It was found that bright tin-lead alloy deposits were obtained by the addition of polyoxyethyleneglycol (average molecular weight 3000) 1.0g/L and formaldehyde 0.6mL/L to pyrophosphate baths containing SnSO4 0.145M, Pb (CH3COO)2 0.055M and K4P2O7 0.5M. The further addition of EDTA to the bath suppressed the preferential deposition of lead and yielded fine-grained deposits at current densities of 10mA/cm2 or less. The optimum EDTA-to-lead concentration ratio for bright tin-lead alloy plating was from 1 to 1.5. The lead content of the deposits increased and the cathode current efficiency decreased with increasing concentrations of EDTA and K4P2O7 in the baths. The lead content of the deposits was virtually the same as that in the baths at current densities from 5 to 15mA/cm2. The cathodic polarization curve for lead deposition was more noble than that for tin without the EDTA and other additives, but shifted to a less-noble potential than that for tin deposition with their addition. It was concluded that the presence of EDTA suppresses the preferential deposition of lead.
