Electrodeposition of Sn–Bi lead-free solders: Effects of complex agents on the composition, adhesion, and dendrite formation

The influences of complex agents, including citric acid, ethylenediaminetetraacetic acid (EDTA), and polyethylene glycol (PEG) on the electrochemical co-deposition of Sn–Bi alloys were systematically investigated through the linear sweeps voltammetric (LSV) analyses. The onset deposition potential of Bi ions is obviously shifted to a negative value, close to that of Sn ions by the simultaneous addition of the above three compounds into the plating solution. Based on the LSV results, deposits plated from the typical solutions with the Sn 4+/Bi 3+ ratio of 1, pH of 6.0, and various combinations of complex agents were characterized by scanning electron microscopic (SEM), energy-dispersive spectroscopic (EDS), and X-ray diffraction (XRD) analyses. The 30Sn–70Bi alloy (in wt%) is successfully plated from the typical plating bath containing 0.4 M citric acid, 1.0 M EDTA, and 0.2 M PEG, suggesting the possibility in plating the Sn–Bi alloys with their composition around the eutectic point (42Sn–58Bi). The adhesion of deposits and the formation of dendrites are respectively improved and inhibited by the synergistic effects of these three complex agents. Contrary to this combination, nearly pure Bi deposits are obtained from the typical plating solutions containing complex agents in the other combinations.