Current density dependence on the mechanism of the electromigration-induced flip-chip Cu pad consumption was observed. A critical current density was determined, which defines the consumption mechanism of Cu pad. Below this critical current density, a constant interfacial compound layer would be established. The mechanism whereby the Cu pad is consumed is the dissolution of the interfacial Cu-Sn compound. Above the critical current density, the interfacial compound layer continues to grow over time. The consumption of the Cu pad was then primary controlled by the total Cu flux in the interfacial Cu-Sn compound layer.