Comparison of electromigration for lead-free solder joints of Cu vs. Ni UBM flip chip structure

Flip chip solder joints made with Cu and Ni underbump metallurgy (UBM) on the chip under current stressing were studied. The effects of material and various thicknesses (5, 10, 15, and 20 mum) of UBM on reliability were investigated. The solder material used was lead-free (Sn4.0Ag0.5Cu). Time to failure of both cases (Cu and Ni UBMs) was forecasted through the physical damage occurring at the bump under current stressing by using two-dimensional Finite Element Method (FEM). The simulation results conclude that thicker UBM can enhance electromigration reliability. Moreover, the comparison of the time to failure between Cu UBM structure with that of Ni UBM structure indicates that the time to failure of the flip chip solder joint with Cu UBM structure is approximately 5% longer than the case of Ni UBM structure due to the lowering current crowding effect at Cu UBM in the solder joint which is clearly shown in the simulations.