The microstructure and adhesion strength of the Sn–0.7Cu/Cu solder joints were examined. The acicular η(Cu6Sn5)-phase was formed inside the solder and at the solder/Cu interface, mostly in the direction normal to the interface. Compared to the Sn–3.5Ag alloy, the Sn–0.7Cu solder showed comparable wettability, but lower microhardness and joint strength. After aging, the acicular η-phase grew into a round and scallop-shaped morphology, thin є(Cu3Sn)-phase evolved just above the Cu substrate, and the joint strength decreased linearly with aging time.