Effect of pulse plating on composition of Sn–Pb coatings deposited in fluoroborate solutions

Galvanostatic pulsed current (PC) and pulse reverse (PR) plating of Sn–Pb coatings onto copper rotating disks from fluoroborate solutions has been conducted to obtain alloy compositions otherwise not achievable by DC plating. PC plating investigated over a wide range of pulse frequencies (20 Hz–200 kHz) and duty cycles produces coatings with compositions that differ only slightly from those obtained by DC plating at the same current density. On the other hand, the use of PR plating can be dramatic, producing Pb contents both well above and below that achievable by DC plating. Varying the frequency from 20 Hz to 200 kHz with the duty cycle and average current density fixed at 80% and 3.78 A dm^–2 yields compositions between 2.5 and 28 wt.% Pb. The Pb:Sn ratio in the deposit is always lower than that in the plating bath when a PR frequency of 20 Hz is imposed, but generally exceeds it at a frequency of 20 kHz. Alloy composition appears to be closely related to the working electrode potential reached during the anodic pulse. A higher frequency leads to less positive potentials during the anodic pulse and shorter anodic pulse times, which may enhance Sn dissolution and enrich the coating in Pb.