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30 SMT Magazine • March 2014 Influence of Microstructure on Mechanical Behavior of Bismuth Pb-Free Solders SummarY: Certain alloys that contain bismuth have shown superior reliability in pCB testing, but they have not been widely used in electronics manufacturing due to their Bi content. this article focuses on the microstructure and behaviour of Bi-containing alloys, and compares this data with that of SaC305. Introduction SAC-Bi and Sn-Ag-Bi alloys have demonstrat- ed superior performance in thermal cycling reli- ability tests of PCBs, such as the National Center for Manufacturing Sciences (NCMS) programs in the 1990s 1, 2 and the Joint Group on Pollu- tion Prevention-Joint Committee on Aging Air- craft program of the early 2000s 3 . Despite these promising results, they have not been widely used in electronics manufacturing due to their Bi content, which has raised concerns for the potential of forming the low-melting point Sn- Pb-Bi eutectic phase (Tm 96°C) in mixed SnPb/ Pb-free soldering. The recently concluded (De- cember 2011) NASA-DoD program Phase II fol- low-on to JCAA-JGPP revived the possible use of Bi-containing alloys with the recommenda- tion that lower reflow temperatures for ternary Sn-Ag-Bi and quaternary SAC-Bi could reduce potential for pad cratering, as well as possess greater resistance to the growth of tin whiskers 4 . At the same time, an explanation for the per- formance of Bi-containing alloys documented in thermal cycling has not been provided, and basic aspects of the metallurgy of these alloys have not been explored to the same extent as more common SAC alloys. The original NCMS project on Pb-free solder 1 elected not to pursue this type of characterization of the alloys on the presumption that mechanical property data for the solders did not necessarily correlate with or provide insight into the reliability of the circuit board, which was the project's motivation. In the present work, the relationship be- tween microstructure, aging and mechanical behavior was studied for Sn-3.4Ag-4.8Bi (SnAg- feaTure by David B. Witkin The aeroSpaCe CorporaTion Influence of Microstructure on Mechanical Behavior of Bismuth Pb-Free Solders