Failure analysis on bad wetting of ENIG surface finish pads

Electroless nickel/immersion gold (ENIG) is an effective surface finish, which guarantees good corrosion resistance, a long shelf life, a flat soldering surface for surface mount technology (SMT) and a good electrical probe surface for in-circuit-test. ENIG has become a popular surface finish for pads of printed circuit boards (PCB) and ball grid array packages (BGA). However, the disadvantages of ENIG surface finish pads cannot be neglected. Compared with other surface finishes, such as organic surface protection (OSP) and hot air solder leveling (HASL), ENIG is always more expensive. Except for the high cost, another significant disadvantage of ENIG surface finish pads is related to the so called “black pad”, which may lead to bad wetting, including nonwetting and dewetting, as well as brittle solder joints. In this paper, the failure analysis on bad wetting of ENIG surface finish pads related to “black pad” is revealed with the aid of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray photoelectric spectroscopy (XPS). Surface of bad wetting solder joints and nickel layer of bare PCB pads was analyzed with OM and SEM/EDS. The bad wetting area of the joints showed black through the inspection of OM. The result of SEM/EDS demonstrated that typical "black pad" morphology of nickel layer with "mud crack" appearance was found on the surface of both the bad wetting area of the solder joints and the nickel layer of bare PCB pads. Depth profiling analysis was performed on the bare PCB pad with XPS. The curve of phosphorus content showed a peak during the profiling process, which indicated a phosphorus rich layer in the gold/nickel coating. The phosphorus rich layer was resulted from the consumption and corrosion of nickel during the immersion gold plating process. Metallographic specimens of the bad wetting solder joint and the bare PCB pad were prepared and the cross sections were analyzed with SEM. Continuous IMC was formed at the interface between the PCB pad and solder. The thicknesses ofIMC layer of the solder joint at the pad side and at the component side were -0.3-0.7 J.1m and -1.8 J.1m, respectively, which are within a reasonable range. It indicates that the bad wetting was irrelevant to parameters during the soldering process. Spikes of the nickel layer were observed in the cross section of the bare PCB pad, which are in accordance to the "mud crack" morphology of black pad. The severe "black pad" defect degraded the solderability of the PCB pads, leading to the bad wetting of the solder joints eventually. Solderability test was performed on the bare PCB following the procedures of J-STD-003B, Test C1. Bad wetting and black pads were found after the solderability test as well, from which it can be inferred that the bad wetting was mainly related to the weak solderability of PCB pads and the "black pad" defect degraded the solderability of ENIG surface finish pads.