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Study Electromigration in SnBiAg / SAC(305) Mixed Solder Alloy
Keywords: Low Temperature Solder, Lead-free solder, Electromigration
Interest has been growing in Sn based solders containing Bi, including SnAgCu solder with a few percent added Bi, and in SAC/Sn57BiAg mixed assemblies. But the mechanical properties of Sn-Bi alloys vary dramatically with Bi concentration. The strength doubles with the addition of up to approximately three percent Bi, then decrease by approximately 25% as the Bi concentration increases to that of the eutectic. In contrast, the ductility of SnBi alloys, as reflected in elongation measurements varies a great deal with Bi concentration, decreasing dramatically with the addition of a few percent of Bi, and displaying maximum values between 30 and 40 % Bi. The indication is that variations in the Bi concentration in Sn can profoundly affect mechanical properties. Processes which affect the distribution of Bi atoms in Sn based solder joints need to be better understood. Current stressing has been shown to be very effective in promoting Bi diffusion in Sn. Therefore examined the effect of current stressing on the distribution of Bi atoms in SAC/Sn57BiAg mixed assemblies. The evolution of the microstructures of SnBiAg / SAC mixed solder joints were studied under current stressing at elevated temperatures and extended times. Certain conditions resulted in the clear segregation of a significant amount of Bi. Results were consistent with previous measurement of the diffusion of Bi in Sn, and the effective charge number of Bi.
Faramarz Hadian, Reaserch assistant
Binghamton University
Binghamton, NY

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