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The different effects of OSP and ENEPIG surface finishes on the electromigration-induced failure mechanism of Sn-3.0Ag-0.5Cu flip chip solder joint were investigated at 150°C under a current density of 1×104 A/cm2. In as-soldered state, the interfacial (Cu0.55Ni0.45)6Sn5 IMC formed on Ni UBM at the chip side in both OSP and ENEPIG joints. However, the EM resistance of the two joints was greatly different when electrons flowed from chip to PCB though they had the same composition of interfacial (Cu, Ni)6Sn5 and the same Ni UBM. For OSP joint, the interfacial (Cu, Ni)6Sn5 and the Ni UBM displayed an excellent EM resistance; and the Cu content of interfacial (Cu, Ni)6Sn5 IMC at the chip side was slightly higher than that of as-reflowed joint. While for ENEPIG joint, the interfacial (Cu, Ni)6Sn5 IMC and Ni UBM were seriously consumed during EM, and the joint failed. The obvious difference of EM-induced failure between the OSP joint and the ENEPIG joint was due to the different effects of surface finishes. Compared with the ENEPIG joint, the OSP joint could offer a Cu source to improve the stability of interfacial (Cu, Ni)6Sn5 IMC, which effectively inhibited the dissolution of Ni during EM.