The effect of Cu content on the microstructure and the vibration deformation mechanisms of a potential lead-free solder, Sn-9Zn-xCu (x = 0.2, 0.5, 0.7, 1.0 wt.%), are examined in this study. Results show that Zn-rich phase and Sn-Zn eutectic decreased, while Cu-Zn intermetallic compound and proeutectic Sn-rich phase increased with increasing the Cu content. For the specimens with high Cu content (0.7Cu and 1.0Cu), hard massive Cu5Zn8 existed mostly amongst the proeutectic Sn-rich phase dendrites, and Zn-rich dispersed unevenly, leading to the deterioration in the tensile strength and ductility. Under a constant vibration force and constant initial-deflection testing, the high Cu specimen with a higher damping capacity was able to absorb more vibration energy and thus possessed a greater vibration fracture resistance. In addition, the lamellar-deformed structures (LDS) and Cu5Zn8 were able to increase the crack tortuosity, which in turn increased the crack propagation resistance.