In this study, an attempt was made to develop an in-situ processing method of Sn-Ag composite solders for the ball grid array in electronic packaging applications. The reinforcing phase(Cu_6Sn_5) was produced by the reaction between Sn in the matrix and Cu powders added. It was found that the shape and size of the phase formed are affected by the processing parameters such as holding time very sensitively. The mechanical properties and the thermal stability of the composite solders were proved to be superior to those of Sn-3.5 %Ag solder.