3-D simulation on current density distribution in flip-chip solder joints with thick Cu UBM under current stressing

In flip-chip solder joints, Cu has been used as a under-bump metallization (UBM) for its excellent wettability with solders. In addition, electromigration has become a crucial reliability concerns for fine-pitch flip chip solder joints. In this paper, 3D finite element method was employed to simulate the current density distribution for the eutectic SnPb solder joints with 5 mum, 10 mum, and 20 mum thick Cu UBM. It was found that the thicker the UBM is, the lower the maximum current density inside the solder. The maximum current density decreased from 4.37 times 10⁴ A/cm² to 7.54 times 10³ A/cm² when the thickness of the UBM changed from 5 mum to 20 mum. Thicker Cu UBM can effectively relieve the current crowding effect inside the solder