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It was found that bright tin deposits were obtained by addition of polyoxyethyleneglycol (PEG) and formaldehyde to pyrophosphate baths containing SnSO4 and K4P2O7. The optimum bath composition and operating condition for bright tin plating were SnSO4 0.2M, K4P2O70.5M, PEG (average molecular weight 3000) 1.0g/L, formaldehyde 0.6mL/L, pH7-9, bath temperature 50°C, and current density 10-20mA/cm2. Cathode current efficiency was over 80% at current densities between 5 and 20mA/cm2, and decreased with increasing concentrations of formaldehyde and K4P2O7. The addition of PEG to an additive-free bath shifted the cathodic polarization curve to a less-noble potential and yielded fine-grained deposits. The addition of formaldehyde to baths containing PEG shifted the polarization curve to a more-noble potential and yielded bright tin deposits. It was concluded that in the electrodeposition of tin from pyrophosphate baths, formaldehyde results in the suppression of the adsorption of PEG on the tin surface and contributes to the brighting of tin deposits only in the presence of PEG.
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