The Uniformity of Initially Deposited Layers in Electro-Tin-Plating and Its Effect on Corrosion Resistance

Abstract

The uniformity of initially deposited layers will affect the corrosion resistance of electro-tin-plate, especially its iron solution value (ISV). The uniformity of initially deposited layers, which can be evaluated by ADC test (Anodic Dissolution Current Test) developed in our laboratory, will vary with change in plating current density and the chemical composition of the plating bath. It is most affected by the concentration of leveling agent. The initially deposited layers become more uniform with increase in the concentration of leveling agent. The result seems to be different from the usual concept of the optimum concentration of leveling agent in the plating bath. However, the present result is effective only in the initial stage of electrodeposition which ends when 1. 2g/m² tin is deposited on the substrate surface. In the following second stage in electroplating, high concentration of the leveling agent gives adverse effect on the corrosion resistance. On the basis of the result above, two-step electro-tin-plating processs has been introduced, which has independently controlled initial and the second plating processes. It is shown that the electro-tin-plate made by this two step process has high corrosion resistance.