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72 SMT007 MAGAZINE I SEPTEMBER 2024 Article by Prabjit Singh, Raiyo Aspandiar, Haley Fu, and L. A. Swaminathan Abstract e high homologous temperature of tin- bismuth alloys and the high electromigration effective valence of bismuth makes tin-bis- muth alloys' microstructures forever chang- ing in their applications, even at temperatures as low as room temperature. While this behav- ior is a serious concern to engineers who must ensure that this forever changing behavior does not have too much of a negative and unaccept- able impact on reliability, it makes for inter- esting research. is paper describes some of the fundamentals an iNEMI team working on Sn-Bi electromigration unearthed during their study and some innovative techniques they Some Fundamentals of Tin-Bismuth Metallurgy and Electromigration developed along the way to better study elec- tromigration in these alloys. Introduction Tin-bismuth has received a lot of attention as being the solder alloy of choice for electronic assemblies requiring low-temperature solder- ing for a number of reasons, including hierar- chical soldering, reducing warpage of assem- blies, reducing damage to components that may be less resistant to high temperatures and simply to take advantage of the energy savings resulting from low-temperature operations 1 . Incorporating tin-bismuth solder joints into assemblies has its own unique challenges aris-