Electromigration in Sn30Ag05Cu flip chip solder joints was investigated under a current density of the order of 104 A/cm2. The transfer of atoms or vacancies and the formation of Joule heating induced by electromigration were studies in terms of microstructural evolution. The formation of pancake-type void at the interface between solder and intermetallic compound was dominated by current concentration and the directional transfer of vacancies induced by electromigration. Series of voids were also found at the interface between Ni（P） finishes and solder joint. The area of voids at substrate side are less than that at chip side. The flux of Cu atoms in the pad has the same direction as the electron flux. The transfer of Cu atoms from pad to solder joint led to the formation of large areas of intermetallic compound in solder joint, and the amount of intermetallic compound increased along the direction of electron flux.