Fast scanning calorimetry (FSC) has a large scanning rate up to 10(5) 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 mu m, 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 beta-Sn dendrites and eutectics exists in the droplets. Fine beta-Sn dendrites were clearly observed in droplets of 300 mu m and 500 mu m. Eutectics containing Ag3Sn phase were also detected by SEM. However, the dendrites and eutectics were difficult to be distinguished in droplets of 25 mu m and 50 mu m, attributing to the higher cooling rate and larger undercooling. (c) 2013 Elsevier B.V. All rights reserved.