at the liquid–liquid/liquid–gas/liquid– solid interface. The applications of surfactants initially involved in cleansing, but further developed in numerous functions. For example, surfactants have been used in remediation of petroleum oil contaminated soil (Karthick et al., 2019), synthesis of photocatalysts (Liang et al., 2019; Shen et al., 2009), pesticide application (Appah et al., 2020), and pharmaceutical drug delivery (França et al., 2018; Panigrahi et al., 2018). In addition, the ease of changing the functions of surfactants through manipulations of concentrations (Nakama, 2017) gives the advantage to use surfactants in various applications. Surfactants have been used to solve the agglomeration issue of nano-sized fillers. Chemicals such as polyoxyethylene (Sharma & Ahn, 2021, Wakeel et al., 2021), cetyltrimethylammonium bromide (Liao et al., 2017), octyl phenyl ether (Poh et al., 2016), polyoxyethylene (Morimoto et al., 2020), lauryl ether (Shrivastava & Das, 2019), and sodium dodecyl sulfate (SDS) (Lee et al., 2020; Lin & Duh, 2007; Yin et al., 2020) are used to treat the MWCNT surfaces, apart from silane coupling agent (Poh et al., 2016). Sodium dodecyl sulfate is a common surfactant from the family of organosulfate compounds mainly used to remove oil and stains. In recent years, this surfactant has been reported to be used in water treatment (Jin et al., 2017; Khan & Zareen, 2006), batteries (Ghavami & Rafiei, 2006; Hosseini et al., 2018) and improving the surface of adsorbents for solid-phase extraction as it has shown promising ability to adsorb on an oppositely charged metal oxide surface, for example, iron oxide (Bagheri et al., 2012; Miranda et al., 2015), alumina (Dobson et al., 2000; Mohamad et al., 2021), calcium oxide (Tamjidi & Esmaeili, 2019), and on natural polymer such as chitosan (Onesippe & Lagerge, 2008). The polar head forms the micelle–water interface, whereas the non-polar tail forms the micelle core in the context of immediate modification of SiO2 with SDS (Muhamad et al., 2015). During micellisation, the form SDS micelle will bind to the individual SiO2 . The micelle formed on the modified fillers reported induced repulsive force between the particles, prevented agglomeration, and improved graphene nanoparticles’ dispersibility in an epoxy matrix (Paramashivaiah & Rajashekhar, 2016). Meanwhile, the wall thicknesses of treated multi-walled carbon nanotubes (MWCNT) are thicker than untreated filler due to the attachment of SDS surfactant (Poh et al., 2016). In addition, the dominant dark spots of MWCNT agglomerations were successfully dispersed throughout the epoxy matrix, indicating a successful effect of SDS surface treatment (Poh et al., 2016). The successful surface modification of SDS to the surface of SiO2 nanoparticles was also reflected in several interactions under Fouriertransform infrared spectroscopy (FTIR) (Muhamad et al., 2015). Vibrations of –OH in the 3464 cm−1 absorption band, with the bending resonance (1626 cm−1 ) and S–O was stretching absorption (1219 cm−1 ) of sulfate group of SDS, demonstrate that SDS has already been deposited onto the surface of SiO2 nanoparticles through hydrophobic contact.

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Jing et al. (2017) looked into the consequence of adding modified Ag-GNSs to Sn-3.0Ag-0.5Cu (SAC305) solder. The addition of 10 mg SDS and 40 ml N, Ndimethylformamide enhanced the adherence of Ag on GNSs in this study. The new, modified Ag-GNSs particle was well separated, with 10 nm Ag particles successfully distributed on the GNSs. Two methods of producing Ag-GNSs/SAC305 composite solder were discussed through mechanical mixing and ball-milling approaches. The intermetallic compound (IMC) growth of the ball-milled Ag-GNSs/SAC was lower than the mechanical mixing and pristine SAC305 after soldering due to a more uniform distribution of the modified Ag-GNSs in the SAC305. Intense collisions between milling medium and solder particles during the milling process generated a large volume of defects that allowed Ag-GNSs to be partially embedded on the solder. Increase loading of Ag-GNSs loading from 0.03 to 0.1 wt.% revealed that thinner IMC was obtained. This indicates that the ability of modified Ag-GNSs to hinder the diffusion of Sn and Cu increased at higher filler loading up to 0.1 wt.%.

3.3 Electroless Plating Metal plating is a covering process of metal on another surface, typically another metal. Metal plating commonly served several purposes, such as improving corrosion resistance (Tang & Zuo, 2008) and wearability (Hiratsuka et al., 2003; Kanta et al., 2009), increasing electrical conductivity (Moazzenchi & Montazer, 2019), and aesthetic purposes (Ogden, 1993; Usui et al., 2018). Metal plating can be in different forms, depending on the applications, such as i.

ii.

iii.

iv.

Physical vapour deposition: Surface modification technique requires removing atoms from a solid source of coating materials through ions bombardment or heating process. This is followed by the deposition of the atoms on the surface of the substrate. Material such as chromium or titanium is commonly used as the source of the coating. This technique produces a very thin coating with a solid bond between the coating and the substrate, capable of producing a layer with high corrosion resistance and abrasive resistance (Rossnagel, 1999; Rossnagel & Kim, 2001). Atomic layer deposition: Another alternative method to produce a high-quality thin film. In this technique, the substrate will be exposed to two alternating vapour reactants until saturation of atoms is achieved. This unique thin film growth mechanism allows the production of a high uniformity and conformity layer that suits the applications in microelectronics (Ritala & Leskelä, 2002). Carburising: A heat treatment process that supersaturated carbon in the austenite phase of low carbon steel to produce a hardened layer with high wear and corrosion resistance (Sun, 2005). Electroplating: The most popular type of metal plating. Capable of electroplating various metals such as silver, gold, tin, copper, chrome, zinc, and nickel.

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It requires two electrically connected electrodes of cathode and anode and electrolyte solution. Direct current will pass through both electrodes through the electrolyte. The anode (source of coating) will be dissociated and deposited on the cathode. Electroplating offers excellent control of the precursor, even at extremely low concentrations (Giurlani et al., 2018). Immersion plating: Also known as dip plating, this deposition technique is regarded as one of the simplest. The process depends on chemical reactions without an external power supply. The driving force for reaction in this technique is the galvanic potential differences between the substrate and the dipping solution of nobler metal ions (Al-Syadi et al., 2021; Harraz et al., 2002; Tsuboi et al., 1998). Electroless plating: Alternative to electroplating with economic advantage. Just like immersion plating, no external current is needed. The process itself involves several simultaneous reactions, including producing negative charges on the substrate. This technique can create a layer with consistent thickness irrespective of the size and shape of the substrate. The coating produced usually has good frictions and excellent corrosion resistance (Loto, 2016).

Electroless plating has gained popularity in metal plating in recent years as significant improvement in binding properties can be achieved by this technique (Chen et al., 2003; Zhu et al., 2011). Aside from offering at a much low capital cost, electroless plating is easy and straightforward (Bulasara et al., 2011), with reproducible results. Besides that, electroless plating can also be applied to non-conductive surfaces such as an epoxy resin (Touyeras et al., 2005), reduced graphene oxide (Zhang et al., 2016), and SiO2 (Kiang et al., 1992). There are varieties of metal that can be deposited using the electroless plating technique, for example, palladium (Baudrand & Bengston, 1995; Cheng & Yeung, 2001), copper (Ali et al., 2020; Xu et al., 2004), silver (Hai et al., 2006; Schaefers et al., 2006), gold (Baudrand & Bengston, 1995; Hu et al., 2008), and nickel (Chen et al., 2003; Hu et al., 2008; Yu et al., 2018). Nickel is regarded as one of the ideal interlayer materials between the reinforcement and the solder in lead-free solder applications. Nickel served as bridging materials that produce stable IMCs with molten Sn as the main element in most leadfree solder. Consequently, the reinforcement phase that can be chemically inert can be distributed well with less agglomeration. This has been effectively hindering the aggregation of GNSs (Wang et al., 2019a, b). Moreover, Ni is recognised to bind with lead-free solder, resulting in the formation of robust IMC throughout the soldering process. It has also been extensively documented that using Ni as a transition layer in the interfacial area improved the overall characteristics of lead-free samples. In particular, adding a Ni layer to GNS significantly suppressed Cu3 Sn expansion (Chen et al., 2016), increased yield and tensile strength of Sn-3.5Ag-0.7Cu solder utilising Ni-modified GNS (Khodabakhshi et al., 2017) and shrinking of the eutectic region for Ni-added graphene-coated Cu/SAC305 solder (Li et al., 2018). Pure Ni electroless plating is the most basic type of electroless plating. This technique rivals electroplating in terms of the quality of the coating produced, especially on the corrosion and wear resistance. The coating finds wide applications in marine

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and harsh environments where most weak acids, alkalis, and other aggressive ions are present. Nickel alloys can also be deposited using electroless plating. Nickel alloys can also be deposited using electroless plating. Coating made of binary or ternary Ni alloys can be tailored depending on the reductant used. Nickel–phosphorus (Ni–P) and nickel–boron (Ni–B) alloys are two of the most common nickel alloys used in electroless plating. Electroless composite coatings are the most advanced type of electroless coating. This type of coating is usually made in combination of Ni alloys with the additions of solid particulates such as SiC (Huang et al., 2021), Al2 O3 (Mirzamohammadi et al., 2017), and zirconia (Bostani et al., 2018). Recent studies (Loto, 2016) highlighted that the coating’s quality and appearance are highly contributed by factors such as bath concentrations, deposition time, operating pH and temperature, and the additives used. Wang (2004) added that the mole ratio of cations and anions could also play an essential role in producing good deposition. In the meantime, Lee et al. (2019) found that depositing MWCNTs at pH 7 resulted in the maximum quantity and biggest size of nanoparticles. The amount of phosphorus was also found to dictate the metallurgical properties of the deposited layer (Baudrand & Bengston, 1995). The pre-treatments, for example, oxidation, hydrophilic treatment, and sensitising before electroless plating, have also led to increased Ni plating on a surface (Chen et al., 2003). To set for practical Ni electroplating bath as by catalytic chemical reduction as water solutions, at least three main components must be present (Loto, 2016), which are (i) source nickel cations, (ii) hypophosphite anions, and (iii) several types of additives such as chelating agent, exaltant, stabilizer, pH regulators, and wetting agent. A simplified synthesis process for the Ni-decorated MWCNT reinforcement has been reported by Lee et al. (2019). The first started with the acid treatment (nitric and sulfuric acids) process with the addition of copper sulfate solution to functionalise the surface of MWCNTs with Cu ions. A swirling and sonication procedure was performed to accelerate the development of the Cu nucleus on the exterior of the MWCNTs. The solution was combined and calcined before the Ni layer was deposited on the Cu nanoparticles. The electroless Ni plating was done at 80 °C, with variation in pH. The microstructure and the elemental properties of the successfully synthesised Ni-MWCNTs were examined to check for the quality of the surface modification process made before being mixed in lead-free solder. Composite solder utilising electroless Ni-plated reinforcements are usually produced through mechanical alloying (Khodabakhshi et al., 2017; Lee et al., 2019) or powder metallurgy route (Pal et al., 2020a, b, 2021). Solders, such as Sn-58Bi, flux, and different percentages of Ni-plated reinforcements (e.g., Ni-MWCNT) are combined in paste mixing equipment (Lee et al., 2019) or a planetary greater energy ball mill in the availability of inert gas in the mechanical alloying process. This is followed by a compaction and sintering process to produce the final product (Khodabakhshi et al., 2017).

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4 Effect of Surface Modification on the Reinforced Particle to the Solder Properties The impact of applying surface-treated reinforcements has been evaluated and investigated with considerable effort. The influence on five intermetallic compound properties, microstructure, temperature, wettability, and mechanical characteristics, on the other hand, was discovered to give significant and vital evidence on the dependability of surface-modified reinforcements in lead-free solder.

4.1 Intermetallic Compound In soldered connections, intermetallic composition serves a critical function. IMC causes the soldered joint to become brittle and weak at extreme temperatures. Joint rupture is more likely at greater temperatures as a result of this scenario. As a matter of fact, research on the impact of incorporating surface-treated reinforcements on IMC is critical. The IMCs of Cu3 Sn and Cu6 Sn5 of SAC305 changed from sharp, discontinuous scallop shaped to uniform and continuous layers with surfactant-modified CNTs addition (Xu et al., 2014). The presence of CNTs pinned the diffusion of Cu towards the interface, while accelerating the Cu migration from substrate to form Cu6 Sn5 , hence retarding the growth of both IMCs. Comparable findings of IMC growth inhibitions were also seen in surfactant-modified CNTs-Sn-0.3Ag-0.7Cu (Zhu et al., 2018), and in electroless Ni-rGO-Sn-2.5Ag-0.5Cu (Huo et al., 2018). The IMC thickness exhibits a linear functional relationship with the square root of the ageing period in ageing research (Wang et al., 2020). The IMC thickness increases as the isothermal ageing interval progress. The growth rate constant of SAC3050.2Ni-CNTs/Cu solder junctions was reported to be relatively lower than that of other experiments in this investigation. As a result, increased Ni-CNTs significantly inhibited the growth of IMC layers. Lee et al. (2019) also reported a comparable finding. When opposed to pristine Sn-58Bi solder, the IMC layer of the 0.1 wt.% Ni-MWCNT composite solder displays a substantial drop in thickness from 6.67 to 4.49 μm after 1000 h of thermal ageing. Mechanism of dual-layers of IMCs formation due to the presence of Ni can be explained in three consecutive stages of (Mehrabi et al., 2016): (i)

(ii)

Dissolution: The mechanism started with the Cu dissolution process from Cu substrate into the molten SAC solder, initiating metallurgical bonds. The concentration gradient of Cu at the interface becomes the driving force to allow Cu diffusion. As a natural outcome, the Cu/liquid interface might have a higher concentration of Cu. Chemical reaction: Given the existence of a substantial driving force for chemical interactions between Cu, Sn, and Ni (from the interface layer), the solid IMC of scallop-type (Cu1-x Nix )6 Sn5 complex structure began to develop.

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Solidification: Diffusion and reaction-type growth processes enable (Cu1-y Niy )3 Sn4 complicated layer development between (Cu1-x Nix )6 Sn5 and the copper substrate over time until full solidification is accomplished.

4.2 Microstructure The main objective in surface modifications of the reinforcement phase in leadfree solders is to improve the adhesion between the reinforcement and solder as the matrix. The microstructural analysis allows monitoring of solder development over time and enables surface modifications to enhance an interfacial reaction to be evaluated. Properties of composite solder are closely related to the size of the grain structure. Meanwhile, in most cases, fine-grained microstructure contributes to better properties of composite solder. Chen et al. (2014) investigated the impact of adding Ni-GNSs reinforcement to the SAC solder microstructure. Significant refinement in β-Sn from phases 31.36 ± 4.12 um while solder with the addition of Ni-GNS has an average grain size of 26.17 ± 3.85um was seen, while the long, needle-like shape of Ag3 Sn transform to a shorter and smaller structure. The same observation of grain refinement was also reported by Li et al. (2017) on the effect of SDS-modified Cu6 Sn5 additions in SnBi solder. Based on the elemental mapping performed, the homogeneous distribution of Cu element was identified across both Bi-rich and Sn-rich phases after the surface modification process on the reinforcement. The absence of agglomeration between reinforcement particles indicates that the surface modification successfully overcame the Van der Waals attraction. The dispersion of components in the lead-free solder throughout the reflow procedure can be limited by adequate reinforcement dispersion. The Zener pinning mechanism inhibits the development of granules and IMCs by inhibiting the mobility of these components (Kotadia et al., 2012; Mokhtari et al., 2012). As a result, the grains and IMCs in the composite solder would become finer after reflowing. Heterogeneous nucleation was also reported contributing to microstructure refinement, where the presence of reinforcement allows for an alternative solidification nucleation site and reduced the amount of undercooling needed (Ahmad et al., 2021). As a result, solidification is to be commenced on a nucleus with smaller critical radius. However, as the concentration of reinforcement reaches a certain critical point, agglomeration of particles is expected. It is well understood that the surface energy of nanoparticles is high due to their corresponding surface area. To reduce this surface energy, particles tend to bond with each other and forms aggregate. This process reduces the nucleation points, which also roughen the microstructure (Li et al., 2017). Huo and co-workers (2021a, b) demonstrated that the combination of pyrolysis and ultrasonic agitation further refined the microstructure of β-Sn and the IMCs. The pyrolysis methodology was used to alter the interface of (T-ZnO) to generate NiO nanoparticles adorned T-ZnO (NiO/T-ZnO) before the ultrasonic agitation procedure to ensure NiO/T-ZnO strengthened Sn1.0Ag0.5Cu composite solder. The volume of

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the IMC was decreased by 50.0% in this experiment. There were no micropores or gaps at the reinforcement/solder contact, confirming that the solder matrix and reinforcement were firmly bound, with a few small-sized IMCs identified at the contact. This demonstrated that during solidification, the NiO/T-ZnO reinforcement functioned as heterogeneous nucleation sites for IMC. The microstructure of Sn3.8Ag0.7Cu (SAC387) solder had a smaller grain diameter and apparent enhancement of compression strength at room temperature with the additions of Au–SiO2 (Mokhtari et al., 2012). This is contributed by the successful grain boundaries movement pinning effect of Zener pinning exerted by the Au–SiO2 as reinforcement phase (Fig. 2). This pinning effect wields pressure that thwarts the grain boundaries’ movement, hence improving the compressive strength. However, at elevated temperature, the pinning effect was sufficiently overcome as the size of the reinforcement phase particles was smaller compared to the growing grains of SAC387. This resulted in an insignificant difference in compression strength for SAC387 and the control sample.

Fig. 2 Zener pinning effect on nanocomposite solder. Reprinted with permission from Mokhtari et al. (2012)

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4.3 Thermal Properties Lead-free solder is a part of a microelectronic assembly exposed to repeated heat cycles during its service life. Therefore, understanding the thermal properties of leadfree solder is critical, especially on the changes that the additions of the reinforcement phase might pose. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are excellent methodologies for determining a variety of thermal properties in leadfree solder. Both DSC and DTA are highly similar techniques with the same applications, including the ability to observe fusion and crystallisation events such as glass transition temperatures and melting temperature (Tm) and study oxidation or other chemical reactions. The greatest distinction is that DSC calculates the amount of heat required to raise the reference temperature. DTA, on the other side, maintains a consistent heat flow for the experiment and reference. Zhu et al. (2018) used DSC to evaluate the thermal characteristics of both virgin and surfactant treated CNTs-SAC composite solder during the scanning period of 200 to 260°. The melting temperature of SAC solder was marginally raised by adding surfactant modified CNTs, from 226.4 to 226.6–226.8 °C. The addition of CNTs with a greater melting temperature resulted in a modest rise in the composite’s melting point. The same reduction in melting temperature based on DSC analysis was also seen in the composite solder system of electroless-Ni modified CNTs/SAC305 (Wang et al., 2019a, b). At the addition of 0.1 and 0.2 wt.% of electroless-Ni modified CNTs, the melting temperature increased from 217–220 °C to 221.2–223.2 °C. However, from DTA study reported for Ni-GNSs-Sn-2.5Ag-0.7Cu composite solder at a heating rate of 10 °C/min up to 360 °C, a small decrease in melting temperature from 233.1 to 225.7 °C was seen. Based on the Lindellman criterion, small changes in melting temperature are directly associated with changes in internal structure (Wang et al., 2019a, b). Therefore, the addition of Ni-GNSs possibly increases the neighbouring grain’s surface energy and suppresses the grains’ growth rate. As a consequence, the composite solder system’s consistency suffers, and the cumulative melting temperature plunges. Nevertheless, the overall changes in melting temperature were considered very small. This indicated that the pristine solder reflow profile might be utilised to fabricate composite solder. This also reveals that the composite lead-free solders can be used as a “drop-in” replacement for the lead-based solders without major modification in the existing production line.

4.4 Wettability Wettability, or the capacity of molten solder to expand and flow, is an important part of the soldering process because it ensures a strong metallurgical connection between

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the substrate and the solder. The contact angle to the substrate is commonly used to determine wettability, with reduced contact angles indicating better wettability (Abtew & Selvaduray, 2000; Nai et al., 2006). Pal et al. (2020a, b) have evaluated the relationship between contact angle and relative contact perimeter (RCP) of electroless-plated Ni–SiC/SAC305 composite solder. The lower contact angle was seen with higher RCP values for the electroless-plated Ni–SiC/SAC305 composite solder, indicating that an increase in RCP has led to an increase in wetting properties. The additions of Ni have been seen to reduce the agglomeration and effectively helped to reduce the contact angle to produce better solder wettability on reinforcement surfaces. The contact angle of SAC305 was significantly reduced from 34.5° to 21.2° with the addition of 1 wt.% electroless-plated Ni–SiC (Pal et al., 2020a, b). At the same time, Nai et al. (2006) demonstrated that an optimum wetting angle was produced at 1.5 vol. % and 0.04 wt.% additions of titanium diboride and CNT in Sn-3.5Ag-0.7Cu, respectively. Higher loading of reinforcements was shown to reduce the wettability due to increased viscosity that inhibits the melt flow.

4.5 Mechanical Properties Solders are used as a connecting substance that maintains mechanical, electrical, and thermal continuity in the face of temperature fluctuations, dampness, and other stressed circumstances. Mechanical properties are one of the most important aspects of reliability that must meet the expected levels in microelectronics. Worse, electronic components are made of different materials with different thermal expansion coefficients (CTE). When subjected to the changes in temperature during on and offapplications, the mismatch in CTE produces stresses and strains to the joints made by the solder materials. Thus, it is critical to guarantee the characteristics of the composite lead-free solder with surface-modified reinforcements which are on par with or superior to those of current lead-free solders. This resulted in various investigations on mechanical properties such as ultimate tensile strength (UTS), shear strength, and hardness to be made. The additions of surface-modified-NiO/T-ZnO in Sn-1.0Ag-0.5Cu show a 35.1% increase in UTS from 27.3 to 32.5 MPa (Huo et al., 2021a, b). The elongation at break of the composite solder increased from 20.8 to 28.1%. The fractured form shifted from ductile–brittle mixed to ductile fracture, according to surface fracture analysis. The number of dimples on the fracture surface of NiO/T-ZnO-Sn-1.0Ag0.5Cu composite solder was also enhanced, demonstrating that the involvement of surface-modified reinforcement served as a heterogeneous nucleation site for greater nucleation concentration. The increase in UTS and elongation at break also revealed that the interface of solder/reinforcement had been successfully improved, which allowed for a practical load-transfer effect. For the SDS-modified Cu6 Sn5 -SnBi system, no significant improvement was seen in UTS (Li et al., 2017). However, progress in interfacial bonding reduces porosity

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in between reinforcement/solder. This allows the value of elastic modulus for the composite solder to grow continuously with the increase in reinforcement loading percentage and able to sustain more tensile stress before deformation. In terms of shear strength, significant increases were reported for the Ni-CNTs/SAC305 (Wang et al., 2020), Ni-MWCNTs/Sn-58Bi (Lee et al., 2019), Cu-GNSs/SAC-reactive element (Zhang et al., 2019), and Au-SiO2/SAC387 (Mokhtari et al., 2012). Wang et al. (2020) reported that the microhardness SAC305 rose linearly with the increased amount of Ni-MWCNT added. The increase of heterogeneous nucleation sites and refinement of grains after the additions of Ni-MWCNT contributed to the improved hardness of SAC305 due to the restriction of dislocation movement between the grain boundaries. Less dislocation movement helped the solder system to become stiffer and increased the hardness.

5 Conclusion Composite solder has attracted great attention in academia and industry based on various special traits that are almost impossible to achieve with normal soldering alloy. However, two main problems of the aggregation of the reinforcement phase and non-wetting issues have become major obstacles in manufacturing composite solder. To overcome this reliability concern, various surface modification techniques have been explored, for example, pyrolysis, chemical route, and metal plating. According to the findings, the surface modification method improved the interfacial interactions between the molten solder and the reinforcing particle. A significant positive effect has been seen in microstructure, IMC growth, wettability, and mechanical properties, with minimum changes in thermal stability. These improvements show great potential for the application of composite solder microelectronics. Acknowledgements The authors appreciate the financial support provided by the Ministry of Higher Education Malaysia through FRGS grant scheme (FRGS/1/2019/TK05/UNIMAP/02/5).

References Abdallah, R., Juaidi, A., Assad, M., Salameh, T., & Manzano-Agugliaro, F. (2020). Energy recovery from waste tires using pyrolysis: Palestine as case of study. Journal of Energies, 13(7), 1817. Abtew, M., & Selvaduray, G. (2000). Lead-free solders in microelectronics. Journal of Materials Science Engineering: R: Reports, 27(5–6), 95–141. Ahmad, I., Nazeri, M. F. M., Salleh, N. A., Kheawhom, S., Erer, A. M., Kurt, A., & Mohamad, A. A. (2021). Selective electrochemical etching of the Sn-3Ag-0.5 Cu/0.07 wt.% graphene nanoparticle composite solder. Arabian Journal of Chemistry, 103392. Al-Syadi, A. M., Faisal, M., Harraz, F. A., Jalalah, M., & Alsaiari, M. (2021). Immersion-plated palladium nanoparticles onto meso-porous silicon layer as novel SERS substrate for sensitive detection of imidacloprid pesticide. Scientific Reports, 11(1), 9174.

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Molecular Dynamic of the Nanoparticle Reinforcement in the Pb-Free Solder During Reflow Soldering Process Mohd Sharizal Abdul Aziz, I. N. Sahrudin, M. S. Rusdi, M. H. H. Ishak, C. Y. Khor, and Mohd Arif Anuar Mohd Salleh

Abstract Nano-reinforcement in Pb-free solder has emerged as a potential alternative to improve lead-free solder’s mechanical and physical properties. With the aids of molecular dynamics simulation software, this study attempts to model the trajectory of doped nickel (Ni) nanoparticles in Sn100C solder during reflow soldering. A model that is capable of simulating Ni nanoparticle movement in Sn solder during three reflow soldering process phases. The simulation of Ni-reinforced solder was conducted at three different temperatures: room temperature (30 °C), soaking phase (150 °C), and reflow phase (250 °C) using LAMMPS software. The simulation provides the visualization of the accumulation and aggregation of Ni nanoparticles in the solder. This study better understands the Ni nanoparticles’ phenomenon in the solder paste during the reflow process. Keywords Nano-reinforcement · Molecular dynamics · Nickel nanoparticle · Simulation and modeling

M. S. A. Aziz · I. N. Sahrudin · M. S. Rusdi School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia M. H. H. Ishak School of Aerospace Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia C. Y. Khor (B) Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis, 02100 Arau, Perlis, Malaysia e-mail: [email protected] M. A. A. M. Salleh Center of Excellence Geopolymer and Green Technology (CEGeoGTech), Universiti Malaysia Perlis, Arau, Perlis, Malaysia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 M. A. A. M. Salleh et al. (eds.), Recent Progress in Lead-Free Solder Technology, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-3-030-93441-5_4

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1 Introduction Tin (Sn), lead (Pb), silver (Ag), copper (Cu), and other metals are common ingredients in solder. Solder has existed in paste, wire, and bar forms. Solders, also, available in the market include SnPb, Sn–Ag–Cu (SAC), Sn–Ag, Sn–Cu, Sn–Ag–Bi, and others. Tin (Sn), silver (Ag), copper (Cu), bismuth (Bi), and lead (Pb) are all chemicals used in leaded solder (Pb). Each solder type has a unique nature that affects the soldering process. For example, SAC305 refers to the solder’s composition of Sn, Ag, and Cu. The numbers after the letters represent the solder’s composition percentage. Surface mount technology (SMT) components are commonly attached to circuit boards using reflow soldering. Components are mounted onto a contact pad after a solder melts process. By hand, heat the soldering iron to melt it, and then let it cool to form a strong bond. Large ovens heat the solder in industrial-scale soldering (Vianco & Feng, 2016). Due to the need to monitor board temperature and time in the wave soldering machine, reflow soldering is the preferred method for SMT over wave soldering. Defective soldering results from failure to maintain the proper wave soldering environment. Scientists, particularly those working in electronic packaging, have noticed a trend in the use of nanoparticles to reinforce lead-free solder. According to research, the growing popularity of lead-free solder is also associated with an increase in the formation of micro-voids in the reflowed solder joint. In order to address these issues, research on the reinforcement of nanoparticles for the lead-free solder has received significant attention in recent years (Liu et al. 2016; Shang et al., 2019). It intends to improve the quality of soldering technology. Previous studies used numerical approaches such as finite element method (FEM), finite volume method (FVM), lattice Boltzmann method (LBM) (Abas et al., 2016a, b), discrete phase method (DPM), and molecular dynamics (MD) to simulate the nano-reinforcement process in solder. In this chapter, MD is used to simulate nanoreinforced lead-free solder during the reflow soldering. The computational simulation was done to save money and time over the complicated experimental procedure.

2 Molecular Dynamics Approach Molecular dynamics (MD) is a technique for analyzing the location of atoms in a given space. MD can also provide information about the thermodynamic and dynamic properties of molecules (Polanski, 2009). Traditionally, the MD was used to model particles’ time-dependent motions (trajectories). In conventional MD implementations, the forces acting on the particles were measured as derivatives of potentials. These forces are incorporated into Newton’s equations of motion, solved iteratively for each particle in the system (Krokhotin & Dokholyan, 2015). MD simulated the molecular structure’s natural movement. The MD procedure’s energy allowed the atoms to move and collide with their neighbors. Only if sufficient thermal energy is

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supplied to the molecule, it crosses the energy barriers in which local minima were separated on the conformational potential energy surface for that molecule (Pitman & Menz, 2006). The MD simulation can also simulate the soldering process. The simulation was used to investigate atomic or molecular movement in a material. There are many software packages for MD simulation, including LAMMPS (Dong et al., 2005), CHARMM (Ong & Liow, 2019), Groningen Machine for Chemical Simulations (GROMACS) (Theng et al., 2017), Nanoscale Molecular Dynamics (NAMD) (Chen et al., 2019), AMBER (Witherspoon et al., 2019), and Desmond (Yadav & Khandelwal 2019; Dong et al.2005). This software used the MEAM to simulate the diffusion of Sn and Ag atoms in lead-free solder during reflow soldering. The lack of distribution between Sn and Ag atoms was due to an insufficient Sn/Ag solder mixture during the low-temperature reflow soldering process. The simulation was later improved by increasing the reflow temperature to increase atom activation energy (Dong et al., 2005). Interdiffusion of Sn and Ag atoms occurred only at high temperatures. Since temperature is a variable in these cases, MD simulation clearly showed how atoms or molecules moved and diffused. Figure 1 summarizes the use of numerical methods to simulate the reflow soldering. FEM can be used to predict temperature profile, better, and stress/strain distribution, study IMC growth and morphology, and perform defect analysis after mechanical testing. To predict, FEM is the best tool for computational modeling. FVM is the best for simulating fluid flow or reflow oven inner condition and has proven to model fluid accurately (Najib et al., 2015). Besides, LBM is the best simulation method for fluid systems because it can easily simulate complex fluid structures’ cases. In the case of solder voids, LBM can capture micro-void formation (Abas et al., 2016a, b). Furthermore, the DPM simulation shows the trajectory of particles in solder. In contrast, MD simulation shows the behavior of atoms and molecules in solder and how they interact during the process. LAMMPS is a free, open-source software that communicates in parallel via the Message Passing Interface (MPI). It utilizes neighbor lists, also known as Verlet lists,

Fig. 1 Summary of numerical methods used to simulate soldering process

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to keep track of nearby particles. LAMMPS has application potential in solid-state materials, soft matter, and coarse-grained or mesoscopic systems (Crozier, 2011).

3 MD Simulation Setup In this study, MD simulation was employed to model the reflow soldering environment for Ni-reinforced solder paste. MD simulation is a computer simulation that uses differential equations of motion to calculate particle positions. In the simulation, the force acting on each particle can be used to determine the inter-atomic potential. However, the MD method constraint produces results with significant discrepancies and length scale limitations. Fi = ∇U

(1)

Equation 1 shows the equation of force acting on a particle (Lee, 2016). Fi is the force exerted on the particle i; while U is the inter-atomic potential between the particles. The acceleration of the particle can be determined from Newton’s second law of motion, given by Eq. 2: Fi = mai

(2)

where m is the mass of the particle and ai is the acceleration for the particle (Lee, 2016). Then, the velocity and position of the particle will be updated from the information on acceleration, which is given by vi = vi0 + ai t

(3)

1 ri = ri0 + vi t + ai t 2 2

(4)

and

where vi and ri are the velocity and the position of particle at time t; while vi0 and ri0 are the initial value of the velocity and the particle’s position, respectively (Lee, 2016). Next, the calculation of Eq. 3 and 4 are repeated with time, t = ∇t for all particles in the system. Figure 2 shows the working principle of MD. In the solder paste, Ni nanoparticles were dispersed. Cu atoms from the Cusubstrate are diffused into the molten solder during reflow soldering, forming Cu3 Sn. Cu atoms must be included in this simulation model. This work was done using the

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Fig. 2 Working principle of MD in the simulation

Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and visualized using Ovito. Figure 2 shows the MD simulation step. From downloading simulation tools to writing LAMMPS input scripts to run MD simulations and collecting output via Ovito for further analysis. This simulation model only used three atoms to simplify it. The Sn100C contained 99.24% Sn, 0.74% Cu, 0.006% Ni, and 0.005% Ge. The function of Ge in Sn100C solder is to reduce bridge formation, rapid wetting, and dross generation. Thus, the Ge was ignored in this simulation. In order to model the composite solder system, Ni nanoparticles were suspended in Sn and Cu atom composition. Sn and Cu atoms were arranged in a face-centered cubic (FCC) with lattice constants of 4.249 and 3.597 Angstroms. The Ni nanoparticles were placed in a sphere with an FCC lattice constant of 3.499. Sodium molecular weight (118.71), copper molecular weight (63.55), and nickel molecular weight (58.693). Six composite solder environments with varying reflow soldering temperatures (30, 150, and 250 °C) and Ni nanoparticle counts (2 and 4) were modeled. In all cases, the Ni nanoparticles are well-dispersed. The inter-atomic potential is the actual MD input. The Sn–Cu–Ni system has six interactions: Sn–Sn, Cu–Cu, Ni–Ni, Sn–Cu, and Sn–Ni. The L-J potential (Devonshire, 1938) was used to describe the inter-atomic interactions. The L-J potential is given by  V (r ) = 4 ∈

σ 12 σ 6 − r r

 (5)

where V and r are the intermolecular potential and distance between the two atoms, respectively, while ∈ and σ are the L-J potential parameters (Atkins & de Paula, 2006). In order to obtain the value of L-J potential parameters for Sn–Cu, Sn–Ni, and Cu–Ni, the cross-interaction potential was calculated using Lorentz Berthelot (LB) mixing rule (Allen, 1987) which is define by i− j =

√

i−i ∗  j− j

(6)

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and σi− j =

σi−i + σ j− j 2

(7)

where i− j and σi− j referred to L-J potential parameters for the interaction between atoms i and j. L-J potential is for the interaction between Sn–Sn, Cu–Cu, Ni–Ni, Sn– Cu, Sn–Ni, and Cu–Ni. The example of the calculation for the interaction between Sn–Cu are shown below:  Sn−Cu =

√  Sn−Sn ∗ Cu−Cu

(8)

 Sn−Cu =

 (0.504) ∗ (9.45)

(9)

 Sn−Cu = 2.182

(10)

and σ Sn−Sn + σCu−Cu 2

(11)

(1.4554) + (2.337) 2

(12)

σ Sn−Cu = σ Sn−Cu =

σ Sn−Cu = 1.8962

(13)

This simulation is considered transient/unsteady. There may be errors in the initial geometry setup. The MD integrator may fail if there are atoms or molecules with higher forces. Thus, reducing energy consumption is required to optimize the system. 10,000 maximum iterations and total force evaluation were used in this study. Then the atoms were given random initial velocities. Initial system velocities were 1000, 5000, and 10,000 m/s. The initial system configurations were not equilibrated. It was necessary to equilibrate from initial configurations to equilibrium before collecting data. The system was simulated for 10,000 time steps under canonical ensemble (NVT). This simulation used a time step of 4 femtoseconds.

4 Solder Fabrication Techniques The reflow soldering process used Sn100C solder powder from Nihon Superior, Japan, no-clean solder paste flux type XF-07–03 from RedRing Solder, and Ni nano-powder (99.9%) with 40 nm particles size from US Research Nanomaterials, Inc, USA (Fig. 3). This lead-free Sn100C solder contains 99.24% Sn, 0.75% Cu,

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Fig. 3 Raw materials used in the solder fabrication: a Sn100C solder powder b Solder flux, and c Nickel nanoparticles

0.006% Ni, and 0.005% Ge. Higher Sn concentration increases solder tensile and shear strengths (Loomans et al., 1994; Suganuma, 2001). It develops oxide-free and mechanically strong soldered joints using the right solder flux (Shipley, 1975). After the reflow process, this solder finish leaves only a slight non-corrosive residue around the solder joints. Previous research found that adding Ni nanoparticles to lead-free solder improved hardness, creep resistance, and wettability (Niranjani et al., 2012; Haseeb et al., 2017; Yao et al., 2008). In the first step, Sn100C, solder flux, and Ni nanoparticles were weighed. The weightage percentages for Sn100C solder and flux were, respectively, 80% and 20%. Ni nanoparticles were 0.01% of the composition of Sn100C powder and solder paste flux. Raw material mass, as in Table 1. Next, all raw materials were combined into the container and manually stirred for 30 min to ensure homogeneity. A greyish solder paste is shown in Fig. 4 using Sn100C solder powder (white) with solder flux and Ni nanoparticles.

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Table 1 The mass of materials used in this study Raw materials

Weightage percentage (wt.%)

Mass (g)

Sn100C

80

16.0

Solder flux

20

4.0

Ni nanoparticles

0.01 of Sn100C and solder flux composition

0.002

Total mass

20.002

Fig. 4 Sn100C solder powder and nano-reinforced solder paste

5 Reflow Soldering Process Firstly, the substrate was prepared before reflow soldering. This process uses a PCB FR-type Cu-substrate (see Fig. 5). This PCB type has excellent electrical isolation and mechanical strength. To begin, the Cu-substrate was cleaned with ethanol. Then the Ni-reinforced solder paste was syringe-printed onto the Cu-substrate. The printed solder samples were then reflowed in the reflow oven (Fig. 6). The samples were then heated to a maximum peak temperature of 250 °C in ambient air using the suggested reflow profile (Fig. 7). The oven used is the Madell Technology TYR108C desktop lead-free reflow oven. This reflow oven is ideal for experimental testing.

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Fig. 5 FR-type Cu-substrate

Fig. 6 The reflow oven: a Desktop reflow oven TYR108C b Desktop reflow oven F4N

Figure 7 shows the preheating phase of reflow soldering, where the temperature was increased from 30 to 150 °C. The next phase of soaking was held at 150 °C for a while before ramping up to the next phase. Then came the crucial reflow phase, where the temperature was again rammed up to 250 °C and let the solder thoroughly wet the substrate. Finally, the solder was cooled to room temperature and solidified. Figure 8 shows a reflowed sample after reflow soldering.

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Fig. 7 The reflow soldering temperature profile (Chellvarajoo et al., 2015)

Fig. 8 The reflowed sample

6 Results and Discussion In the MD simulation, the effect of temperature on the particle trajectory was investigated. For each simulated model, Ni nanoparticles were trajected at different distances and directions. As shown in Figs. 9 and 10, the models simulated at a temperature of 30 °C produced the shortest trajectory distance for both models with 2 and 4 Ni nanoparticles. Due to the presence of kinetic energy in the system, the trajectory direction of the Ni nanoparticles at 250 °C exhibited chaotic behavior for both models. In contrast, under the same conditions of 150 and 250 °C, the Ni nanoparticles exhibited the same behavior in both models.

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Fig. 9 The trajectories of 2 Ni nanoparticles at different simulation temperatures a 30 °C, b 150 °C, and c 250 °C

For the purpose of examining the effect of temperature on the particle trajectory, only one atom was selected from among all of the models. The atom with the number 31977 was selected from the first nanoparticle of each model. This atom was placed in a position similar to the initial position to observe the atom’s direction and behavior. Figures 11 and 12 show the image of the trajectory atom derived from the simulation results for cases 2 and 4, respectively, as depicted in the simulation results. The model that was simulated at 250 °C moved in a zig-zag pattern. In contrast, the models that were simulated at 30 and 150 °C moved in a relatively straight path. The zig-zag movement of each particle resulted in the formation of a line of fibrously identical trajectories. It was discovered that the trajectory distance of the particles for models at 30 °C (Fig. 13a–c) was the shortest when compared to models at 150 and 250 °C. A nearly identical trend was observed for the trajectory distance at 150 and 250 °C.

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Fig. 10 The trajectories of 4 Ni nanoparticles at different simulation temperatures a 30 °C, b 150 °C, and c 250 °C

The temperature also impacted the particle trajectory distance and behavior and the other factors mentioned above. In the context of Brownian motion, higher temperatures result in a particle’s kinetic energy being increased by an equal amount. As a result, the greater the distance traveled by the particle. The particle moved in a random direction most of the time. This circumstance explained how the particle was connected to the Brownian motion. The zig-zag movement of the particle was also caused by higher kinetic energy due to higher temperature, which increased the excitement of the particles even further. The temperature of the system also affected the Ni atom’s speed. Its motion revealed a massive distance as the simulation temperature rose. Given the same simulation time (t = 10), it is clear that the Ni atom at 250 °C is faster than those at 30 and 150 °C. Figure 14 shows the speed of a single Ni atom.

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Fig. 11 The trajectory line of Ni atom number 31977 at t = 0 for the simulation model with 2 Ni numbers of nanoparticles at a 30 °C, b 150 °C, and c 250 °C

Moreover, the closely packed nanoparticle at the beginning breaks apart after the simulation process at the temperature of 250 °C as validated in Fig. 15. Thermal expansion refers to the expansion of matter when the substance is heated and contracted when cooled. The particles did not expand, but the space between the particles increased. The higher temperature increased the velocity of the particles, which then increased the space occupied by the particle.

7 Conclusion This chapter presents a simulation method for nano-reinforced solder to investigate its complexity and atomic behavior. MD simulations of Ni-reinforced solder at three different reflow soldering temperatures of 30 °C (room temperature), 150 °C (soaking phase), and 250 °C (reflow phase) were successfully conducted using the LAMMPS software. This study discussed the trajectory of the Ni nanoparticles and their agglomeration behavior. The Brownian and Van der Waals forces influenced the agglomeration of Ni atoms. The trajectory distance of Ni nanoparticles was found to be the longest at 250 °C and the shortest at 30 °C. The surrounding temperature also affected the speed of the moving atom. Brownian motion is responsible for

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Fig. 12 The trajectory line of Ni atom number 31977 at t = 0 for simulation the model with 4 Ni number of nanoparticles at a 30 °C, b 150 °C, and c 250 °C

this phenomenon; additionally, the particle’s kinetic energy increases as the temperature increases. The particle movement is highly dependent on the temperature of the substance. As a result, the moving particle’s distance is more remarkable. Moreover, the fabrication of nano-reinforced solder materials Sn100C and the reflow soldering process were also discussed in this chapter. This chapter is expected to provide a profound understanding of the molecular dynamics in nano-reinforced solder material via the simulation technique.

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Trajectory Distance of Ni Atom in X-axis

(a) Trajectory Distance (Angstrom)

8

7.5

7.2

7.6

7.5

7 6

5.5

5.4

5 4 3 2 1 0 30

150

250

Simulaon Temperature (◦C)

2 Ni Nanoparcles

4 Ni Nanoparcles

Trajectory Distance of Ni Atom in Y-axis

(b) Trajectory Distance (Angstrom)

8

7.6

7.2

7

7.6

7 6

5.3

5

5 4 3 2 1 0 30

150

250

Simulaon Temperature (◦C)

2 Ni Nanoparcles

(c)

4 Ni Nanoparcles

Trajectory Distance of Ni Atom in Z-axis Trajectory Distance (Angstrom)

9 8

7.2

7

7.8

7.5

7 6

5.5

5

5 4 3 2 1 0 30

150

250

Simulaon Temperature (◦C)

2 Ni Nanoparcles

4 Ni Nanoparcles

Fig. 13 The trajectory distance of Ni atom number 31977 in a x-axis, b y-axis, and c z-axis

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0.8 0.71

0.76

0.72

0.7

Speed ( /s)

0.6

0.53

0.53

0.5 0.4 0.3 0.2 0.1 0 30

150

250

Simulaon Temperature (◦C)

2 Ni Nan

cles

4 Ni Nan

cles

Fig. 14 The speed of Ni atom number 31977 for each model

Fig. 15 The image of Ni nanoparticle 1 simulated at 250 °C in a initial frame (t = 0 s) and b final frame (t = 10 s)

Acknowledgements Acknowledgement to Ministry of Higher Education Malaysia for Fundamental Research Grant Scheme with Project Code FRGS/1/2020/TK0/USM/03/6. The authors would also like to thank Universiti Sains Malaysia for providing technical support.

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Recent Progress in Transient Liquid Phase (TLP) Solder for Next Generation Power Electronics Flora Somidin, Rita Mohd Said, Norainiza Saud, and Mohd Arif Anuar Mohd Salleh

Abstract There has been an increasing demand for more reliable high-temperature resistant solder joints in recent years. Developing a reliable high-temperature solder joint not containing lead (Pb) is crucial for advancing more environmentally friendly next generation power electronics. An innovative bonding approach called transient liquid phase (TLP) bonding is a promising method to produce a high-temperature Pb-free solder joint. This chapter reviews the generation power electronics applications using the TLP bonding method. In addition, the article presents a brief understanding of the advancement in soldering technology through the utilisation of the TLP bonding concept and its challenges. Keywords Lead-free high-temperature solders · Power electronics · Transient liquid phase (TLP) bonding

1 Introduction The motivation for developing high-temperature Pb-free solders has increased notably in recent years. The high-Pb (≥85 wt.%Pb) solder alloys are mostly used in integrated circuit (IC) components for high-temperature and high-reliability applications, such as die-attach technology for semiconductor packaging. However, the usage of Pb has been strictly banned in consumer goods because of its toxicity towards the environment and human health (World Health Organization, 2019). Nevertheless, the lack of alternatives for more stable high temperature Pb-free solders has made it difficult for the electronics industry to fully replace high Pb-content solders as die attachments for power electronic applications. In general, the main driving force for the development of higher temperature interconnect materials is the demand for more stable electronic packages. Unfortunately, most electronic packaging systems are currently being deployed closer to heating F. Somidin (B) · R. M. Said · N. Saud · M. A. A. M. Salleh Center of Excellence Geopolymer & Green Technology (CeGeoGTech), Universiti Malaysia Perlis (UniMAP), Taman Muhibbah, Jejawi, 02600 Arau, Perlis, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 M. A. A. M. Salleh et al. (eds.), Recent Progress in Lead-Free Solder Technology, Topics in Mining, Metallurgy and Materials Engineering, https://doi.org/10.1007/978-3-030-93441-5_5

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Fig. 1 Die-attach interconnect in integrated circuit (IC) packaging system

sources (Kang et al., 2020; Watson & Castro, 2015). This is a particular problem in developing electric and hybrid-electric vehicles, in which electromechanical packaging systems are installed close to the engine. Hence, the search for more reliable higher temperature Pb-solders is crucial for the interconnects manufacturing industries as they need to keep pace with the emerging technology trends while at the same time satisfying environmental policy requirements. High-temperature solders are mainly used in power semiconductor IC packaging as die-attached interconnects, as shown in Fig. 1. Die-attach material is in the first level (Level 1) of the IC packaging system. The interconnection materials between the silicon die and package carrier (see Fig. 1) are subjected to multiple heating cycles during subsequent assembly processes. Nowadays, the need for repeated bonding has increased because of the integration of the bonded structure in an IC package. Multiple exposures to soldering temperatures increase the likelihood of damage to the internal structures in an IC package and warpage that can cause solder joints to open or bridge defects. Thus, more reliable solder joints that can sustain repeated bonding processes are needed to be developed. High-temperature Pb-solders with melting temperatures in the range between 260 and 400 °C in the market are still very limited. Most of the readily available options for high temperature solders can be grouped as either bismuth (Bi)-rich alloys (Cho et al., 2016), zinc (Zn)-rich alloys (Niu & Lin, 2016) or Au-rich alloys (Chidambaram et al., 2010; Wang et al., 2020). Some of the reliable high-temperature Pb-free solders options today used expensive elements, for example, gold (Au) and germanium (Ge), such as Au–12 wt.%Ge alloy with a melting point of 356 °C (Wang et al., 2020), which limits their use in many potential applications. Generally, there are still a very limited number of alloying systems available for high-temperature Pbfree solders development. Furthermore, a new alloy system with more cost-effective options should be introduced as high-temperature interconnects for future power electronics. One of the best alloy systems for less expensive high-temperature solders is Sn–Cu (tin–copper) alloys (Zhao et al., 2019). Today, a broad range of Pb-solders is based on Sn-rich alloys due to their ability to wet the widely used Cu-substrate in printed circuit

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board (PCB) packaging systems. This is because of the low material cost, availability and deep understanding of this alloy system with respect to soldering application. The eutectic composition of this alloy is at Sn–0.7 wt.%Cu with a melting temperature of 227 °C and has been widely used in mid-range temperature soldering applications. Copper is the most commonly used metal substrate in the PCB package. The interfacial intermetallic compounds (IMCs) formed between the solder and substrate are Cu3 Sn and Cu6 Sn5 through thermal processing (Annuar et al., 2017). The intermetallic compound (IMC) of Cu3 Sn exists as an orthorhombic structure (ε-phase, Cmcm) at a temperature below 676 °C (Saunders & Miodownik, 1990). At higher temperatures, it changes to cubic γ-Cu3 Sn of the Fm-3 m structure. At the same time, the Cu6 Sn5 exists as a monoclinic η -Cu6 Sn5 structure (C12/c1) at below 186 °C and transforms to hexagonal η-Cu6 Sn5 (P63 /mmc) at a higher temperature. It is interesting to note that the temperatures at which melting commences for Cu3 Sn and Cu6 Sn5 are about 720 and 415 °C, respectively, which suits the temperature requirement for die-attach materials. Furthermore, the high-temperature Sn–Cu IMCs can be grown on Cu-substrates. Therefore, a high-temperature solder joint with Sn–Cu alloy is possible if entire solder joints can be transformed to a complete IMCs structure as depicted in Fig. 2. Specifically, the Cu/Sn–Cu solder/Cu joint can be processed at a lower temperature and held in the liquid–solid region until the Cu/Cu3 Sn–Cu6 Sn5 /Cu joint is complete. This process is called transient liquid phase (TLP) bonding (Cook & Sorensen, 2011; Jung et al., 2018; Sun et al., 2019). The reaction is done at the lower melting point temperature of the process material but will result in a higher re-melt temperature after processing. In the case of Sn–Cu solder, the TLP bonding depends on the formation of a liquid Sn that is transient in nature. In other words, a solder joint can be processed at a relatively low temperature

Fig. 2 Schematic diagram on microstructure evolution in TLP bonding of a solder joint

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using TLP bonding concept, which resulted in the formation of a new in-situ material with a higher re-melting temperature. The process of TLP bonding is also known as the solid–liquid interdiffusion process between the atoms of two materials with different melting temperature elements, which are low and high melting temperature elements. This bonding method involves a complete consumption of low-melting-point metal (i.e., Sn) and the formation of high-melting-point IMCs (Jung et al., 2018; Kang et al., 2020). Alternatively, TLP bonding can become a novel joining approach used in high power electronic devices that utilise a low processing temperature while resulting in high-temperature bonding.

2 Transient Liquid Phase (TLP) Bonding Concepts The basic concept of TLP bonding is illustrated in Fig. 3, which shows the arrangement of TLP joints systems before and after the thermal processing. As depicted in the illustration, the initial arrangement can be in the form of powder-based or layer-based mixtures. At the same time, thermal processing can be done either by liquid-state sintering or soldering (Greve et al., 2015). In the solid-state mode of A and B phases before any heating takes place, the lowmelting temperature constituent (denoted as A) and the high-temperature constituent (denoted as B) are usually arranged in close proximity so bonding can easily take place after it changes to a liquid–solid mode later on. As the temperature rises from room temperature to above the melting temperature (Tm ) of the transient phase (Tm A), phase-A melts and wets the surfaces of phase-B, which remain solid at this processing temperature (Tp ). As interdiffusion occurs between liquid-A and solid-B, a new solid-solution phase of A + B grows between the liquid and solid phases. Suppose Tp is held isothermally for an extended period. In that case, phase-A will eventually be fully consumed if enough B is present in the joint, and any of the left-over solid phase-B stays as a

Fig. 3 Process flow in TLP bonding

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solid in the joint. As a result, the final joint will have a higher re-melt temperature as the process is not reversible in the solid-state condition. However, the new IMCs of a solid solution of A + B-phase will continue to grow towards higher-B composition until it reaches the maximum solute concentration of the systems. Basically, there are four major stages involved in TLP bonding concepts for soldering. Stage 1 is the heating stage in which the component is heated from room temperature to the melting point of the filler metal (for example, eutectic solder). Stage 2 is the dissolution stage, wherein in the case of a low-melting point of solder mixed with high-melting particles (for example, Cu), the Cu particles will dissolve into the liquid phase (eutectic solder) and increase the liquid phase size. The liquid phase increase in size to maintain the mass balance. The next stage is the isothermal solidification stage, where the liquid phase solidifies. Finally, stage 4 is the homogenisation of the solder joint and will occur wherever solid-state diffusion takes place. Transient liquid phase bonding steps consist of the isothermal solidification of an initial molten that joins through the liquid’s disappearance resulting from the interaction between the molten solder and the substrate and the formation of solid intermetallic phases. In general, the initial arrangement of the solder joint is important in the TLP bonding process. Accordingly, the two most prominent structures of initial arrangements for TLP-based solder joints are layer-based and powder-based mixtures. These two types of arrangement are discussed in the following section.

2.1 Solid–Liquid Interdiffusion (SLID) Bonding Transient Liquid Phase (TLP) bonding was initially known as a combination of diffusion bonding and brazing (Jung et al., 2018; Lee et al., 2005). In electronic packaging, TLP joint is established due to isothermal solidification of the IMCs that formed at the solder-substrate interface during the bonding process. These IMCs have higher melting temperatures than the bonding temperature. This type of bonding is also known as solid–liquid interdiffusion (SLID) bonding. This is a layer-based mixture or sandwiched type of TLP bonding. The basic principle of SLID bonding is depicted in Fig. 4. In this approach, a low-melting temperature of a similar metal is sandwiched and melted between a high-temperature base material to create a joint. The liquid phase developed at the interlayer of metal and base material during isothermal heating will result in the formation of IMC with a high melting temperature through the diffusion–reaction between metal and substratum. As a result, bonding is formed at a temperature lower than the original base metal. There are five processes to complete a SLID structure: wetting, alloying, liquid diffusion, solid diffusion and gradual solidification (Sun et al., 2020). Isothermal solidification occurs at bonding temperature during the solid–liquid interdiffusion process. The IMCs layer constantly grows by maintaining the isothermal heating

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Fig. 4 Schematic diagram for basic SLID bonding principal

temperature, as shown in Fig. 4c and d. The liquid phase disappears afterwards, and homogenisation occurs.

2.2 TLP Bonding via Multiple Reflows Technique Transient liquid phase bonding can be established by reflowing the solder joint multiple times using a typical reflow profile, as shown in Fig. 5. The intermetallic growth occurred while the solder was in the molten state. The longer the duration of elevated reflow temperature, the more intermetallic growth occurred. Typically, reflow soldering is the most common technique for producing solder interconnects at the component and board assembly. Most of the interconnects will go through more than one reflow cycle in which the subsequent reflow profile is usually similar to the first one. Sometimes, the components were subjected up to six times reflow cycles. The formation of peritectic Cu3 Sn IMCs can occur after multiple reflow processes (Mohd Said et al., 2020). It is found that the peritectic microstructure consists of monoclinic Cu3 Sn surrounded by hexagonal Cu6 Sn5 . This phenomenon is likely to be called an ‘island structure’. The schematic diagram of the peritectic IMCs formation is depicted in Fig. 6.

2.3 TLP Bonding for Metal Powder Transient liquid soldering for powder metal usually occurs in paste (Li et al., 2015; Min et al., 2021; Mohd Said et al., 2020). At first, powder metals and flux with specific compositions were mixed to produce a paste. Then, the mixing process was conducted in a solder paste mixer. The size of the metal powder plays an important role. If the

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Fig. 5 Typical reflow profile for Pb-free solders

Fig. 6 Peritectic IMC formation

size is too big, the printing process will be more difficult, while agglomeration of the powder will take place if the particle size is too small (Jung et al., 2018); once the paste is printed on the substrate, the paste then can be soldered via the conventional soldering method. To produce TLP bonding, the soldering process can either be using SLID technique or multiple reflow technique. The schematic diagram of the IMCs formation for metal paste is shown in Fig. 7. The sintering TLP bonding method is also applicable in the form of solder paste (Li et al., 2015). This technique is different from traditional solder reflow processes. Typically, solder joints are formed through the wetting process followed by solid– liquid interfacial reaction and solidification process. However, sintering is a process where bonding is achieved through atomic diffusion and particle consolidation, as illustrated in Fig. 8. The sintering process can be carried out at a temperature below the melting point of the metal paste. Usually, the temperature is below 300 °C. To

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Fig. 7 TLP bonding concept in the solder paste

Fig. 8 Sintering process for solder paste

increase density and reduce the formation of the voids, pressure should be applied during the sintering process.

2.4 TLP Bonding in Metal Preform In TLP bonding using a metal preform, a porous metal made by high temperature metal such as Cu or Ag is sandwiched between low-temperature metal layers such as Sn. Prior to this process, the metal performance was fabricated via the powder metallurgy technique. The metal powders were mixed, compacted and sintered and formed a metal performance. Then, the heating or soldering process is carried out. This method is actually the combination of the sintering method and TLP bonding technique. The solid Sn melts and flows into the Cu particles’ gap through capillary action (Shao et al., 2018a), and the diffusion reactions formed the Cu–Sn IMCs skeleton, as illustrated in Fig. 9. However, voids are the major drawbacks of this tech-

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Fig. 9 Schematic diagram of the novel TLP bonding totally through capillary action: a preparation of hybrid preform by pressing Cu powders on Sn foils; b assembly of the samples; c densification of the porous Cu layer by flowing liquid Sn; d formation of Cu–Sn IMCs interconnection through Cu/Sn diffusion–reaction

nique, where voids in electrical connection are detrimental to the whole circuit electrically and mechanically (Yunus et al., 2003). However, pressure could be applied during the heating or soldering process, and it could reduce the formation of the voids and increase the mechanical properties (Mokhtari, 2019).

3 Recent Studies on Transient Liquid Phase (TLP) Solders Systems There have been a few attempts to develop high-temperature solder bonding using the TLP bonding concept in recent years (Kang et al., 2020; Sun et al., 2020). Table 1 lists the recent studies on TLP solder joints system and their methods of achieving high-temperature joints. It can be seen that most of the existing studies use layerbased methods to grow TLP joints from low temperature existing solder alloy systems that consist of Sn-based alloys, such as Sn–Cu, Sn–Cu–Ni, Sn–Ag, and Sn–Ag–Cu. At the same time, most of the metallic substrates that used to be sandwiched and soldered with the TLP solders were Cu and/or Ni elements. This is mainly due to standards PCBs that use Cu and Ni as their most prominent top metallic layers. Therefore, these joints systems of Sn-based alloys and metallic substrate can produce high temperature phases (i.e., Cu3 Sn, Cu6 Sn5 , etc.) after being processed using TLP bonding concepts. Accordingly, the TLP joint is expected to have a robust creep resistance suitable for high-temperature applications, such as power electronics (Li et al., 2014).

SiC–Ni–Ag/Sn–Cu/Cu

Cu/Cu@Sn/Cu

Cu/Sn–Ag/Cu–Zn

SiC–Ni/Sn/Ni

Ji et al. (2016)

Chen et al. (2017)

Chen and Duh (2017)

Li et al. (2017)

Foil

Pressed Sn–3.5 wt.%Ag solder ball into 10 μm foil thickness

Pressed powder mixture of Cu@Sn particles (Cu particle coated with Sn)

Foil

Preforms of multi-layer Sn–Ag foils

Si/Sn–Ag–Sn/Si

Bajwa and Wilde (2016)

Foil

Pressed powder mixture of Ag–40 wt.%Sn with average particle sizes of