The insertion of Pd layer, i.e, ENEPIG, was found to be capable of effectively retarding the electromigration-induced Ni consumption in the flip-chip solder joints. During soldering process, the Pd layer was transformed to the (Pd,Ni)Sn4 phase at the interface. In this study, several interfacial structures, Sn/Pd(1 or 3μm)/Ni and Sn/Cu(3μm)/Pd(1μm)/Ni, were prepared to form the (Pd,Ni)Sn4 with different Ni contents. The electromigration test was conducted at 180oC and at a current density of 5000 A/cm2. In the Sn/Pd(1 or 3μm)/Ni sample, the (Pd,Ni)Sn4 layer with a high Ni solubility (~17at.%) was massively migrated into the solder matrix and the Ni substrate was considerably consumed. In contrast, in the Sn/Cu(3μm)/Pd(1μm)/Ni sample without the occurrence of rapid Pd dissolution, the (Pd,Ni)Sn4 layer with a low Ni content (< 3at.%) remained layer-structured, indicating that it can effectively retard electromigration. The (Pd,Ni)Sn4 phase clearly exhibited a better electromigration resistance when the Ni content decreased.