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Temperature and current density are two crucial factors in electromigration study. In order to investigate the effects of these two crucial factors deeply and separately. A temperature control device, employing forced convection with a constant direction, was adopted to control the chip temperature so that the chip temperature can be decoupled from the applied current density. With this temperature control device, the electromigration effect on UBM (Under Bump Metallurgy) consumption and interfacial reactions in Cu/Sn2.6Ag/Cu flip chip solder joints was investigated under varied extra-high current densities of 3.5 × 104 A/cm2 and 4.5 × 104 A/cm2, respectively, at the chip temperature of 50 ± 1°C. Compared with those without temperature control, the results showed that the joint exhibited a much longer life (over 1500 h) under 3.5 × 104 A/cm2 current stressing, and the life time for 4.5 × 104 A/cm2 was 1862 h. Results also indicated a unique "sawtooth-type" Cu UBM morphology was retained at the cathode/chip or substrate side when the solder joints stressed with current density of 3.5 × 104 A/cm2. Both sawtooth-type morphology and void formation were observed at the cathode/chip side when the current density increases to 4.5 × 104 A/cm2. The evolutions and distributions of these "sawtooth-type" Cu UBM morphology resulting from electromigration will be discussed and examined at this study. It was successfully demonstrated that the temperature control device was able to decouple the applied current density and the chip temperature.