Observation of the solidification microstructure of Sn3.5Ag droplets prepared by CDCA technique

The Sn3.5Ag droplets in various sizes have been prepared by the consumable-electrode direct current arc technique. The cooling rates of the droplets have been evaluated based on Newton’s cooling law and it shows that the cooling rates of the droplets increase dramatically from 6.84 × 10² to 2.52 × 10⁵ K/s as the droplet size decreases from 830 to 43 μm. The range of cooling rate is close to that of laser soldering (up to 10⁴ K/s). It has been found that the β-Sn dendrites are refined as the cooling rate increase, and when the cooling rate is 1.30 × 10⁵ K/s, which corresponding to the size scale smaller than 60 μm, the dendrites nearly disappear in the droplets. In addition, it has been observed that the high cooling rate could successfully avoid the precipitation of plate-like Ag₃Sn and promote the formation of nanoparticles which are desirable in practical application. These nanoparticles uniformly distribute in the Sn matrix and the average size of nanoparticles in different droplets is 46.4 nm (in 380–830 μm droplets), 57.2 nm (in 150–250 μm droplets), 60.6 nm (in 96–150 μm droplets), 63.2 nm (in 43–60 μm droplets) and 65.3 nm (in droplets of <43 μm), respectively. According to the dispersion-strengthening effect, the existence of nanoparticles would be beneficial to improve the mechanical property of the Sn3.5Ag solder alloy.