The surface finish is the thin protective coating on exposed copper pads that keeps them solderable and prevents oxidation. It looks like a minor line item on a fab quote, but the wrong choice shows up later as poor coplanarity, tombstoning, or short shelf life. Here is how the four common finishes compare.
The contenders
- HASL (leaded) — hot air solder leveling with tin-lead. Cheap and forgiving, but uneven surface and RoHS-restricted.
- Lead-free HASL — the RoHS-compliant version, slightly less coplanar and higher process temperature.
- ENIG — electroless nickel / immersion gold. Flat, fine-pitch friendly, longer shelf life.
- Hard gold — electroplated gold over nickel, built for wear on contacts and gold fingers.
Side by side
| Finish | Flatness | Cost | Shelf life | Best for |
|---|---|---|---|---|
| HASL (leaded) | Poor | Lowest | ~12 mo | Through-hole, coarse pitch |
| Lead-free HASL | Poor-fair | Low | ~12 mo | RoHS, coarse pitch |
| ENIG | Excellent | Medium-high | ~12 mo+ | Fine-pitch BGA, HDI |
| Hard gold | Excellent | Highest | Very long | Edge contacts, ATE |
Choosing
Flatness is the deciding factor for fine-pitch assembly: HASL's uneven bumps make 0.4 mm BGAs and QFNs unreliable, which is why ENIG dominates dense boards. Hard gold is reserved for surfaces that see mechanical wear — connector fingers and test-board contacts — because it is expensive and not ideal as a general solderable finish (thick gold can embrittle joints).
For most modern boards, ENIG is the safe default; HASL survives where cost rules and pitch is coarse; hard gold is a contact-wear solution, not a general finish.