FAQ
TL;DR: On ENIG pads, gold should be 1.6–4.0 µin; “The gold thickness shall be 1.6 to 4.0 µin.” If you see black residue, it’s often nickel-corrosion/contamination; clean only for emergency builds and otherwise reject the lot. [I‑Connect007, 2023]
Why it matters: This FAQ helps assemblers and engineers diagnose black residue on ENIG pads, decide when to rework vs. RMA, and ship prototypes fast without killing reliability.
Quick Facts
- IPC‑4552A targets Au 1.6–4.0 µin and Ni 3–6 µm; it also adds a corrosion evaluation chart to control black-pad risk. [I‑Connect007, 2023]
- Black pad = nickel corrosion during immersion gold; look for dark pads and “mud cracks,” and don’t try to “cure” it. [Circuits Assembly, 2017]
- Field fix under deadline: wipe with acetone or Chemtronics Electro‑Wash, then light mechanical rub; proceed with caution. [Elektroda, Joe Wolin, post #21659045]
- Overheating can crack nickel and worsen non-wetting; control dwell time and iron temperature during rework. [Circuits Assembly, 2024]
- If boards arrive black straight from packaging, treat as a fabrication defect and escalate immediately. [Elektroda, Bob Casiano, post #21659049]
What actually causes the black residue on ENIG pads?
Most cases stem from “black pad,” a corrosion of the nickel during the immersion‑gold step. It forms a brittle Ni3P layer that inhibits intermetallic formation, leading to poor wetting and weak joints. Visible signs include dark pads and fine “mud cracks.” You cannot fix true black pad; the correct action is to reject the boards. [Circuits Assembly, 2017]
Is the gold really vaporizing when I solder?
No. At typical hand‑solder temps, gold does not vaporize. Excessive heat and aggressive flux can darken residues and expose copper if the finish is thin or damaged. In the thread, the assembler corrected an overheated iron and observed copper beneath the black areas, indicating finish loss rather than gold vapor. [Elektroda, Bob Casiano, post #21659051]
How do I quickly clean black contamination to finish a prototype?
Three-step emergency method: 1) Wipe pads with acetone or Chemtronics Electro‑Wash. 2) Lightly scrub with a fiberglass pen or lint‑free swab until bright. 3) Flux and retin with fresh solder, then assemble. This is a stopgap for deadlines, not a cure for black pad. [Elektroda, Joe Wolin, post #21659045]
Should I return ENIG boards that show black film out of the bag?
Yes. Boards arriving with blackened pads are unacceptable. As one expert advised, “Return the boards … or get your money back.” This indicates poor plating or contamination, and assembly risks latent failures. Push the fabricator for re-make under IPC‑4552 controls. [Elektroda, Olin Lathrop, post #21659050]
What is ENIG in simple terms?
ENIG is electroless nickel covered by a thin immersion gold layer. Gold protects nickel from oxidation; nickel provides solderability and acts as a diffusion barrier. ENIG offers flatness for fine pitch, long shelf life, and good electrical performance, but can suffer black pad if the process is miscontrolled. [Circuits Assembly, 2024]
What is “black pad” in one sentence?
Black pad is nickel corrosion that occurs during immersion‑gold plating, producing a phosphorus‑rich, brittle layer that prevents proper solder wetting. [Circuits Assembly, 2017]
What ENIG thickness should I expect on reliable boards?
Per industry guidance, expect roughly 3–5 µin gold over 120–240 µin nickel, balancing solderability and corrosion control. “The function of the gold deposit is sacrificial,” so thicker isn’t better. This statistical window aligns with IPC‑4552 practices. [Circuits Assembly, 2006]
Does using thicker gold prevent black pad?
No. IPC‑4552A sets an upper Au limit because excessive immersion gold can attack nickel and increase corrosion risk. Control both nickel quality and gold thickness within spec. Quote: “The gold thickness shall be 1.6 to 4.0 µin.” [I‑Connect007, 2023]
How can I distinguish black pad from flux ‘black tar’?
Black pad shows dark pads and micro “mud cracks,” with brittle joints and non‑wetting. Black tar appears as a soft black deposit linked to nickel plating issues during wave/selective soldering; it looks similar but does not necessarily reduce mechanical strength the same way. Microscopy helps differentiate. [Circuits Assembly, 2015]
What if I can’t return the boards—what risks remain after cleaning?
Surface cleaning may restore wetting for shipment, but internal nickel corrosion persists. Expect higher risk of intermittent opens and shock‑sensitive joints. “Boards with black pad cannot be ‘cured’;” use them only if risks are accepted and documented, and plan a clean production remanufacture. [Circuits Assembly, 2017]
Why did my pads look black before any soldering or flux?
In the cited case, pads were black straight from packaging and in some spots after heating. That points to factory process defects, not assembly. Treat this as a supplier quality issue and request corrective action or remake. [Elektroda, Bob Casiano, post #21659049]
What iron/rework practices reduce ENIG damage?
Use the lowest effective tip temperature, minimize dwell time, and avoid repeated heating. ENIG nickel is susceptible to thermal shock; overheating can crack nickel and worsen adhesion and wetting issues. Preheat assemblies and use fresh flux to reduce contact time. [Circuits Assembly, 2024]
