The formation of Intermetallic compounds (IMCs) at the interface between an Sn-9Zn-1.5Ag-xBi solder alloy and a Cu substrate dipped at 250°C and heat-treated at 150°C for various times has been investigated by an optical microscope (OM), an X-ray diffractometer (XRD) and a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The OM result shows the flat and smooth surface for the Sn-9Zn-1.5Ag-1Bi solder alloy and Cu substrate after dipping at 250°C. The phases of IMCs formed are Cu₆Sn₅ and Cu₅Zn₈ for both lead-free solder alloys. After thermal treatment at 250°C for 200 h, the phases of IMCs are Cu₆Sn₅, Cu₅Zn₈ and Ag₃Sn. The Cu₆Sn₅ has a scallop morphology, and is located at the interface between the solder and Cu substrate. The adhesion strength for the Sn-9Zn-1.5Ag-1Bi lead-free solder alloy is higher than the Sn-9Zn-1.5Ag solder alloy. After being heat-treated at 150°C, the adhesion strength of the Sn-9Zn-1.5Ag-1Bi solder alloy decreases from 12.67±0.45 to 6.92±0.38 MPa after thermal treatment for 200 h.