The pH effect on black spots in surface finish: Electroless nickel immersion gold

In order to understand the black spot generation after electroless nickel immersion gold (ENIG) plating, we investigated the pH effect with a combined approach of experiments and computer aided engineering (CAE). As the pH is increased in IG plating solution, the deprotonation of citric acid as chelating agent is enhanced to stabilize the solution by producing Ni-citrate complex ion. For the substitution reaction between nickel and gold, excess citrate ions (deprotonated citric acids) are adsorbed along nodal boundaries of Ni-P layer to decrease the surface reactivity. Since the low reactivity decreases the overall growth rate, the resulting homogeneous Au layer growth avoids the unfavorable galvanic cell corrosion to control black spot. Based on molecular orbital method and kinetic Monte Carlo calculation, our computational approach well explained the capability of citric acid as chelating agent and the Au growth rate along the nodal boundaries of Ni-P layer depending on the surface reactivity.