Fast scanning calorimetric measurements and microstructure observation of rapid solidified Sn3.5Ag solder droplets

Fast scanning calorimetry (FSC) has a large scanning rate up to 105 K/s which makes it possible to simulate laser soldering. In this paper, the cooling rate and size dependence of undercooling for Sn3.5Ag droplets were studied. The undercooling increased slightly with increasing cooling rate. When the droplet size was smaller than 25 μm, undercooling increased dramatically. For investigating the solidification microstructure at high cooling rates, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were applied. In comparison with the bulk alloys, refined solidification structure instead of coarse β-Sn dendrites and eutectics exists in the droplets. Fine β-Sn dendrites were clearly observed in droplets of 300 μm and 500 μm. Eutectics containing Ag3Sn phase were also detected by SEM. However, the dendrites and eutectics were difficult to be distinguished in droplets of 25 μm and 50 μm, attributing to the higher cooling rate and larger undercooling.