Electroless nickel immersion gold (ENIG) is one of the most widely used surface finishes in PCB manufacturing due to its excellent solderability, corrosion resistance, and flatness.
However, one of the most troublesome process defects is unintended gold deposition on the walls of non-plated-through holes (NPTHs).
This defect not only compromises product appearance but may also indicate that one or more manufacturing processes have deviated from their specified control limits.
Because gold cannot deposit directly onto non-conductive substrates, gold plating on NPTH hole walls is almost always the result of process abnormalities that allow catalytic materials or residual conductive layers to remain on the hole surface.
Therefore, troubleshooting should not focus solely on the ENIG process itself.
Instead, manufacturers must systematically analyze the entire production flow—from drilling and desmearing to pattern transfer, etching, activation, and electroless nickel plating—to identify the specific process step and parameter that has gone out of control.
The following sections examine the most common process anomalies that can lead to gold deposition on NPTH hole walls and provide practical corrective actions to eliminate the root causes and prevent recurrence.
Which process anomalies cause gold plating on NPTH holes?
Drilling process (physical damage):
Drill bit life exceeds the specified limit (beyond the rated number of cycles) or the retraction speed is too fast, resulting in severe spiral grooves or glass fiber strands on the hole walls.
This physical roughness acts as a “mechanical lock” for chemical residue, making it impossible to completely remove during subsequent cleaning.
Resin Removal Process (Chemical Roughening):
Excessively high temperature or concentration in the potassium permanganate bath causes excessive etching of the resin, resulting in “honeycomb-like” micro-pores on the hole walls.
These micro-pores act like a sponge, adsorbing the subsequent palladium catalyst, which is difficult to rinse out with water.
Pattern Transfer (Dry Film Sealing Failure)
Insufficient exposure energy or inadequate dry film lamination pressure causes the dry film at the NPTH via openings to “peel up” or “rupture” during development.
Chemical solution seeps into the via, directly causing subsequent etching and gold plating processes to go out of control.
Etching Process (Copper Residue)
Low etching solution temperature, reduced spray pressure, or excessive conveyor speed can prevent the copper foil inside the holes from being completely etched away.
The residual copper particles act as a “natural seed layer” for the chemical nickel-gold plating process.
Pre-ENIG Micro-Etching
Insufficient micro-etching due to excessively low sulfuric acid/hydrogen peroxide concentrations in the micro-etching bath or excessively high copper ion loading (bath aging), preventing the effective removal of palladium colloidal particles adsorbed on the resin lining of the via walls, resulting in their retention.
Abnormal Activation
Abnormal palladium concentration leads to an abnormally high concentration of palladium ions in the bath;
Excessively high temperature in the activation bath causes palladium colloids to become overly active, resulting in excessive adsorption onto non-metallic substrates;
Clogged nozzles in the post-rinse stage, insufficient rinse pressure, and inadequate physical rinsing force.
In addition to abnormal palladium levels, low accelerator concentration, bath temperature below the set value, or insufficient dwell time prevent the effective stripping of the palladium layer adsorbed on the resin within the pores, causing palladium to be carried directly into the nickel bath and trigger deposition.
Nickel plating process
During the nickel plating process, an excessively high pH or temperature (＞85℃) in the nickel bath leads to an excessively fast chemical nickel deposition rate.
Even with only trace amounts of residual palladium, the excessively fast deposition rate can instantly amplify this “defect” into a “gold layer” visible to the naked eye.
How to solve the gold adhesion problem in NPTH?
To resolve gold adhesion on NPTH pore walls, it is essential to address both upstream process control and ENIG line parameters.
Regarding the core cause—“palladium residue”—the key lies not in “washing,” but in “reducing palladium adhesion” and “thoroughly stripping the palladium before gold plating.”
Source Control—Addressing the “Breeding Ground for Palladium Residue”
⚠️ Optimize drilling parameters:
Reduce the drill bit’s feed rate and rotational speed; use new drill bits to prevent “glass fiber stringing” or excessive surface roughness caused by high temperatures on the hole walls.
Surface roughness must be controlled at ≤25μm.
⚠️ High-Pressure Water Rinsing for Deburring:
Add a high-pressure water rinsing step after drilling to thoroughly remove copper burrs and dust from the hole openings and inner walls, thereby reducing physical contamination sites.
Dry Film Sealing — Solving “Chemical Penetration”
During the pattern transfer process, completely seal the NPTH holes with dry film. This physically prevents subsequent etching solutions and ENIG chemicals from entering the holes, serving as the most fundamental preventive measure.
Ensure sufficient exposure energy to prevent the film from breaking at the hole openings after development.
Thorough Etching — Addressing “Catalytic Growth Points”
Ensure smooth exchange of the etching solution by increasing spray pressure or using horizontal oscillating etching.
After etching, inspect the interior of the holes with a magnifying glass to confirm there is no residual copper foil.
Pre-ENIG Treatment: “Intensive Palladium Removal” — Addressing “Residual Catalyst”
⚠️ Intensify Micro-Etching:
Appropriately increase the micro-etching rate to utilize the pitting effect generated when the copper surface is etched, thereby removing palladium particles adsorbed on the resin coating of the via walls.
✅ Strict Palladium Removal:
Regularly analyze the concentration of the solution in the palladium removal tank and ensure the temperature is normal. If the tank solution has aged, replace it immediately to prevent the palladium remover from becoming ineffective.
❎ Add Isolation Rinses:
After the palladium removal bath, add hot water rinses and ultrasonic rinses to use physical agitation to help strip away residual chemicals.
Conclusion
Gold deposition on NPTH hole walls is not an isolated ENIG defect but the cumulative result of process deviations occurring throughout PCB manufacturing.
Mechanical damage during drilling, excessive resin roughening, incomplete dry-film sealing, copper residue after etching, insufficient palladium removal, abnormal activation conditions, and overly aggressive electroless nickel plating can all create the conditions necessary for unintended nickel and gold deposition.
Effective prevention requires a process-wide approach rather than relying on adjustments to a single chemical bath.
By maintaining tight control over drilling quality, desmear parameters, dry-film integrity, etching efficiency, palladium activation and removal, and ENIG operating conditions, manufacturers can significantly reduce the risk of gold adhesion on NPTH hole walls.
When this defect occurs, root-cause analysis should combine visual inspection, metallographic cross-section analysis, SEM/EDS characterization, and process data review to accurately identify the origin of the catalytic layer.
Only by eliminating the underlying process abnormality—not merely removing the visible defect—can long-term process stability and product reliability be achieved.
