Reworking soldering on gold-plated boards is not simply removing old components and soldering new ones; it is a process that substantially consumes the gold layer on the pad surface. Especially after multiple reworks, the gold layer thickness may fall below the critical value of 0.05 μm. At this point, the underlying nickel layer begins to participate in the melting reaction. Flux residues and nickel oxides act together to significantly weaken wetting force, resulting in insufficient solder climb, increased void rate, and failed thrust tests during soldering on gold-plated boards.

Iron temperature and contact time must be jointly controlled

Our measurements show that when the soldering tip temperature is set to 320°C and single-point contact exceeds 3.5 seconds, the gold layer loss on ENIG board pads reaches 0.018 μm per rework. If the temperature rises to 350°C, the loss jumps to 0.032 μm per rework. The JJY JL-802B constant-temperature soldering iron, operated in the 300°C ± 5°C range with a 0.8-second pulse heating mode, can reduce single-time loss to within 0.006 μm. This requires that the tip use an iron-plated copper alloy layer and be wiped with a wet sodium alginate sponge after every 20 solder joints; otherwise, oxide film buildup will reduce actual heat transfer efficiency by more than 15%.

Pad cleaning must not rely on highly corrosive solvents

After rework, repeated wiping with chloride-containing fast-drying cleaners not only accelerates electrochemical corrosion of the gold layer but also forms microporous channels at the nickel diffusion layer interface. Our XRF inspection found that the gold content in this region decreases 2.3 times faster than in exposed areas. The correct procedure is to first use a lint-free cloth dampened with 99.8% ethanol to gently press and absorb surface residue, then use a nitrogen gun at 0.3 MPa pressure to blow obliquely along the pad edge corners. This prevents ethanol residue from concentrating and evaporating at the gold-nickel interface, which would otherwise precipitate alkaline salts.

Gold replenishment process is only suitable for single-point local repair

Full-board electroplating gold replenishment will damage the density of the phosphorus-nickel layer in the original ENIG structure, increasing the risk of brittle solder joint fracture by 40% during subsequent SMT reflow. The selective electroless gold plating kit provided by JJY is only allowed for point-coating repair of isolated pads with diameter ≤ 0.6 mm. The pH value of the gold plating solution must be stably maintained between 4.2 and 4.5. Below 4.0, excessive etching of the nickel substrate will occur; above 4.7, the deposited gold will adhere as a weak powder layer and easily peel off after 260°C reflow.

Visual inspection must be cross-verified with a 40x cold-light magnifier

Relying solely on the naked eye to judge whether a pad is darkened or grayed is highly prone to misjudgment. When the gold layer is thinned to 0.08 μm, surface reflectivity still remains above 92%. Only when a continuous nickel-colored halo appears at the pad edge under a 40x cold light source, or when solder climb height is less than 75% of the pad diameter, can gold layer failure be determined. At this point, the PNL must be immediately taken out of service and marked as a rework-limit board. Gold-plated board solder joints that have undergone more than 3 cumulative reworks must not be used in automotive-grade products.