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.
