Black Pad Susceptibility of the Electroless Ni Films on the Cu UBM

The occurrence of black pad in the electroless Ni film during the immersion gold process is related to the surface morphology of the Ni(P) film. A nonuniform distribution of the nodule size and curvature is the crucial factor. Large nodules with small surface curvatures had higher P concentration and did not corrode, while small nodules with large surface curvatures had lower P concentration and corroded. Experiments using different types of Cu substrates suggest that the Ni(P) film black pad susceptibility increased with the defect density and/or the residual stress in the underlying substrate. Annealing the Cu substrate before the electroless Ni plating greatly reduced the black pad formation.