Phase field simulation of the microstructural evolution and electromigration-induced phase segregation in line-type Cu/Sn-Bi/Cu solder interconnects

The separation state and inhomogeneous nature of microstructure in Sn-Bi based solders can reduce the reliability of solder interconnects obviously. Notably, continuously scaling down of the feature size of solder interconnects may greatly promote the electromigation and segregation of Bi atoms in Cu/Sn-Bi/Cu interconnects under electric current stressing, which will induce the inhomogeneity of the microstructure and decrease the reliability of solder interconnects inevitably. In this study, a phase field model is developed to simulate the microstructure of the eutectic Sn-Bi solder in a line-type Cu/Sn-Bi/Cu interconnect. Further, the phase field equation is coupled with the Laplace equation governing the electric potential to simulate the segregation of the Bi-rich phase in line-type Cu/Sn-Bi/Cu interconnects under electric current stressing. The simulated evolution and coarsening behavior of the eutectic microstructure are consistent with experimental studies. Under electric current stressing, Bi atoms migrate towards the anode side, and finally a Bi-rich phase region forms at the anode side, while a Sn-rich phase layer forms at the cathode side. Moreover, by reversing the direction of electric current, the Bi-rich phase layer is formed at the anode side starts to dissolve, and finally a continues Bi-rich phase layer is formed on the opposite side.