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14 SMT Magazine • August 2017 mission and when the lead-free electronics ap- peared to become a reality (RoHS 1 in 2002/2004, RoHS2 in 2011/2013). Variations of EU RoHS ad- opted by other countries were later deployed glob- ally in the mid-2000s and thereafter. It is cheerfully intriguing to observe that some esteemed engineers and scientists have al- ways showed an interest in novel lead-free sol- der materials including B-bearing lead-free al- loys. This is vividly demonstrated by the atten- dance at my lectures covering lead-free and the role of Bi, which were delivered in the Profes- sional Development Courses offered by NEP- CON West and NEPCON Japan in 1997–2003, IPC lead-free roadshows in 2003–2013, and at in-house programs at OEMs and NASA facili- ties over the last two decades, as well as recently (2017) by SMTA webtorial programs . Fifteen years passed by; how is the lead-free electronics doing—its track record during this fifteen-year run and its outlook? And what is the role of Bi in the lead-free arena marching forward? SAC system, specifically, SAC305 (Sn3.0Ag0.5Cu), has been widely known and used as a de facto "standard" alloy in the in- dustry for the past 15 years. However, during this period, for a designed performance or other purposes, different alloys outside the SAC sys- tem have been successfully used by some OEMs at their discretion, albeit in special low-volume applications. On the subject of Bi in the lead-free arena, one important point worth noting is to differen- tiate two separate Bi-containing lead-free alloy systems—Sn-based alloy system vs. Bi-based al- loy system. The two systems bear separate met- allurgical phenomena, thus physical properties and mechanical behavior, which in turn their respective intended applications and product service environments. Bismuth can be introduced either through the supply chain (not by design) or by design. With the deliberation of all relevant parameters, theoretical and practical, Bi plays a potent role in electronic solder interconnections. The prop- er use of Bi can benefit the performance and re- liability of electronic package and assembly in- cluding solder joint performance, tin whisker mitigation, among others. 1,2 By the same token, its improper use could impart deleterious effects to solder joints, thus product reliability. An adequate understanding of the properties and performance parameters of Bi is critical to the product reliability. This se- ries will discuss the relevant areas of Bi to help dispel misconceptions and to demonstrate per- formance criteria related to Bi. The goal is to help achieve the desired level of performance and product reliability. Topics to be Covered This series will highlight the following topics: • Bi: Characteristics, resources, safety data • Bi effects in 63Sn37Pb solder joint – Physical properties – Mechanical behavior • Effects of Bi from component coating and PCB surface finish – Dissolving into solder joint – Estimation of concentration of Bi in solder joint – Effect of compositional change • Bi effects in SAC solder joint (SnAgCuBi) – Compositional change – Stress vs. strain – Fatigue behavior • Bi effects in other Pb-free alloys (SnCu, SnAg, SnAgIn) – Stress vs. strain – Fatigue performance • Historically established Bi-containing electronic solder alloys THE ROLE OF BISMUTH (BI) IN ELECTRONICS: A PRELUDE " It is cheerfully intriguing to observe that some esteemed engineers and scientists have always showed an interest in novel lead-free solder materials including B-bearing lead-free alloys. "

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