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 mean time to failure and failure modes were found to be strongly dependent on the change in current density. The composite solder joints did not fail after 1 month stressed at 4.07×104 A/cm2, but failed after just 10 h of current stressing at 4.58×104 A/cm2. At a slightly higher current stressing of 5.00×104 A/cm2, the composite solder joints failed after only 0.6 h due to melting. Precipitation and growth of Cu6Sn5 at the cathode caused the Cu under bump metallurgy to be quickly consumed and resulted in void formation at the contact area. The void reduced the contact area and displaced the electrical path, affecting the current crowding and Joule heating inside the solder bump. Significant Joule heating inside solder bumps can cause melting of the solder and quick failure. The effect of void propagation on current crowding and Joule heating was confirmed by simulation.