Investigation of surface defects of electroless Ni plating by solder resist dissolution on the ENIG process

Abstract

Defects, such as a black pads and pinholes, are generated on the Ni/Au interface after the electroless nickel immersion gold (ENIG) process. The contamination of plating solutions, including dissolution of the solder resist (SR), side reaction products, and impurities (Cu, Ni, drag-in), can be a cause of the black pad phenomenon. In this study, electroless Ni plating was investigated to determine the correlation between SR dissolution and black pad generation. Three different SR components, monomer, hardener, and photoinitiator were used to examine the effects of the properties of the coating layer. The surface morphology, plating rate, and phosphorus (P) content of the electroless Ni layer with SR additives were analyzed. The black pad mechanism by SR dissolution was examined by electrochemical analysis including the open circuit voltage (OCV) and cyclic voltammetry (CV) in a three-electrode cell. When a few ppm of SR was added, the OCV increased compared to the pure solution. CV revealed a change in hysteresis loop in the oxidation region upon the addition of SR. The contamination in the solution promotes the oxidation reaction rather than the reduction reaction. Therefore, the content of P in the contaminated solution was higher than that of the pure solution, and defects, such as black pad and pinholes, were observed.