SMTA International Conference Proceedings

Electromigration Behavior of SAC(305) / SnBiAg Mixed Solder Alloy Assemblies

Authors: Faramarz Hadian, Mohammed Genanu, Eric Cotts
Company: Department of Physics, Binghamton University
Date Published: 9/22/2019   Conference: SMTA International

Abstract: Interest has been growing in Sn based solders containing Bi, including SnAgCu solder with up to three weight percent Bi, and in SAC/Sn57Bi1Ag mixed assemblies. But the mechanical properties of Sn-Bi alloys vary dramatically with Bi concentration. The strength increases by more than fifty percent with the addition of only three weight percent Bi, then decreases 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 40wt% 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 we examined the effect of current stressing on the distribution of Bi atoms in SAC/Sn-57Bi-1Ag mixed assemblies. The evolution of the microstructures of SAC/SnBiAg 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.

Key Words: 

Low temperature solders, mixed assembly, electromigration, and diffusion

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