The Evolution of Microstructures and Mechanical Properties of SnAgCu/Cu Weld Interface during Isothermal Aging

Abstract

Based on the model of the diffusion flux ratio of Cu and Sn at the Cu/Cu3Sn/Cu6Sn5/Sn interface, the evolution of microstructures, the behavior of formation and growth of intermetallic compound (IMC) for Sn95.5Ag3.8Cu0.7 solder aged at 150 °C were studied. The nano indentation was applied to measure the hardness and elastic modulus of the IMC. The tensile strength and the low cyclic fatigue properties of the weld joint were also measured. The results showed that the thickness increments of Cu3Sn and Cu6Sn5 phases of IMC are mainly controlled by diffusion mechanism. The growth rates of Cu3Sn and Cu6Sn5 are 7.86×10-19 m2/s and 7.80×10-17 m2/s, respectively. The hardness and elastic modulus of IMC enhance with increasing aging time. For 24h aging time, the microstructure of the Cu6Sn5 keeps scalloped shape and the elastic modulus of IMC layer are similar to the copper substrate, which result in high fatigue resistance of the welded joint and its tensile strength of 69.50 MPa. With the mechanical hardening cumulative effects resulting from aging and cyclic strain, the fatigue performance and tensile strength of the welded joint gradually worse with the further increase of the aging time.