Effects of Aging on Microstructure and Mechanical Properties of Sn-Ag-Cu-Bi Solder Alloys

In this study, the effects of aging and Bi content on the microstructural evolution and mechanical properties of Sn-Ag-Cu-Bi alloys were investigated. Samples of three types of alloys, SAC405, SAC405+2%Bi, and SAC405+3%Bi, were cast and subjected to aging at 25°C, 75°C, and 125°C for up to 90 days. For samples from each of the temperature and storage durations, the microstructure was investigated with SEM and the mechanical properties, including hardness and creep strength, were studied by nano-indentation. The results revealed that the addition of Bi induced pronounced changes in the size and distribution of intermetallic compounds (IMCs) and subsequently the mechanical properties. 2% to 3% Bi addition to SAC405 can greatly enhance the creep strength of SAC405 with aging as the Bi caused IMCs to be finely dispersed in Sn matrix. Moreover, Bi content of 2% to 3% in SAC405 was able to retain the hardness as it was in the as-cast condition. The results provide important information on solder selection for applications where solder joint fatigue life is main consideration.