The dissolution properties of Cu in molten Sn-Cu-Ni and Sn-Zn alloys were investigated and compared with those of Cu in SnAg -Cu alloys. In the Sn-Cu-Ni alloys with small amounts of Ni, the dissolution rate of Cu decreases with increasing Ni content when the Ni content is below 0.15 mass%. The dissolution rate is proportional to the difference between the saturated solute concentration in liquid (C s) and the solute concentration in liquid (C), (C s ϪC), which is estimated from the isothermal section of the Cu-Sn-Ni ternary phase diagram. In the Sn-Zn alloy, the Cu-Zn reaction layer forms at the solder/Cu interface. The layer prevents Cu dissolution and thus the dissolution rate of Cu is dramatically reduced.