Corrosion of the joining metallurgy in multilayer substrates during processing

Difficulties encountered with respect to solder wettability and the ability of gold wire to bond to nickel-and-gold plated, thick-film, molybdenum bonding pads were traced to stains observed on the pads after the plating processes. These strains were, in turn, found to be due to galvanic corrosion occurring during processing. The mechanism of stain formation is explained in terms of the lowered ability of nickel to passivate itself upon alloying with molybdenum during the diffusion step following nickel plating, the pitting nature of the corrosion cell, and the insolubility of the corrosion product