Bare copper oxidises. A surface finish is the thin layer that keeps it solderable until the board is assembled — and the choice affects your cost, your shelf life, your yield on fine-pitch parts, and in one case whether your product can pass a medical or automotive audit at all.
Two jobs, and nothing else.
A surface finish has to do exactly two things: protect the copper from oxidising between fabrication and assembly, and present a surface the solder will wet reliably during reflow. Every finish is a different trade-off between how well it does those two jobs, how flat it is, how long it lasts, and what it costs.
The one that most often gets chosen wrongly for the wrong reason is HASL — because it is the cheapest, and because someone has been using it since the 1990s.
Read the row that matches your components and your deadline, then check the cost column before you commit.
| Finish | Surface | Shelf life | Good for | Avoid when | Relative cost |
|---|---|---|---|---|---|
| HASL hot air solder levelling |
Uneven — solder mounds | 12 months+ | Through-hole boards, large pads, cost-driven builds | Fine pitch under 0.5 mm, BGAs, anything needing a flat surface | Lowest |
| ENIG electroless nickel immersion gold |
Very flat | 12 months+ | Fine pitch, BGA, QFN, press-fit, long storage, most modern assemblies | You need to weld or wire-bond to it, or you need the absolute lowest price | High |
| OSP organic solderability preservative |
Flat, invisible | 6 months — the shortest | High-volume builds assembled soon after fabrication; consumer boards | The board will sit in storage, or you need to probe/test it repeatedly | Low |
| Immersion silver | Very flat | 6–12 months, tarnishes | Fine pitch and BGA where ENIG's cost is hard to justify | Handling and packaging are not controlled — it tarnishes and stains | Medium |
| Immersion tin | Flat | 6–12 months | Press-fit connectors, some automotive boards | You are worried about tin whiskers, or the board will be reflowed more than about twice | Medium |
| ENEPIG electroless nickel electroless palladium immersion gold |
Very flat | 12 months+ | Gold and aluminium wire bonding, high-reliability medical and aerospace | Standard solder-only assembly — you are paying for a capability you will not use | Highest |
All six of these are available on Hongrong boards. If you are unsure which your application needs, tell us the operating environment and the assembly process and we will recommend one — ask us.
HASL leaves an uneven surface — solder mounds. On a 0.5 mm pitch part that unevenness is enough to lift a pad off its paste and produce an open joint. ENIG is deposited chemically and is essentially flat, which is why it became the default for BGA and QFN assembly. If your board has any fine-pitch component, HASL is the wrong choice regardless of price.
OSP is a thin organic film that protects the copper until the first reflow — at which point it is consumed. Its shelf life is the shortest of the six, around six months. If your boards are fabricated in batches and consumed slowly, or you plan to run them through the line twice, OSP will cause solderability problems and you will not find out until assembly. ENIG or immersion silver store far better.
"Black pad" is a defect where the electroless nickel layer beneath the gold corrodes during processing, producing a brittle, non-wetting surface that looks perfectly fine. It is a process-control problem, not an inherent property of ENIG — but it means who deposits the ENIG matters as much as the choice of ENIG. If you are buying ENIG boards from an unknown supplier and seeing sporadic solder-joint failures on BGAs, this is one of the first things to rule out.
ENEPIG exists mainly to solve two problems at once: it removes the black-pad risk of thick ENIG, and it is compatible with gold and aluminium wire bonding. If your assembly is standard solder reflow, ENEPIG costs more and buys you nothing. If you are bonding bare die or wires to the board, it is often not optional.
The surface finish belongs in the fabrication notes of your Gerber package, not in an email. If it is not specified, many fabricators will build the cheapest option that satisfies the drawing — which is how a board with 0.4 mm pitch parts ends up with HASL. State the finish explicitly, and state the finished copper thickness alongside it.
Setup, stencils, component minimums and test fixtures — and how to keep unit cost sane between 1 and 1,000 pieces.
The difference between Class I, II and III, and which products genuinely need the strictest one.
Layers, line width, copper weight, drill sizes and every other limit we build to.
Tell us the smallest pitch on your board and where it is being assembled. We will come back with a recommendation and a price — within 24 hours.
