In advanced electronic products, electromigration-induced failure is one of the most serious problems in fine pitch flip chip solder joints because the design rule in devices requires high current density through small solder joints for high performance and miniaturization. The failure mode induced by electromigration in the flip chip solder joint is unique, owing to the loss of under bump metallurgy (UBM) and the interfacial void formation at the cathode contact interface. In this study, Electromigration of flip chip solder joints has been investigated under a constant density of 2.45×104 A/cm2 at 120 °C. The in-situ marker displacements during the electromigration test was measured and found to show a rough linear change as a function of time. Scanning electron microscopic images of the cross section of samples showed the existence of voids at the interface between Al interconnection and under bump metallurgy. The void movement was matched with the marker displacements during the electromigration test, and voids moved to the cathode interface between Al interconnection and under bump metallurgy in the downward electron flow (from chip to substrate) joint. The mechanism of electromigration-induced void migration and failure in the flip chip are discussed. During electromigration, a flux of atoms is driven from the cathode to the anode or a flux of vacancies in the opposite direction. It can lead to two possible mechanisms of void migration. First, if we regard the void as a rigid marker of diffusion, it will be displaced towards the cathode by the atomic flux in the electromigration, Second, if we consider surface diffusion on the void surface, electromigration will drive atoms on the top surface of the void to the bottom surface of the void, and consequently the void will move towards the cathode.