The evolution of microstructure and microhardness of Sn–Ag and Sn–Cu solders during high temperature aging

The changes in microstructure and microhardness of Sn–0.5Ag, Sn–1.0Ag, and Sn–0.7Cu Pb-free solders were investigated during high temperature aging at 200 °C. As-solidified microstructures, revealed by cross-polarized light microscopy, consist of relatively large Sn grains in both Sn–Ag and Sn–Cu solders. Upon aging at 200 °C, 2 h, Sn grains become smaller compared to the as-solidified ones. In addition, the microhardness of Sn–Ag solders increases after 200 °C, 2 h aging, while that of Sn–Cu solder decreases. Detailed observation of the coarsening and redistribution of intermetallic particles in each system further explains this response of mechanical properties during high temperature aging. To investigate the effect of aging temperature, solders were aged at a lower temperature, 150 °C for up to 1000 h and compared with aging at 200 °C. The microstructural changes during the high temperature aging were characterized in terms of Sn grain size, crystal orientation, and IMC growth kinetics, and were further correlated with the changes of their mechanical properties.