The “Black Pad phenomenon” has been rarely occurred after ENIG(Electroless Nickel Immersion Gold) process for surface finish of PCBs(Printed Circuit Boards) or wire bonding pads whose conducting layers are typically made of thin copper foils. In this study, the effect of Cu on black Pad formation was examined by the experiment using Al UBM as well as Cu UBM. Ni(P) films on Cu UBM were mainly corroded at the nodule boundaries, on the other hand, Ni(P) films on Al UBM had pitting corrosion in the middle of the nodules after IG process. The properties of both films were studied by further analysis with SEM, TEM, and EDS. Ni(P) films on Cu UBM had various P content across nodules and nodule boundaries, however, Ni(P) films on Al UBM had the nodules with uniform phosphorous content. In addition, in case of Ni(P) films on Cu UBM, it was found that nodules with different P content exhibited different nodular shapes. A nodule with higher P content was comparatively large in appearance, large nodule diameter and small curvature. It was composed of many minor nodules with the same curvature, which comprised a major nodule surface. On the other hand, the lower P nodules consisted of separate small nodules with larger curvatures. They tend to be corroded preferentially because of the size effect and the presence of big crevices. The relation between the P content and the nodule curvature of the Ni(P) film could be explained by the simple equation.
Furthermore, it was ascertain that Al and Cu atoms from UBM affected the corrosion property of Ni-P films by ENIG processing on Cu/Al double layered UBM. In addition, the effects of the other properties of the UBMs were studied using various copper based UBMs which included copper blocks, copper foils and sputtered coppers. Topography of the UBMs influenced upon the morphology of Ni-P films and texture of UBMs were impacted on the propensity of corrosion of Ni-P films. Furthermore, ENIG process on aged copper foils proved that the stress in the substrates affected the propensity of the corrosion of Ni-P films.
In addition, ternary Ni-W-P alloys were proposed as a solution to work out black pad phenomenon. Ternary Ni-W-P films were investigated using corrosion potential measurement and corrosion couple experiments. It was revealed that Ni-4W-5P films had better resistance to corrosion than Ni-5P films from both results.
