Fabrication of Sn-3.5Ag eutectic alloy powder by annealing sub-micrometer Sn@Ag powder prepared by citric acid-assisted Ag immersion plating

A Sn-3.5Ag eutectic alloy powder has been developed by chemically synthesizing sub-micrometer Sn@Ag powder at room temperature. This synthesis was achieved by first obtaining a submicrometer Sn powder for the core using a modified variant of the polyol method, and then coating this with a uniformly thin and continuous Ag layer through immersion plating in 5.20 mM citric acid. The citric acid was found to play multiple roles in the Ag coating process, acting as a chelating agent, a reducing agent and a stabilizer to ensure coating uniformity; and as such, the amount used has an immense influence on the coating quality of the Ag shells. It was later verified by transmission electron microscopy and X-ray diffraction analysis that the coated Ag layer transfers to the Sn core via diffusion to form an Ag₃Sn phase at room temperature. Differential scanning calorimetry also revealed that the synthesized Sn@Ag powder is nearly transformed into Sn-3.5Ag eutectic alloy powder upon annealing three times at a temperature of up to 250 °C, as evidenced by a single melting peak at 220. 5 °C. It was inferred from this that Sn- l3.5Ag eutectic alloy powder can be successfully prepared through the synthesis of core Sn powders by a modified polyol method, immersion plating using citric acid, and annealing, in that order.