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A Study on Black Pad Phenomenon in Electroless Ni(P) Films
Keywords: Black Pad, UBM, ENIG
The “black pad” phenomenon known to occur during the electroless nickel immersion gold (ENIG) process has been a serious reliability problem in the microelectronics packaging industry for decades. The phenomenon was generally ascribed to hyper-corrosion of nickel-phosphorous (Ni-P) film during the IG process, however concrete evidences were rare due to difficulties associated with the sample reproducibility. In this study, it was found that the phosphorous content of the Ni-P film varies in the submicron scale. The nodular variation of the P content induces potential difference large enough to drive galvanic corrosion when the film is exposed to the electrolyte containing gold cyanide. Corrosion properties were investigated by measuring corrosion rate and corrosion potential of the Ni-P films with various P content. The corrosion potential of the films increased with P content. Therefore, the films having lower P content are relatively corroded and those having higher P content are protected. Subsequent corrosion couple experiments using Ni-P films with different P content demonstrated that preferential corrosion occurred at the low P side. The galvanic current between two films increased with the difference in the P content(ΔP), so that the propensity of forming black pad also got severe with the P content difference. The phosphorous content of the films was measured by EDS. The images of the surface of the corroded films were obtained by SEM, and the property of the cross-section of the films before and after corrosion was analyzed by FIB and TEM.
J.H. Kim, Student
Daejeon 305-701,
South Korea

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