Abstract
The leadframes for power transistors are generally of copper alloy strip plated with nickel and silver, and copper strike plating is usually applied to the nickel surface to improve the adhesion of the silver on nickel after thermal treatment.
Accordingly, the mechanism by which copper strike plating increased interface adhesion was investigated by Auger electron spectroscopy.
The results can be summarized as follows:
(1) The poor adhesion of silver plating on nickel after thermal treatment in air was due to the penetration of oxygen into the silver layer and to the oxidation of the nickel surface.
(2) The copper in the strike plated layer diffused into the silver plating after thermal treatment above a certain temperature. The diffused copper fixed the permeated oxygen at a position close to the silver surface, thereby inhibiting the oxidation of the nickel surface and improving the adhesion of the silver.
(3) The copper in the strike plated layer also diffused into the nickel plating after thermal treatment, and the diffused copper is thought to act as an adhesive between the silver and nickel layers.
