PCB Surface Finish: ENIG, ENEPIG, HASL, OSP, Silver & Tin

A PCB surface finish is the coating on exposed copper pads that prevents oxidation and gives a solderable, sometimes bondable, surface. The choice trades off pad flatness, shelf life, solder-joint reliability, wire-bonding, high-frequency loss, lead-free compatibility and cost.

We offer all major finishes: HASL, lead-free HASL, ENIG, ENEPIG, OSP, immersion tin, immersion silver and electrolytic hard gold, with selective combinations such as ENIG plus hard-gold fingers on one board.

HASL (hot air solder leveling) dips the board in molten solder and levels it with hot air. It is robust and low cost, but the surface is uneven, so it is a poor fit for fine-pitch BGAs below about 0.8 mm and for via in pad. ENIG (electroless nickel immersion gold) plates a nickel barrier and a thin gold layer. It is flat, solders well on fine pitch and keeps for 12 months or more, at a higher cost. Choose HASL or lead-free HASL for through-hole and coarse-pitch boards where cost matters most; choose ENIG for BGAs, QFNs, via in pad and boards that will be stored before assembly.

ENIG deposits 3–6 µm of electroless nickel and 0.05–0.1 µm of immersion gold. The nickel is the barrier against copper diffusion, and the gold protects the nickel and gives an excellent solderable surface. It is the most specified lead-free finish for complex assemblies: flat, coplanar pads for fine-pitch BGA and QFN, 12+ month shelf life and good joint reliability.

The known risk is black pad, a nickel-phosphorus corrosion defect that causes brittle joints. Mid-phosphorus chemistry (7–9% P) and good process control have largely eliminated it. If the risk must be removed entirely, specify ENEPIG.

Hot air solder leveling dips the board in molten solder and levels it with hot-air knives. Traditional HASL uses tin-lead; lead-free HASL uses a SAC or tin-copper alloy. It offers excellent solderability, long shelf life and low cost, but pads are uneven, thicker at the edges and thinner in the middle. That makes it unsuitable for BGAs below about 0.8 mm pitch and for QFN thermal pads. For through-hole and coarse-pitch SMT boards, lead-free HASL is still a cost-effective choice, and the thermal shock of the process should be considered on thick or HDI boards.

Immersion tin deposits a thin tin layer directly on copper, giving flat, solderable pads at moderate cost and working well with press-fit connectors. Shelf life is moderate, around 6 months. Tin-whisker growth over time is the concern, so it is acceptable for most commercial electronics but avoided in aerospace, military and other high-reliability hardware.

Electrolytic hard gold, gold with a small amount of cobalt or nickel over a nickel underlayer, is used for edge connectors, keypad contacts and other surfaces that see repeated insertion or wear. It is usually applied selectively, for example ENIG on pads with hard-gold fingers on the connector edge, with beveling as required.