Localised aqueous corrosion of electroless nickel immersion gold-coated copper

Electroless nickel (Ni) immersion gold (Au), commonly referred to by the acronym ENIG, is the most common protective coating applied on the exposed copper (Cu) traces of printed circuit boards (PCBs). In this work, we elucidate the local corrosion mechanism of the ENIG-Cu system by applying microscopic, surface analysis and electrochemical techniques with high spatial resolution to provide a comprehensive understanding of the complex local corrosion mechanism of the ENIG-Cu system. The corrosion initiation is highly localised and associated with pores or micro-defects in the Au layer. The corrosion initiates by the dissolution of the underlying Ni layer, being less noble than Au. The dissolution propagates in lateral and perpendicular directions relative to the surface in an elliptical fashion. With time, the direction of corrosion propagation changes to a predominantly lateral attack of the Ni layer. The corrosion process is governed by the cathode/anode ratio of the Au/Ni galvanic couple.