Influence of Ag content on the formation and growth of intermetallic compounds in Sn–Ag–Cu solder

The interfacial reactions and growth kinetics of Sn–Ag–Cu were investigated during solid-state aging. The effects of chemical composition on the interface structure and the growth of interface under high temperature and high humidity were experimentally and numerically studied. Three solders Sn–0.4Ag–0.7Cu, Sn–0.7Ag–0.7Cu, and Sn–1.0Ag–0.7Cu were used to conduct several experiments. In order to determine the long-term reliability of the solder joints, thermally accelerated aging tests were performed for 0, 10, 30, 50, 100, and 350 h at 85 °C and 85% relative humidity. The surface morphology, thickness, and distribution of compounds were observed by scanning electron microscopy. The results showed that with increasing Ag content, the growth of Cu6Sn5 layer suppressed. The growth kinetics of intermetallic compound (IMC, Cu6Sn5 + Cu3Sn) remained a diffusion-controlled process during the isothermal aging, and Cu6Sn5 gradually transformed into a Cu3Sn IMC. The scallop-like shape disappeared at later aging stages, suggesting a change in the growth mechanism to the steady growth in the perpendicular direction to the interface. In addition, the results revealed that Ag effectively slowed the growth kinetics of Cu6Sn5. The apparent activation energy Q for Cu6Sn5 + Cu3Sn IMC of interfaces at Sn–0.4Ag–0.7Cu/Cu, Sn–0.7Ag–0.7Cu/Cu, and Sn–1.0Ag–0.7Cu/Cu was calculated to be 69.11, 88.90, and 56.61 kJ/mol, respectively.