Composition and Microstructure Control of Tin-Bismuth Alloys in the Pulse Plating Process

Pulse plating of Sn-Bi alloys were investigated from a bath containing three compounds, including citric acid (CA), ethylenediaminetetraacetic acid (EDTA), and polyethylene glycol (PEG400) at p H = 1. Based on the linear sweep voltammetric (LSV) analysis, the large gap in the onset deposition potential between Sn and Bi metals led to displacement deposition at the interface between metallic Sn atoms and Bi 3+ ions in the time-off (t off ) period. Consequently, the pulse-plating parameters (i.e., duty percentage in the pulse cycle and pulse frequency) were adjusted to affect the double-layer charging effect and the mass transport process to control the composition and morphology of alloys. This pulse plating method provides a reliable route to obtain uniform Sn-Bi alloys of high Sn contents (e.g., 80Sn-20Bi and 90Sn-10Bi) and the eutectic 42Sn-58Bi deposit. Various Sn-Bi deposits were characterized by the scanning electron microscopic (SEM), energy-dispersive spectroscopic (EDS), and X-ray diffraction (XRD) analyses.