Effect of low concentrations of Pb on Sn electrodeposition in methyl sulphonic acid solutions

The influence of Pb²⁺ on the initial stages of Sn electrodeposition onto copper from methyl sulphonic acid solutions in the absence of organic additives has been investigated. Particular attention is placed on the effect of Pb²⁺ at concentrations much lower than previously reported and where no lead is detected in the resulting deposit. Linear sweep scans and i-t transients show that Pb²⁺ catalyzes Sn plating at Pb²⁺:Sn²⁺ molar concentration ratios between about 1:1000 and 20:1000. This effect can be significant, with as much as a 30% increase in the amount of tin deposited over that obtained in the absence of Pb²⁺ when the concentration ratio is 2:1000 or 5:1000. At concentration ratios above 20:1000, the effect disappears and, in fact, Pb²⁺ has an inhibitory effect on Sn²⁺ reduction. Under conditions where Pb²⁺ enhances Sn electrodeposition, it also begins to affect electrocrystallization by promoting faster nucleation and a more 2-dimensional coating that more rapidly covers the substrate. This trend continues as the Pb²⁺ concentration is further increased and the catalytic effect disappears.