Due to asymmetric structure of Copper (Cu) interconnects, Cu electromigration (EM) is related to the electron flow direction. In various technological nodes, Cu EM performance with electron down-flow direction is generally worse than that with electron up-flow direction due to the presence of voiding underneath the via. This study investigates Cu EM behavior with electron down-flow. The failure modes were verified and optimized processes were provided to improve EM performance. Additionally, layout geometry effect on electron down-flow EM was discussed.