Black Oxide Residue on ENIG PCB Pads—What Causes It and How to Remove Efficiently?

A prototype batch of ENIG (Electroless Nickel Immersion Gold) PCBs exhibited black oxide residue on the pads, which appeared after soldering and was difficult to remove by scraping. The black film is likely a contamination or poor plating quality rather than actual gold vaporization, which would require extreme heat. The discoloration was present straight from the board house packaging and worsened with excessive soldering iron temperature. Underneath the black residue, copper was exposed, indicating the gold layer was compromised or absent. The issue is attributed to manufacturing defects or improper plating processes at the PCB fabricator. Due to time constraints, returning the boards was not an option, so manual removal by rubbing or using solvents was considered. Suggested cleaning methods include acetone or specialized cleaners like Chemtronics Electro-Wash. The consensus is that such contamination is unacceptable for production boards and should be addressed with the manufacturer for future batches.
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