Morphological evolution and migration behavior of the microvoid in Sn/Cu interconnects under electrical field studied by phase-field simulation

Microvoids usually form at the interface between the Sn-based solder and Cu substrate during aging process. The existence and growth (electromigration and coalescence) of the microvoids can decrease the reliability of solder joints, in particular for the joints undergoing electrical current stressing of high density. In this paper, a diffuse interface model is employed to simulate the morphological evolution and migration behavior of microvoids in the solder interconnect consisting of the Sn-based solder and Cu substrate (i.e., Sn/Cu system), under electrical fields. Simulations take into account the coupled effect of surface diffusion and electrical field. The order parameter equation and electrical field equation are solved by using the finite difference method. The validity of this method is confirmed by the agreement of the evolution of noncircular microvoids driven by surface energy with that predicted theoretically. Simulation results show that the electrical field has significant influence on the morphological evolution of microvoids. The circular microvoids migrate from the region with high electric potential to the region with low potential under electrical field. Moreover, the migration velocity of the microvoid is constant. With increasing the voltage, the migration rate increases and under a high voltage the severe migration of microvoid may lead to a failure of the solder interconnect by the electromigration or coalescence of microvoids.