Low-Temperature Soldering Morgana Ribas, et al. Alpha Assembly Solutions The Printed Circuit Assembler’s Guide to...™ Low-Temperature Soldering Morgana Ribas, Tom Hunsinger, Traian Cucu, Ramakrishna H V, Garian Lim and Mike Murphy ALPHA ASSEMBLY SOLUTIONS © 2018 BR Publishing, Inc. All rights reserved. BR Publishing, Inc. dba: I-Connect007 PO Box 1908 Rohnert Park, CA 94927 U.S.A. eBook ISBN: 978-0-9998648-4-5 Visit I-007eBooks.com for more books in this series. I-Connect007.com About the Authors Morgana Ribas, Ph.D. Manager, Metals Technology Group—R&D Alpha Assembly Solutions Morgana is a metallurgical engineer with a master's degree in extractive metallurgy from UFRGS in Brazil and a Ph.D. from Rice University in the United States. Currently, Dr. Ribas is manager for the metals technology group at Alpha Assembly Solutions. Her work has appeared in more than 50 publications, including technical journals, confer- ence proceedings, and patents. Tom Hunsinger Vice President of Global Marketing Alpha Assembly Solutions Tom has over 13 years of experience in the elec- tronics assembly industry. He has held several positions in both the United States and Asia while working to provide innovative products and assembly solutions. Traian Cucu, Ph.D. Group Leader, Global Applications and Technologies Expert Group (GATE)—R&D Alpha Assembly Solutions Traian received his master’s degree in electrical and power engineering from Politehnica Univer- sity of Timisoara in Romania, and his Ph.D. in electronics, telecommunications, and information technologies from Polytechnic University of Bucha- rest, also in Romania. He has been working in the electronic assembly industry since 1999 and had his work published in more than 40 publications, including technical journals, conference proceedings, and application notes. About the Authors Ramakrishna H V, Ph.D. Manager of Chemistry Alpha Assembly Solutions Dr. Ramakrishna H V received both a bachelor’s and master’s degree in polymer science and a Ph.D. in polymer science and technology from the University of Mysore in India. With more than 10 years of experience in the formulation of soldering materials, he is currently the manager for solder paste development activities at Alpha Assembly Solutions, India. Garian Lim Interconnect Solution Portfolio Manager Alpha Advanced Materials Garian is the interconnect solution portfolio manager for Alpha Advanced Materials (AAM). He is responsible for soldering product solutions for semiconductor pack- aging applications. He has more than 10 years of expe- rience working with materials in the semiconductor assembly market. Garian has a bachelor’s and master’s degree in mechanical engineering from Nanyang Tech- nological University of Singapore. Mike Murphy Director of Marketing—Core Products Alpha Assembly Solutions Mike has been in the electronics assembly industry for over 20 years in a variety of technical and commercial roles. He has presented at numerous industry events and published many technical articles. In his current role, Mike oversees a global team of portfolio manage- ment professionals who are responsible for the total life cycle management of a diverse range of specialty mate- rials for the global electronics assembly market. Peer Reviewers Dr. Raiyo Aspandiar Senior Engineer Intel Corporation Raiyo Aspandiar has worked at Intel Corporation in the boards and system assembly facility in Hills- boro, Oregon, since 1983. He was part of the team that introduced surface mount technology to Intel. Over the years, he has participated in the develop- ment of PCBs and assembly processes for moth- erboards and mobile modules, which contained a myriad of packages for the Intel microprocessors, chip sets, and connectors. Currently, Raiyo is working on and low-temperature soldering development. Raiyo has published more than 65 technical papers and is the joint holder of 15 patents in the electronics packaging and manufacturing field. He is also a former member of the SMTA Board of Directors and the SMTA Technical Committee and received the SMTA Member of Technical Distinction Award in 2009. He is a graduate of Stanford University and the Indian Institute of Technology Bombay. Happy Holden Consulting Technical Editor I-Connect007 Happy Holden is the retired director of electronics and innovations for Gentex Corporation. Happy is the former chief technical officer for the world’s largest PCB fabricator, Hon Hai Precision Indus- tries (Foxconn). Prior to Foxconn, Happy was the senior PCB technologist for Mentor Graphics and the advanced technology manager at Nan Ya/Westwood Associates and Merix. Happy previously worked at Hewlett-Packard for over 28 years as director of PCB R&D and manufacturing engineering manager. He has been involved in advanced PCB technologies for over 47 years. Contents 1 Introduction Chapter 1 3 Low-Temperature Soldering Chapter 2 9 History and Overview of Low-Temperature Solders Chapter 3 19 Second Generation Low-Temperature Solders Chapter 4 29 Using the Right Chemistry Chapter 5 39 Advanced Applications for Low-Temperature Solders 49 Conclusion 50 Glossary 54 References 56 About Alpha Assembly Solutions Introduction With the movement to use lead-free solder in electronics assembly, a great deal of research and focus has been placed into the search for viable alternatives to SnPb alloys. It is clear that the domi- nant alloys used in modern electronics assembly are variants on the SnAgCu (commonly known as SAC) alloy system. Over time, however, several factors have combined to contribute to an increase in the use of alternative alloys with lower melting points. This book provides an introduction to the evolution of modern low-tempera- ture soldering, illustrates the importance of chemistry when devel- oping low-temperature solder (LTS), and discusses advanced and emerging applications where lower melting point alloys can provide unique assembly solutions. 1 Chapter 1 Low-Temperature Soldering Ever since the printed circuit board (PCB) assembly industry moved to lead-free assembly processes, low-temperature soldering has been a hot topic and a top priority on the new alloys agenda. But what is the definition of low-tempera- ture alloys? The basic structure of the assembly remains the same as SnPb materials. An alloy (usually containing tin) is used to create a joint between a pad and a component termination. The joint plays a mechanical and elec- trical role, and sometimes a thermal role. Figure 1.1 shows a typical joint formed with a SAC305 alloy used to assemble a surface mount device (SMD) capacitor to a PCB. Figure 1.2 illustrates a cross section of a typical lead-free joint. Figure 1.1: SMD assembled with solder paste. The concept of low-temperature solder materials has seen slight changes ever since the transition from lead-containing to lead-free assembly processes. It all started with the adoption of the SAC family of lead-free alloys when a low-temper- ature process was understood to be any process using alloys with lower processing temperatures (lower melting points) than the SAC family. This meant everything below 220°C. Figure 1.2: Cross-section of a lead-free SMD joint. 3 Table 1.1: Lead-free alloys commonly used in the assembly industry. Table 1.1 provides examples of solidus/liquidus temperatures for common SAC and low-temperature alloys used in the industry. This is an informative non-exhaustive list, as there are many more alloys used in the field. As the assembly industry was look-ing for alternatives for lead-free alloys that would deliver a performance close to or better than SAC alloys, a more granular hierarchy came into use, although nothing has been standardized or generally agreed upon across the industry. Figure 1.3 presents a proposed hierarchy and set of definitions. The joint in lead-free processes (both SAC and low-temperature alloys) is still formed in the same way as the lead-containing process. The connection is made between the leads/contacts and sometimes spheres of a component and a copper pad. The same pad/component finishes are used with the obser- vation that all the lead-containing materials have been replaced by lead-free materials. SAC alloys are typically used instead of lead-containing alloys for the hot air solder leveling (HASL) process. As a result, the joint will have the same Figure 1.3: Classification of the assembly process based on the peak reflow temperature. 4 type of intermetallic compound (IMC) (SnCu, SnNi, etc.) (Figure 1.4). There will be a noticeable difference in the intermetallic thickness between a SAC process and a low-temperature assembly process. When compared with a joint formed with a SAC alloy in a SAC assembly process, the joint formed with a low-temperature, high bismuth content alloy will have a thinner intermetallic. Figure 1.4: Lead-free joint structure with an SnNi IMC layer. There have been numerous studies looking to find the right lead-free, low-temperature alloy for the assembly industry. High bismuth content alloys (35–60% bismuth) have emerged as the top choice, as the sum of their properties best serves the needs of the assembly industry today. Recently, more families of alloys with liquidus temperatures below or very close to 200°C have come under scrutiny as the industry aims to improve solder performance even further without increasing the assembly temperatures. Figure 1.5 shows the BiSn phase diagram [1]. Although lead-free alloys containing bismuth were initially considered during the transition to lead-free materials, they have dropped from the list of candi- dates due to the lower melting ternary SnBiPb phase. Once the industry moved to lead-free materials, however, the concern of the low-melting ternary SnBiPb phase was reduced, and bismuth containing alloys became interesting again. Figure 1.5: BiSn phase diagram [1]. 5 Looking at the assembly processes, high bismuth content alloys could theoret- ically be used in both wave soldering processes and surface mount technology (SMT).