We have studied the effect of thickness of Cu under bump metallization (UBM) from 5 μm, 10 μm to 50 μm on electromigration induced failure mechanism in flip chip solder joints. In the case of 5 μm Cu UBM, due to the direct current crowding effect at the UBM/solder interface, the failure mode induced by electromigration was the loss of UBM and the interfacial void formation at the cathode contact interface between the interconnect line and the solder bump. The current crowding effect in flip chip solder joints were reduced when the Cu UBM thickness was increased to 10 μm, and the flip chip joint with 10 μm Cu UBM showed much longer mean-time-to-failure than that with 5 μm Cu UBM because the 10 μm Cu UBM was enough to contain the current crowding inside the UBM. However, even when the current distribution was uniform in the solder area in this case, the final failure mode was the same as the case of 5 μm UBM. The failure occurred by a two-stage consumption of the 10 μm thick Cu UBM in the joint where electrons flowed from the chip to the substrate. In the first stage, the 10 μm Cu UBM dissolved layer by layer at the entire Cu UBM/solder interface. After half of Cu UBM was dissolved, the asymmetrical dissolution of Cu UBM was concentrated at the corner where electrons entered from Al interconnect to Cu UBM. A small number of Kirkendall voids were found in the Cu 3Sn layer during the electromigration testing, but they were not a serious concern in this case. When the Cu UBM was 50 μm thick, more uniform distribution of current density was obtained in the solder bump, and the flip chip joint showed a very strong resistance against electromigration-induced failure. In the case of a 50 μm thick Cu UBM combined with a 20 μm height shallow solder interconnect, the flip chip joint did not fail after 720 hours of current stressing at 100°C with a current density of 6.75 × 10 4 A/cm 2 calculated at the pad opening on the Si chip side. The reduction of current crowding in the solder region by using thick Cu UBM and the small ratio of Sn to Cu in the bump structure improved the reliability against electromigration- induced failure. The effect of Cu thickness on reducing current crowding during electromigration has been confirmed by simulation. However, in the case of 50 μm thick Cu UBM, Kirkendall void formation was found to be much more serious and the formation was enhanced by electromigration at the Cu/Cu 3Sn interface due to the large Cu/Sn ratio. Furthermore, a very large temperature gradient exists across the shallow solder interconnects, leading to thermomigration. Electromigration accompanied by thermomigration could replace current crowding as a serious reliability issue in using 50 μm thick Cu UBM based interconnects. To provide a comparison for the microstructure evolution caused by electromigration and by thermal annealing alone, a flip chip joint with 50 μm thick Cu UBM and with 20 μm height shallow solder bump was investigated without any current stressing but with aging at 150°C for 720 hours. In the case of thermal aging, thick Intermetallic Compounds (IMCs) were observed not only under the Cu column bumps but also on the sidewall finish of the substrate because there was no electromigration and no temperature gradient for thermomigration between the chip and the substrate. The Kirkendall void formation was not very serious in the case of thermal aging as compared to the case of electromigration because Sn moved to both sides to form IMCs during thermal aging.