ENIG vs HASL vs OSP: Choosing a PCB Surface Finish
The surface finish decides solderability, shelf life and whether fine-pitch assembly is possible. A practical comparison of the six finishes we run, with the cases where each one wins.
For fine-pitch SMT and BGA, choose ENIG. For low-cost boards that go straight into assembly, choose OSP. For through-hole boards that may be reworked in the field, choose lead-free HASL. Those three cover the large majority of orders we build.
A surface finish has one job: protect the exposed copper from oxidation while staying solderable. Everything else, including flatness, shelf life, contact resistance and cost, is a consequence of how each finish achieves that.
01Flatness decides fine-pitch capability
HASL is a hot-dip process, and the levelling step leaves a slightly rounded, variable deposit. On a 0.4 mm pitch BGA pad that variation is enough to cause inconsistent paste release and occasional bridging. ENIG, OSP, immersion silver and immersion tin are all chemically deposited and stay flat, which is why they are the default for fine-pitch work.
Flatness also matters for stencil design. A flat finish lets you use a thinner stencil and a smaller aperture, which improves paste release on small pads.
02Finish comparison
The table summarises the trade-offs. Shelf life assumes sealed vacuum packing with desiccant and storage below 25 degrees Celsius at under 60 percent relative humidity.
| ENIG | Flat, 12 month shelf life, good for fine pitch and BGA, wire bondable, higher cost |
|---|---|
| Lead-free HASL | Rounded surface, 12 month shelf life, robust and reworkable, low cost, not for fine pitch |
| OSP | Very flat, 6 month shelf life, low cost, single reflow window, cannot be handled with bare hands |
| Immersion silver | Very flat, 6 to 12 month shelf life, good RF performance, sensitive to sulphur and handling |
| Immersion tin | Flat, 6 month shelf life, good for press-fit connectors, tin whisker consideration |
| Hard gold | Wear resistant, used on edge connectors and contact fingers, 200 plus mating cycles |
03Where each one fails
Every finish has a failure mode worth knowing before you specify it.
- ENIG: black pad, a brittle nickel-phosphorus interface caused by an over-aggressive plating bath. It shows as a clean fracture at the solder joint and is controlled by bath monitoring, not by design
- HASL: pad coplanarity variation, which shows up as insufficient solder on small pads
- OSP: the coating is consumed in the first reflow, so a board that needs a second pass or a field repair loses its protection
- Immersion silver: tarnishing in sulphur-rich environments, and fingerprint contamination if handled without gloves
- Immersion tin: reduced solderability if stored past six months, and tin whisker growth on long-life designs
- Hard gold: not suitable for soldering directly, because gold dissolves into the joint and embrittles it above roughly three percent gold by weight
04Combining finishes on one board
It is common and entirely practical to run two finishes on one board: ENIG across the SMT areas and hard gold on the edge connector fingers. This is a two-step plating process with an extra peel-back, so it adds cost and lead time, but it is standard practice for boards that both mount components and plug into a backplane.
If you need this, mark the gold areas clearly on the assembly drawing and give the gold thickness, usually 30 micro-inches for connector fingers that mate repeatedly.
Key takeaway
Pick the finish from your assembly process, not from cost alone: ENIG for fine pitch, OSP for a single fast assembly pass, HASL for reworkable through-hole boards, and hard gold only where a contact surface is needed.
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