Electromigration induced microstructure evolution and damage in asymmetric Cu/Sn-58Bi/Cu solder interconnect under current stressing

The electromigration induced microstructure evolution and damage in asymmetric Cu/Sn-58Bi/Cu solder interconnects were investigated by in-situ SEM observation, focused ion beam (FIB) microanalysis and finite element (FE) simulation. The SEM results show that the electromigration-induced local degradation of microstructures, i.e., segregation of Bi-rich phase and formation of microcracks, in the asymmetric solder interconnects is much severer than that in the symmetrical ones. FIB-SEM microanalysis reveals that the microregional heterogeneity in electrical resistance along different electron flowing paths is the key factor leading to non-uniform current distribution and the resultant electromigration damage. Theoretical analysis and FE simulation results manifest that the current crowding easily occurs at the local part with smaller resistance in an asymmetric solder interconnect. All results indicate that the asymmetric shape of the solder interconnect brings about the difference of the electrical resistance between the different microregions and further results in the severe electromigration damage.