We evaluated the melting property and the interfacial structure of the joints between Sn-Ag solder and Au/Ni-Co plating. It was confirmed that addition of Co in Ni plating and existence of Au plating effectively accelerated the reaction and the Sn-Ag solder completely transformed to the intermetallic phases with higher melting temperature. It is known that Ni3Sn4 layer is formed as the interface reaction layer, but (Ni, Co)Sn2 layer is formed by increasing the ratio of Co in Ni-xCo. The addition of Co is effective in rapidly forming the intermetellic compounds in the inside of Sn-Ag solder probably due to the formation of (Ni, Co)Sn2 reaction layer with higher diffusivity of Ni. Furthermore, the existence of Au plating is effective in forming the intermetellic compounds of (Au, Ni)Sn4 or (Au, Ni, Co)Sn4 which consumes more molar fraction of Sn than Ni3Sn4. This process is expected to replace the packaging technology using high temperature solders.