Abstract: Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of electroless nickel immersion gold processing printed circuit boards (PCB-ENIG) in a simulated electrolyte solution (0.1 mol. L^-1 NaHSO₃), and the morphology, composition of corrosion products as well as the failure mechanism of the plating layer were analyzed with the aid of stereo microscopy and scanning electron microscopy (SEM) combined with energy dispersive spectrometry (EDS). It is found that the corrosion resistance of PCB-ENIG in NaHSO₃ solution is relative poor. Partial discolor appears in PCB-ENIG just immersing for 12 h, along with micro-crack generation. The electrolyte can directly erode the Cu substrate through micro-cracks, forming dendritic crystalline products around these micro-cracks, whose main composition should be Cu₂S. Continuous generation of these corrosion products results in large transverse shear stress between the Ni intermediate layer and Cu substrate, and eventually bubbling and shedding of the Ni plating layer occur.