Chemical Degradation
An oxidation phenomenon occurs during electroless nickel immersion gold plating when the gold bath aggressively attacks the underlying nickel layer. The resulting reaction leads to black pad nickel corrosion, which depletes the nickel and leaves behind a dark, hyper-phosphorous residue on the surface of the solder pad. Oxidation occurs along the nickel grain boundaries, forming deep micro-fissures that degrade the mechanical strength of the pad.
These structural compromises remain hidden beneath the gold deposit until subsequent assembly steps or stress testing expose the defect. A high ratio of gold to nickel ions in the plating chemistry accelerates this galvanic corrosion, especially when the immersion gold process is run at a high temperature or for an extended period.
Microscopic Detection
Inspection of the finished printed circuit board requires cross-sectional analysis or high-power scanning electron microscopy to identify the characteristic micro-fissures in the nickel boundary. While optical inspection can identify the dark pad appearance after gold stripping, it cannot determine the depth of the boundary degradation. Measuring the phosphorus content of the nickel layer through energy dispersive spectroscopy provides an additional quantitative measure of the corrosion severity.
If the phosphorus level exceeds fifteen percent in the affected boundary, the risk of solder failure rises.
Joint Failure
The primary consequence of the degraded boundary is the formation of brittle solder joints that fail under low mechanical stress. During reflow, the solder cannot form a continuous intermetallic compound with the corroded nickel surface. Solder joints fail along the interface when the assembly undergoes thermal cycling or physical shock.
This brittle fracture results in intermittent or complete electrical opens in the field.
