The potential of the newly developed Sn–9Zn solder paste as a lead-free solder, especially focusing on the stability at high temperature, was examined. The initial interface strength between Sn–9Zn and Cu, about 50 MPa by the tensile test, is higher than other interfaces such as Sn–37Pb/Cu. While the Sn–9Zn/Cu interface maintains the high strength level after heat exposure at 125 °C, the heat exposure at 150 °C degrades strength seriously. The degradation at 150 °C is caused by dissipation and by disruption of the Cu–Zn reaction layer at the interface. Where the Cu–Zn layer is eroded to form a whole, Sn directly reacts with a Cu substrate to form a thick Sn–Cu reaction region. Such an interfacial morphology change causes the serious degradation. With the Ni/Pd/Au coating on a Cu substrate, the interface becomes much stronger than the direct interface. Even after heat exposure at 150 °C, strength degradation is not so significant. Zn segregates into the coating layer. During high-temperature exposure, Ni and Pd diffuse each other. Zn also diffuses into the coating layer to form compounds, and as a result, a depleted zone of Zn is formed in the solder close to the interface.