The formation and evolution of the intermetallic compound (IMCs) between SnAgCu lead-free solder and Cu substrate, after isothermal aging at 150°C for 24, 48, 120, 240 and 480 hours, were studied. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint during aging. The IMC phases were identified by energy dispersive X-ray (EDX). The results showed that IMCs layer of Cu6Sn5 was formed at the interface of solder and Cu during reflowing. With the increase of aging time, the grain size of the interfacial Cu6Sn5 increased and the morphology of the interfacial Cu6Sn5 column was changed from scallop-like to needle-like and then to rod-like and finally to particles. At the same time, the rod-like Ag3Sn phase formed at the interface of solder and the IMCs layer of Cu6Sn5 with the aging time increased. In addition, large Cu6Sn5 formed in the solder with the aging time increased. The tensile strength was measured for the solder joints. The results showed that the tensile strength increases slightly at beginning and then decreases with the aging time. SEM was used to observe the fracture surface and it showed that the fracture position moved from solder matrix to the interfacial Cu6Sn5 IMCs layer with the aging time increased. The weakening of the solder matrix is caused by the coarsening of the eutectic solder structures. The weakening of the interfacial IMCs layer is caused by the evolution of morphology and size of the interface Cu6Sn5 layer.