Effects of microstructure on vacancy and stress distributions in micro joints under current stressing

The vacancy diffusion and stress evolution in SnPb and SnCu micro solder joints under current stressing are studied based on simulated microstructure from a phase field model. The vacancies are driven to the cathode by electric current and accumulated in the phase with lower vacancy formation energy. Stress concentration is predicted at the interfaces between phases, which is more severe when more vacancy-plated atom pairs are generated or annihilated. Compared to the Sn15Cu joint, there are more vacancies accumulated and annihilated at the cathode of the Sn27Pb joint and thus resulting in a higher von Mises stress. The effects of phase morphology on electromigration are further investigated. It is found that the decrease of the amount of interface due to phase coarsening in the Sn37Pb micro joint can accelerate vacancy accumulation. As a result, during electromigration the stress can quickly increase in the joint aged for a longer time. In addition, the connectivity of the Pb-rich phase also affects the electromigration behavior. A well-interconnected network of Pb-rich phase can accelerate the vacancy accumulation and thus stress concentration. Due to the combined effect of the connectivity of Pb-rich phase and the amount of interface, the maximum stresses caused by electromigration in three joints of different compositions, i.e. Sn47Pb, Sn37Pb, and Sn27Pb, are in the order of Sn37Pb>Sn47Pb>Sn27Pb.