The interfacial reaction between eutectic SnBi and Cu was studied by TEM after the sample was reflowed and aged in solid state, respectively. The microstructural evolution at the SnBi/Cu interface during reflowed and solid-state aged process was analyzed. The results show that there are two layers of intermetallic compounds (IMCs), Cu3Sn and Cu6Sn5, located at the interface between solder and Cu after the sample was reflowed. The segregation of Bi at the interface between Cu3Sn and Cu was observed. Furthermore, the segregation of Bi induced the formation of voids at Cu3 Sn/Cu interface during solid-state aging process.