PCB surface finishes: what the pads are plated with
The pads on a board are plated with one of about six things. Telling them apart takes a glance, and it changes how you grade the board and what you should ignore.
Identifying the finish
| Finish | Appearance | Thickness |
|---|---|---|
| Hard gold | Bright, saturated brass-yellow. Only on edge fingers and contact pads, never the whole board | 15–50 µin gold over ~100 µin nickel |
| ENIG | Flat, dull to satin, pale greyish gold. Uniform, no doming. Never bright brassy | 2–5 µin gold over 118–236 µin nickel |
| ENEPIG | Almost identical to ENIG, sometimes marginally whiter | 1.2–2.8 µin gold, 2–12 µin palladium |
| Immersion silver | Flat, bright white-silver when fresh. Tarnishes to iridescent purple and blue-brown from the pad edges, then grey | 4–12 µin silver |
| HASL | Bright silver with domed, uneven pads and a visible meniscus. Looks like solder because it is solder | Variable |
| Immersion tin | Flat, dull matte grey, slightly chalky. Goes cloudy quickly | 20–50 µin tin |
| OSP | Bare copper colour — pink, salmon or orange. Oxidises to dull brown | Organic film |
The distinction that matters
Everything hinges on telling hard gold from ENIG flash. They are both gold and they differ by five to twenty-five times in thickness.
The rule is positional as much as visual: if the gold is everywhere on the board, it is ENIG and it is not worth stripping. If the gold is bright and confined to a fingered edge, it is hard gold and it is the most valuable thing on the board.
Two confirmations. Hard gold is electrolytic, so it needs a plating bus — look for the tie-bar stubs at the board edge where the bus was routed away. And the colour differs plainly once you have seen both: hard gold is saturated brass-yellow, ENIG is pale and greyish because two to five microinches is semi-transparent over the nickel underneath.
How much gold is actually in ENIG
At three microinches and roughly 10% exposed pad area — soldermask covers the rest — a square foot of board carries about 0.014 g of gold per side. Across a typical board that works out to about 0.025 g of gold per pound, or roughly 56 g per tonne.
That figure lands squarely inside published low-to-mid-grade board assays, which is a good sign the arithmetic is right. It also means something important: on a modern connector-poor board, the ENIG surface is most of the gold that is left.
Why stripping it is a losing trade
- The nickel is 40 to 120 times thicker than the gold. Immersion gold does not lift off — you have to dissolve or undercut the whole nickel layer to release it, so you are chemically processing about fifty times more nickel by volume than the gold you recover.
- It is the thinnest gold on the board. Every other gold source — fingers, connectors, pins, bond wire — is a better target.
- You destroy the board's sale value to get it. To accumulate one troy ounce you process about 1,250 lb of board. That same material sold intact at a low mid-grade rate is around four thousand dollars — roughly what the ounce is worth — and you keep the copper, silver and palladium as well.
- The smelter pays you for it anyway. Assay-based settlement returns 98% of that gold. Stripping is not additive; it substitutes for a payment you were already receiving.
- Hazardous waste. Cyanide or nitric systems, nickel-laden liquor, and yields well under 100% on masked, rough surfaces.
Where the same effort does pay
Gold fingers at thirty microinches, plated both sides, with no soldermask and no ambiguity. One square inch is about 0.019 g — roughly $2.48 at current spot. The tool list is a pair of shears. That ten-fold thickness advantage plus trivial mechanical separation is exactly why the market pays around $90 a pound for fingers and $3.25 for mid-grade board.
Questions
How can I tell ENIG from gold fingers?
ENIG is pale, dull and everywhere on the board. Hard gold on fingers is bright brass-yellow and confined to a card edge. Hard gold is five to twenty-five times thicker and is the only one worth recovering separately.
Is it worth stripping gold plating from circuit boards?
No. ENIG contributes about 0.025 g per pound of board, buried under nickel forty to a hundred times thicker, and you would need roughly 1,250 lb of board per troy ounce — material that is worth about the same sold intact, with the copper and silver still in it.
What is immersion silver on a PCB?
A four to twelve microinch silver finish that looks bright white when fresh and tarnishes to iridescent purple-brown from the pad edges. The tarnish pattern is a reliable way to identify it, but the silver content is trivial by weight.