Flip chip on flexible substrate technology has attracted much attention in the electronics industry for portable applications because the reduction of packaging volume coupled with long-term reliability is a challenging issue. In this study, the electromigration behavior of flip chip Sn96 Ag3.5 Cu0.5 solder joints on flexible substrates has been investigated under a constant current density of 4× 104 A cm2 at room temperature. The in situ resistance change of the circuit during the electromigration test was measured and found to show stepwise changes before the final failure. Scanning electron microscopic images of the cross section of samples showed the existence of voids in the bulk of the solder joints before electromigration and the movement of these voids after electromigration. The void movement was matched with the resistance change during the electromigration test, and voids moved from within the bulk of solder bump to the cathode interface between Al interconnection and under bump metallurgy in the downward electron flow (from chip to substrate) joints. The mechanisms of electromigration-induced void migration and failure in the Pb-free flip chip on flexible substrates are discussed.