Phase Field Simulation of Segregation of the Bi-riched Phase in Cu/Sn-Bi/Cu Solder Interconnects under Electric Current Stressing

The migration and segregation of Bi atoms in Cu/Sn-Bi/Cu solder interconnects under electric current stressing usually induce the formation of Bi-riched layers at the anode side, which can cause the failure of solder interconnects easily. In our study, the microstructure evolution of the eutectic Sn-Bi solder in a Cu/Sn-Bi/Cu flip chip solder joint and a right-angle Cu/Sn-Bi/Cu solder interconnect is simulated by phase field method. Further, the phase field equation is coupled with the Laplace equation governing the electric potential to simulate the segregation of the Bi-riched phase in Cu/Sn-Bi/Cu solder interconnects, in which the Bi-riched phase and Sn-riched phase inhomogeneously distribute. The coarsening behavior of the eutectic microstructure can also be seen in the simulation results, and the phases grow and become coarse along multiple directions in the flip chip solder interconnect compared with that in the right-angle solder interconnect. Under electric current stressing, Bi atoms migrate towards the anode side, while Sn atoms move in the opposite direction, and finally a segregated layer of Bi-riched phase is formed at the anode side, which is consistent with the experimental studies. The phase separation first occurs near the bottom corner of the right-angle solder interconnect. Moreover, the inhomogeneity of microstructures can influence the current density distribution, and the electromigration and segregation of the Bi-riched phase can induce the increase of both the voltage and resistance.