An electroless plating process includes providing a semiconductor substrate which has a substrate and a copper pillar disposed on the substrate; providing a tin-silver plating solution includes 0.1-50 wt % tin and 1×105-2 wt % silver; and performing a reduction reaction, wherein the semiconductor substrate is disposed in the tin-silver plating solution for making tin and silver of the tin-silver plating solution deposit jointly on the copper pillar surface to form a tin-silver co-deposition layer. The tin-silver co-deposition layer is able to enhance the coupling strength between the copper pillar of the semiconductor substrate and the other semiconductor substrate and is also able to reduce the time and cost of the process performing tin-plating and silver-plating separately.