A semiconductor device includes a semiconductor substrate, a pad region on the semiconductor substrate, a passivation layer over the semiconductor substrate and at least a portion of the pad region, and a bump structure overlying the pad region. The passivation layer has an opening defined therein to expose at least another portion of the pad region. The bump structure is electrically connected to the pad region via the opening. The bump structure includes a copper layer and a SnAg layer overlying the copper layer. The SnAg layer has a melting temperature higher than the eutectic temperature of Sn and Ag.