Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126°C, 136°C, 158°C, 172°C, and 185°C. At 126°C and 136°C, voids formed at the interface of Cu 6Sn5 intermetallic compounds and the solder layer. However, at temperature 158°C and above, extensive Cu dissolution and thickening of Cu6Sn5 occurred, and few voids were observed. We proposed a model considering the flux divergency at the interface. At temperatures below 131°C, the electromigration flux leaving the interface is larger than the in-coming flux. Yet, the in-coming Cu electromigration flux surpasses the out-going flux at temperatures above 131°C. This model successfully explains the experimental results.