A PCB surface finish is the final metallurgical coating applied to exposed copper pads and traces. Its role is to protect copper from oxidation, provide a solderable surface for assembly, improve contact reliability for connectors and test points, support special processes like wire bonding, and influence planarity, fine-pitch capability and corrosion resistance.
Why surface finish matters
- Solderability & assembly yield — wetting behavior, voiding tendencies and lead-free reflow compatibility.
- Pad planarity for fine pitch — 0.5 mm pitch and below requires flat pads to avoid bridging.
- Reliability & corrosion resistance — long storage and harsh environments demand the right finish.
- Contact performance — edge fingers and connectors need hard gold, not solderable finishes.
Common finish types
ENIG (Electroless Nickel Immersion Gold)
Electroless nickel plated onto copper, capped with a thin immersion gold layer to protect the nickel. Excellent planarity for fine-pitch BGAs/QFNs, good shelf life, consistent solderability. Choose a qualified fabricator to avoid "black pad" (nickel hyper-corrosion).
ENEPIG (Electroless Ni / Electroless Pd / Immersion Au)
Adds a palladium barrier between nickel and gold. Excellent for wire bonding (especially gold wire) and soldering on the same board. Reduces black-pad risk. Higher cost than ENIG.
HASL — Lead-Free & Leaded
Board is dipped into molten solder, then hot-air knives blow off excess. Cost-effective and robust for through-hole and larger SMT. Surface is not perfectly flat — risky for fine-pitch BGAs and 0.5 mm QFNs. Lead-free is the modern default.
Immersion Silver (ImAg)
Thin silver layer deposited by immersion reaction. Good planarity and electrical performance for some high-speed/RF applications. Sensitive to sulfur-rich environments (creep corrosion); requires sealed packaging.
Immersion Tin (ImSn)
Very flat surface, good solderability when fresh. Watch for tin whiskers, intermetallic growth and shorter shelf life. Useful for press-fit and certain specialized assembly flows.
OSP (Organic Solderability Preservative)
Thin organic coating that protects copper and burns off during reflow, allowing solder to wet directly to copper. Very flat, low cost, environmentally friendly. Best for high-volume SMT with tight logistics. Sensitive to handling and limited reflow cycles.
Hard Gold (Electroplated)
Thicker, harder electroplated gold over nickel. Excellent wear resistance for edge connectors (gold fingers), test pads and high-mating-cycle contacts. Not intended as a general solderable finish; often applied selectively.
Engineering selection table
| Finish | Planarity | Fine-pitch SMT | Shelf life | Cost | Best use case |
|---|---|---|---|---|---|
| ENIG | Excellent | Excellent | Good | Medium | Fine-pitch SMT, premium general |
| ENEPIG | Excellent | Excellent | Good | High | Wire bonding + soldering |
| Lead-Free HASL | Moderate | Fair | Good | Low | General-purpose, through-hole |
| Immersion Silver | Excellent | Good–Excellent | Medium | Medium | Fine pitch / some RF |
| Immersion Tin | Excellent | Good | Medium | Medium | Flat SMT pads (controlled storage) |
| OSP | Excellent | Excellent | Short–Medium | Low | High-volume SMT, fast-turn builds |
| Hard Gold | Excellent | N/A | Very Good | High | Edge connectors, wear surfaces |
How to choose
- ENIG — fine-pitch BGA/QFN, longer storage, premium balance.
- OSP — lowest cost, high-volume SMT, fast time from fab to assembly.
- Lead-Free HASL — cost-effective, through-hole-heavy boards.
- ENEPIG — wire bonding plus soldering, advanced packaging.
- Hard Gold — edge fingers, repeated mating cycles.
Storage & handling
FAQ
Which finish is best for BGA?
Typically ENIG or ENEPIG — excellent planarity and consistent solderability for fine pitch.
What's the cheapest surface finish?
Usually lead-free HASL or OSP. OSP is very cost-effective for high-volume SMT.
Do I need hard gold for edge connectors?
Yes — hard gold withstands repeated insertion. ENIG is solderable and flat but not as wear-resistant.
Need help on your build?
Talk to a Sunrise PCB engineer.