Interaction effect between electromigration and microstructure evolution in BGA structure Cu/Sn-58Bi/Cu solder interconnects

Dimension of solder interconnects (or joints) and pitches has been continuously scaling down, resulting in inhomogeneous microstructure and severe electromigration (EM) effect in solder interconnects. In this study, the interaction effect between electromigration and microstructure evolution in ball grid array (BGA) structure Cu/Sn-58Bi/Cu solder interconnects under a direct current density of 1.5×10⁸ A/m² is studied by cellular automaton (CA) modeling embedded with finite element (FE) simulation. Results show that, compared with configuration or geometry of BGA interconnects, the influence of inhomogeneous eutectic phases on the distribution of current density is more obvious. The current density in the Sn-rich phase is much higher than that in the Bi-rich phase. Bi atoms in Sn-rich phase are more prone to migrate to the anode first, rather than migrating directly along the interface between Bi-rich phase and Sn-rich phase. Consequently, the damage in the Sn-rich phase, rather than at phase interfaces or in the Bi-rich phase, could usually be observed in experiments. By employing the criterion of EM induced atomic flux of Bi in FE analysis under CA rules, simulation results of Bi-rich phase segregation are consistent with the experimental observation under current stressing.