Effects of NCF and UBM Materials on Electromigration Reliabilities of Sn-Ag Microbumps for Advanced 3D Packaging

The effect of non-conductive films (NCF) and under-bump metallization (UBM) materials on the electromigration (EM) failure mechanism of Sn-Ag microbumps was investigated under stress conditions at current densities ranging from 0.5~1.3 × 105 A/cm2 at 150°C. In the case of NCF microbumps, the EM failure mechanism were almost similar to that involving Cu/Ni/Sn-Ag microbumps with NCF and non-NCF. However, the NCF applications increased EM lifetime for the Cu/Ni/Sn-Ag microbumps. In the case of Ni and Cu UBM microbumps, the EM failure mechanism varied between Sn-Ag microbumps with Ni UBM and Cu UBM. No EM-induced failure was observed in Cu UBM microbump. However, EM-induced voids were found on the cathode side of the Al trace in the Ni UBM microbump. The three-dimensional (3-D) finite element method was used to simulate the current density and temperature distributions of the microbumps. The results showed that the Joule heating of the Ni UBM microbump was higher than that of the Cu UBM microbump. Thus, the increased Joule heating due to thicker Ni UBM resulted in a negative effect on the EM failure time. Therefore, the Cu UBM microbumps are expected to display enhanced EM resistance.