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The electromigration of flip chip solder joints consisting of 97Pb-3Sn and 37Pb-63Sn composite solders was studied under high current densities at room temperature. The MTTF (mean time to failure) and failure modes were found to be strongly dependant on the change of current density. The composite solder joints did not failed after one month stressed at 4.07 × 104A/cm2, but they failed after 10 hours of current stressing at 4.58 × 104 A/cm2. At a slightly higher current stressing of 5.00 × 104 A/cm2, they failed after only 0.6 hours by the melting of the composite solder bumps. Due to the precipitation and growth of Cu6Sn5 intermetallic compound at the cathode side, the Cu under bump metallurgy (UBM) was quickly consumed and followed by void formation at the contact area. The void reduced the contact area and displaced the electrical path, thus it affects greatly the current crowding and Joule heating inside the solder bump. A large Joule heating inside solder bumps can cause melting of the solder bump and the failure occurred quickly. The effect of void propagation on current crowding and Joule heating has been confirmed by simulation.