The electromigration behavior of Cu/SAC305/Cu joints was investigated. Current stressing tests were conducted at a certain temperature and current density for various hours. The diffusion characteristics of the elements near the soldering interface and IMC (intermetallic compound) growth mechanism were studied. Results indicate that the growth of IMC presents a parabola dependence on time under thermal-electrical coupling at the anode. The thickness of IMC increases at first and then decreases. Meanwhile, the morphology of IMC changes obviously at the cathode. The diffusion of the elements at the soldering interface could be divided into two stages. At the initial stage, the high element concentration gradient in the soldering interface plays a leading role to the element diffusion. The IMC shows a similar growth trend at the anode and cathode. At the second stage, the concentration gradient in the soldering interface decreases. The electron-wind force plays a crucial role to the element diffusion. Accordingly, the thickness of IMC decreases at the cathode but increases at the anode.