This study investigated the thermal effect on the interfacial IMC (Cu3Sn and Cu6Sn5) formation at the interface between pure tin and a copper substrate at 125degC. During thermal aging, the morphology of Cu6Sn5 transformed from a scallop to a layer shape. The thickness of IMCs (Cu3Sn, Cu6Sn5) changed with an increase in aging time. It was observed that scallop Cu6Sn5 decomposed during heat aging for up to 420 hours; in contrast, upper Cu3Sn grew rapidly. Between 420 hours and 2693 hours, both Cu3Sn and Cu6Sn5 increased with increasing aging time. However, between 2693 hours and 2854 hours of thermal aging, the shrinkage of Cu3Sn and the growth of Cu6Sn5 were induced as a result of the segregation of voids formed at the Cu3Sn/Cu interface. On a free surface, Cu6Sn5 and Cu3Sn tended to grow in the X-Y and Z planes, respectively.