Electromigration in solder joints causes a void formation between intermetallic compounds (IMC) and solder at the cathode. The effect of electromigration in mechanical test of Cu wires joined by solder was performed. The current density of electromigration was 1∼5×103 A/cm2. The working temperature was 100∼150°C. Tensile stress and shear stress were applied either before or after electromigration. The tensile strain rate was 3 μm/min. We observed that, without electromigration, tensile stress caused a ductile break at the middle of solders because the solder was softer. On the other hand, if combined with electromigration, a brittle failure always occurred at the cathodes interface during tensile test. The ultimate tensile strength decreased with longer electromigration time or higher current density. In shear test, the daisy chain of solders failed alternatively at the cathodes after electromigration.