The Cu consumption in flip-chip Sn∕Cu solder joint structures, both with and without current stressing was studied. The Cu consumption activation energy without current stressing was determined to be 0.93eV. With current stressing, the Cu consumption activation energy decreased to 0.66eV. We believe that the enhancement of Cu consumption by current stressing is due to the electromigration (EM) Cu atoms dissolved in the Sn to electromigrate instantaneously toward the anode side, which means that the cathode’s Cu–Sn compound layer dissolves into the Sn to satisfy the Cu solubility limit. The EM-induced Cu consumption is controlled by either the dissolution of the cathode Cu–Sn compound or by EM Cu flux in Sn. The temperature defines that the controlling step of Cu consumption was found and determined to be 129°C by equating the dissolution flux in the Cu–Sn compound and the EM Cu flux in Sn. When the EM temperature is higher than the critical temperature, the Cu–Sn compound dissolution process controls the Cu consumption. In this case, a constant interfacial Cu–Sn compound thickness remains at the cathode interface, which is determined by the balance between the Cu–Sn compound dissolution and the chemical Cu flux. If the EM temperature is lower than the critical temperature, the EM Cu flux in the Sn controls the Cu consumption. The growth of Cu–Sn compound at cathode is controlled by the EM Cu flux in Sn and the chemical Cu flux.

Polarity void formation has been observed at the ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ interfaces in current-stressed $\mathrm{Cu}∕\mathrm{Sn}∕\mathrm{Cu}$ flip-chip joints. Electromigration voids were observed at the ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ anode interface, but no voids were found at the ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ cathode interface. The backstress in the anode’s Cu trace line caused a vacancy backflow toward the ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ anode interface, which led to serious void formation. In the opposite ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ cathode interface, Cu atoms in the cathode’s Cu trace line electromigrated toward the ${\mathrm{Cu}}_3\mathrm{Sn}∕\mathrm{Cu}$ cathode interface, which alleviated or healed possible Kirkendall void formation due to the normal annealing process.

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.