Failure mechanism of Cu/SnAg/Ni microbumps in three-dimensional integral circuits under electromigration

In this study, we use the Cu/SnAg/Ni/Cu joint with a thickness of 7μm/8μm/2μm /5μm and 7μm/16μm/2μm /5μm. The diameter of the solder joint is about 30 μm and the UBM opening is 18 μm in diameter with 60 μm pitches. In order to measure the resistance changes, it was fabricated which constitute four probes. By using this method we can measure the resistance changes in microbump excluding the resistance changes in the wiring trace to make it more sensitive to subtle microstructure changes. We investigated the mechanisms of electromigration in microbumps with different stages when resistance increased 20%, 50% and 100% of the initial resistance. Solder joints were stressed with 0.56 A on 150°C hot plate. The average current density is 8 × 10⁴ A/cm², calculated based on the UBM opening. We ground the samples with 1000, 2000, and 4000 sand paper and then polished them with 1 μm, 0.3 μm, and 0.05 μm Al₂O₃. We observed their morphologies with SEM and measured their component with EDX. In the bump with electron flow upward, we observed that microbumps were transformed into Cu₆Sn₅ IMCs and Cu₃Sn IMCs joints. In addition, voids were formed at the interface between nickel and solder. We found that both void formation and UBM dissolution were occurred. In the bump with electron flow downward, microbumps were transformed into IMC joints without void formation. To sum up, this study provides a better understanding on the relationship between the behavior of resistance curve and the morphology changes in microbumps.