Phase field study of the combined effects of electromigration and thermomigration on phase segregation and physical properties of Sn58Bi solder joints under electric current stressing coupled with temperature gradient

The obvious coarsening and segregation of the Bi-riched phase can greatly affect the reliability of Sn58Bi solder joints. Noticeably, continuous decrease in the size of solder joints induces a significantly enhanced strengthening effect of electric current stressing and temperature gradient on the microstructure evolution, which may easily lead to the inhomogeneity and segregation of the microstructure in Sn58Bi solder joints. In this study, the microstructural evolution of two Sn58Bi solder joints in a daisy-chained structure under electric current stressing coupled with temperature gradient is simulated by a phase field model. The simulation results show that the electric current stressing can drive the Bi atoms to migrate to the anode sides, while the temperature gradient drives the Bi atoms to migrate to the cold ends of solder joints. Moreover, the temperature gradient can assist or counteract electric current stressing on the diffusion and directional migration of Bi atoms, affecting the microstructure evolution. The simulation results of the microstructure evolution are in good agreement with the experimental observation. In addition, the change in physical properties of Sn58Bi solder joints induced by the inhomogeneous microstructure is quantitatively analyzed.