• Chemical residues and solution contamination

The ENIG process involves multiple chemical plating processes (such as micro-etching, activation, chemical nickel plating, and immersion gold). If the chemical residues of the previous process (such as micro-etching solution and activation solution) are not thoroughly cleaned, the subsequent plating solution will be contaminated. The contaminated plating solution may cause uneven growth of the nickel layer or introduce impurities, causing local corrosion in subsequent use and forming black areas. In addition, improper control of the pH value, temperature or gold ion concentration of the gold plating solution may also lead to poor adhesion of the gold layer or exposure of the nickel layer, accelerating the oxidation of nickel.

• Nickel layer corrosion (main cause)

The black disk phenomenon of ENIG usually originates from the corrosion of the nickel layer. The root cause is the uneven distribution of phosphorus content in the nickel plating layer or defects in the plating structure. If the phosphorus content is too low (<7%) during the nickel plating process, the nickel layer is prone to form an easily oxidized crystalline structure, which reacts with oxygen and moisture in the air to form black nickel oxide (NiO/Ni(OH)₂) during subsequent storage or use. In addition, if the porosity of the nickel plating layer is too high (such as the plating layer is too thin or the crystal is rough), the internal copper layer will be exposed, and the copper and nickel will form a galvanic effect (electrochemical corrosion), further accelerating the blackening of the nickel layer.

• Immersion gold process problems

If the thickness of the immersion gold layer is insufficient (usually 0.05-0.1μm is required) or the gold layer is discontinuous, the nickel layer will be directly exposed to the environment, accelerating oxidation. If the pH value of the gold plating solution is too high (>5.0) or the temperature is too high (>90℃), it may cause partial dissolution of the nickel layer or decrease the adhesion of the gold layer, making the nickel layer more susceptible to corrosion due to contact with air.

• Process control and environmental factors

1. Incomplete cleaning: If the water washing after ENIG is not sufficient, the residual chemical agents (such as the complexing agent in the gold plating solution) may continue to corrode the nickel layer.
2. Insufficient drying: If the PCB is not completely dried after cleaning (such as residual moisture), the oxidation of the nickel layer will be accelerated.
3. Improper storage environment: If the PCB after ENIG is exposed to a high temperature and high humidity environment (such as humidity>60% RH or temperature>30℃) for a long time, the reaction between the nickel layer and oxygen and moisture will intensify, resulting in the deterioration of the black disk phenomenon.

• Material and design issues

1. Poor substrate compatibility: The surface treatment of some special substrates (such as high-frequency boards) is incompatible with the ENIG plating solution, which may result in poor bonding of the nickel plating layer, easy to fall off or corrode.
2. Pad design defects: tiny burrs or excessive roughness on the edge of the pad may damage the integrity of the plating layer and expose the nickel layer locally.

• Strengthen cleaning and water washing: Use multi-stage countercurrent rinsing (such as 3-5 levels of pure water rinsing) to ensure that chemical residues from previous processes such as micro-etching and activation are completely removed; add final pure water rinsing and hot air drying after ENIG (temperature 80-100℃, time ≥5 minutes).
• Precisely control nickel plating parameters: adjust the phosphorus content of the nickel plating solution (controlled at 8-12%), pH value (4.5-5.0) and temperature (85-90℃) to ensure that the nickel layer is dense and has low porosity; regularly analyze the composition of the plating solution (such as nickel ions, sodium hypophosphite concentration) to avoid the introduction of impurities.
• Optimize the immersion gold process: control the pH value (4.5-5.0), temperature (85-90℃) and gold ion concentration (0.1-0.3g/L) of the gold plating solution to ensure that the gold layer thickness is ≥0.05μm and the coverage is complete; replace the gold plating solution regularly to avoid the aging solution causing the gold layer adhesion to decrease.

• Select high-purity copper foil (purity ≥99.9%) and low-stress substrate to ensure the bonding strength of the nickel plating layer;
• Verify the compatibility of the substrate with the ENIG plating solution (such as through the "plating bonding strength test") to avoid the plating layer from falling off due to material conflict.

• The ENIG-posted PCB needs to be temporarily stored in an environment of ≤40% RH and 22-26℃ to avoid high temperature and high humidity accelerating the oxidation of the nickel layer;
• For ENIG PCBs stored for a long time, it is recommended to use vacuum packaging or nitrogen protection to isolate oxygen and moisture.

• Online AOI detection: Identify color anomalies on the pad surface (such as black spots or gray areas), and confirm whether it is nickel layer corrosion by combining X-ray or slice analysis;
• Regular process verification: Conduct "nickel layer corrosion resistance test" (such as neutral salt spray test for 48 hours) before each batch of production to monitor the stability of the coating;
• Record environmental parameters (humidity, temperature) and process parameters (plating solution composition, temperature), and optimize the process window through data analysis.

The black pad phenomenon in the ENIG process is essentially surface oxidation caused by nickel layer corrosion or gold layer failure, which needs to be prevented by optimizing plating solution control, strengthening cleaning and drying, and strict environmental management. The core measures include precise control of nickel/gold plating parameters, improving the density of the plating layer, and avoiding exposure of the nickel layer to the environment, thereby ensuring welding reliability and long-term electrical performance.