As the size of consumptive electronic products continues to shrink, Flip Chip ICs also have to miniaturization. However, shrinking the size of solder bump is similar to the change of current density. We will discuss the effect of solder bump by elecromigration under different current density. In this study, we applied current stress of 0.8A、1.0A and 1.2A respectively on lead-free SnAg solder joints at 100℃. And we observed the cross-section of specimen for various stages and analyzed the failure modes for each solder bump . In experimental result, for the bump without current stressing, we found the formation of void causing by Cu UBM consumption which was affected by thermomigration effect. And we demonstrated (Cu,Ni)6Sn5 accumulation in current crowding region for the bump with an upward electron flow. Then, we found not only the formation of pancake-type void, but also destruction of IMC on the bump with a higher current density downward electron flow.