A Study on the Mechanism of Black Pad Formation during Electroless Nickel Immersion Gold process

Abstract

The “Black Pad” phenomenon referring to blackening of electrolessly plated nickel-phosphorous (Ni(P)) films after the immersion gold (IG) process was reproduced using pure chemicals for the first time. Chemical composition of the electrolyte most suitable for the black pad reproduction was proposed, and it was found that submicron scale nodular variation of the P content of the Ni(P) film induced potential differences large enough to drive galvanic corrosion when exposed to the electrolyte; gold cyanide solution in this case. Instead of conventional potential measurement techniques, corrosion couple experiments using Ni(P) films with different P content were performed to substantiate preferential corrosion, which occurred at the lower P side with the propensity of forming black pad getting more severe with the P content difference (ΔP) between films. Subsequent AFM analysis showed that variations of nodular shapes and curvatures among nodules led to submicron scale variations of the P content in the film, which could promote galvanic corrosion.