Root Cause of Black Pad Failure of Solder Joints with Electroless Nickel/Immersion Gold Plating

Publications on the so called black pad defect, seen in electroless nickel/immersion gold (ENIG) surface finishes, often cite high phosphorous (P) content in the nickel (Ni) plating as a key factor in the defect. Therefore, one proposed solution is to decrease the P content in the electroless Ni plating. In a contrary approach, some researchers propose a high P content to avoid the black pad defect. In the present work, the solder reaction with ENIG plating and the resulting interfacial structures were studied. Focused ion beam (FIB) was used to polish the cross sections to reveal details of the microstructure of the ENIG plated pad with and without soldering. High speed pull testing of solder joints was performed to expose the pad surface. Results of SEM/EDX analysis of the cross sections and fractured pad surfaces support the suggestion in the literature that black pad is the result of galvanic hyper-corrosion of the plated electroless Ni by the gold (Au) plating bath. High P content in the fractured surface on the pad side is not the signature of black pad. New criteria are proposed for diagnosing black pad in ENIG