This study investigated the thermal effect on the interfacial IMC (Cu 3Sn and CU6Sns) formation at the interface between pure tin and a copper substrate at 125°C. During thermal aging, the morphology of CU6SnS transformed from a scallop to a layer shape. The thickness of IMCs (Cu3Sn,CU6Sns) changed with an increase in aging time. It was observed that scallop CU6Sns decomposed during heat aging for up to 420 hours; in contrast, upper CU3Sn grew rapidly. Between 420 hours and 2693 hours, both CU3Sn and CU6Sns increased with increasing aging time. However, between 2693 hours and 2854 hours of thermal aging, the shrinkage of CU3Sn and the growth of CU6Sns were induced as a result of the segregation of voids formed at the CU 3Sn/CU interface. On a free surface, CU6SnS and CU 3Sn tended to grow in the XY and Z planes, respectively.