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).