This study investigates the electromigration (EM) effect under a high current density (104 A/cm2) on the different interfacial compound phases at Sn(Cu) solder/electroless nickel immersion gold (ENIG) interfaces. The interfacial Ni3Sn4 phase at the Sn-0.7 wt.%Cu/ENIG joint interface was quickly depleted after a short period (50 h) of current stressing. The inference drawn is that the Ni atoms in the Ni3Sn4 phase at the joint interface are likely forced out under current stressing; however, the ternary (Cu,Ni)6Sn5 compound effectively reduces the EM-driven Ni flux into the Sn bump; thus, a significantly lower Ni(P) consumption was observed at the Sn-1 wt.%Cu/ENIG interface. The EM-induced Ni(P) dissolution rates in the Sn-0.2 wt.%Cu/ENIG and Sn-1 wt.%Cu/ENIG cases were calculated to be 0.028 μm/h and 0.018 μm/h, respectively. In addition, significant EM-assisted Ni3P formation was observed for the current-stressed Sn-0.2 wt.%Cu/ENIG and Sn-0.7 wt.%Cu/ENIG cases; however, for the Sn-1 wt.%Cu/ENIG case, formation of a Ni3P layer was scarcely observed. Moreover, the initial (Cu,Ni)6Sn5 that formed at the interface appeared compact with a layer-type structure, which reduced the EM-driven Ni diffusion.