Effects of Plating Conditions on the Microstructure of 80Sn-20Pb Electrodeposits from an Organic Sulfonate Bath

Abstract

Alloy deposits of 80Sn-20Pb were electroplated on a Cu-based lead frame alloy (PMC-102) from an organic sulphonate bath using direct or pulsed currents and the effects of the plating parameters on the microstructure of the electrodeposits were investigated. At a cathodic current density of 10 A dm(-2) the addition of a grain-refining additive to the bath increased the cathodic polarization for the deposition of 80Sn-20Pb to about 560 mV greater than that in the bath without the additive, thereby producing an extremely fine deposit. The grain morphology of the electrodeposits formed by pulsed currents was also extremely fine at low duty cycle and low frequency but gradually coarsened with increasing duty cycle and/or frequency, approaching that of the deposit formed by direct current. The electrodeposits of 80Sn-20Pb were identified as a mixture of Pb and beta-Sn crystals. While the Pb crystals were randomly oriented, the beta-Sn crystals exhibited {200} preferred orientation which gradually transformed to {220} with decreasing duty cycle and/or frequency of pulsed plating.