PCB Surface Finishes Explained: ENIG, HASL, OSP and More

Detailed Explanation

Bare copper oxidises quickly in air — within hours in humid conditions. An oxidised pad is poorly solderable or unsolderable: the flux in solder paste cannot reliably remove heavy copper oxide, and the solder joint either doesn't form or produces a weak, high-resistance connection. A surface finish prevents this by protecting the copper between fabrication and soldering.

The choice of surface finish also affects pad flatness (important for fine-pitch components), shelf life (time between fabrication and use), RoHS compliance, cost, and process temperature requirements.

HASL — Hot Air Solder Levelling

How it works: The bare board is dipped in molten solder, then the excess is removed by hot-air knives, leaving a thin solder coating on the exposed pads.

Variants:

• Leaded HASL (Sn63/Pb37): Traditional, lower melting point (183°C), well-understood, but not RoHS compliant.
• Lead-free HASL (LF-HASL): Uses SAC305 or similar alloys (melting point ~217°C); RoHS compliant but requires higher process temperatures during levelling.

Pros: Cheapest surface finish; long shelf life (12+ months); excellent solderability; well-understood.

Cons: Uneven surface — the solder buildup varies across a pad, which causes coplanarity problems with fine-pitch ICs (0.5 mm pitch QFPs, BGAs). Not suitable for 0.5 mm pitch or smaller components. The thermal shock of the dipping process can stress vias and laminates on thin or multilayer boards.

Use when: Large-pitch through-hole or SMT components (0.8 mm pitch and above), cost is the primary constraint, no fine-pitch ICs, leaded process is acceptable (or LF-HASL is specified for RoHS).

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ENIG — Electroless Nickel Immersion Gold

How it works: A thin layer of nickel (typically 3–6 µm) is deposited onto the copper by electroless (autocatalytic) plating, followed by a very thin immersion gold layer (0.05–0.125 µm). The nickel acts as a barrier between the copper and the gold (which dissolves into the solder on first reflow); the gold protects the nickel from oxidation during storage.

Pros: Flat, uniform surface — essential for BGAs and fine-pitch ICs. Long shelf life (12+ months). Excellent gold contact surface for edge connectors, test points, and press-fit connectors. RoHS compliant.

Cons: More expensive than HASL or OSP. Risk of black pad defect if fab house process chemistry is not well controlled (see FAQ above). The nickel layer can become brittle if too thick.

Use when: Fine-pitch SMT components (0.5 mm pitch or below), BGAs, boards with edge-connector fingers, boards that will be stored before assembly, or any board where pad flatness is required. ENIG is the default recommendation for most modern product designs.

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OSP — Organic Solderability Preservative

How it works: A thin (~0.2–0.5 µm) organic compound (usually a benzimidazole or imidazole derivative) is applied to the copper pad surface. It bonds to the copper and prevents oxidation. The OSP coating decomposes during solder reflow, exposing clean copper for joint formation.

Pros: Very flat surface (essentially the bare pad surface, as the OSP is an organic film, not a metal deposit). Cheapest flat-surface option. RoHS compliant. Environmentally low-impact process.

Cons: Shortest shelf life — typically 6–12 months in original sealed packaging; pads may degrade quickly if the packaging is opened and the boards are exposed to humidity. The organic coating is fragile and can be damaged by handling, fingerprints, or multiple reflow cycles. Not suitable for edge connectors, test points, or press-fit connectors (the organic film is not a good contact surface). Visual inspection is difficult (OSP-coated pads look identical to bare copper).

Use when: High-volume production with controlled storage and quick assembly (boards go from fab to reflow within weeks), cost is paramount, no connectors or test contact surfaces in the design.

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Immersion Silver (IAg)

How it works: A thin silver layer (0.1–0.4 µm) is deposited onto the copper by immersion plating (displacement reaction — silver ions displace copper ions at the surface). No nickel layer is used; the silver is deposited directly on copper.

Pros: Very flat surface. Excellent solderability. Good for fine-pitch components. RoHS compliant. Less expensive than ENIG.

Cons: Silver tarnishes in air containing sulfur compounds (found in some rubber bands, cardboard, and air pollution). Tarnished silver is harder to solder. Shelf life is typically 6–12 months in sealed packaging. Silver migration (electromigration) can occur on high-voltage boards with closely spaced traces in humid environments. Less widely available than ENIG from Australian fab houses.

Use when: Fine-pitch SMT where OSP shelf life is a concern and ENIG cost must be avoided; RF/microwave boards where silver's good electrical conductivity at high frequency matters.

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Immersion Tin (ISn)

How it works: Thin tin layer (0.8–1.2 µm) deposited by immersion plating directly onto copper.

Pros: Flat surface. Good solderability. RoHS compliant.

Cons: Tin whiskers — tin spontaneously grows microscopic filaments (whiskers) that can cause short circuits on fine-pitch boards, particularly in high-temperature environments. This is a significant reliability concern for space, military, and automotive applications. Also sensitive to handling (tin oxide forms quickly). Not widely used for general SMT designs in Australia; specialist applications only.

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Comparison Summary

Choosing a Surface Finish for a New Design

For most modern product designs:

1. ENIG if any of the following apply: fine-pitch ICs or BGAs, boards stored before assembly, edge connectors or gold-contact test points, or cosmetic consistency matters.
2. OSP if: the design is all standard-pitch SMT (no BGAs, no sub-0.5 mm pitch), the assembly turnaround is fast (boards assembled within weeks of fabrication), and cost must be minimised.
3. LF-HASL only for: large-pitch, simple boards where cost is the only constraint and fine-pitch components are absent. Rarely the right choice for new designs.

Design Considerations

• ENIG edge fingers require different plating thickness: PCB edge connectors typically need a heavier gold specification (hard gold, ENEPIG, or additional gold plating) than standard ENIG provides — the thin immersion gold in standard ENIG wears through quickly with repeated connector insertions. Specify hard gold or ENEPIG if the design includes an edge-card connector.
• OSP boards require careful assembly scheduling: An OSP board opened from its packaging and left on a shelf for weeks before assembly may have degraded solderable surfaces — coordinate fabrication timing with the assembly schedule.
• Discuss finish options with your specific fab house: Availability and pricing vary. Australian contract manufacturers often stock ENIG and LF-HASL as standard options; OSP and immersion silver may be special orders with longer lead times.

Common Mistakes

• Specifying standard ENIG for a board that also needs edge-card connector contacts — the immersion gold thickness is not wear-resistant; specify hard gold or ENEPIG for those contacts instead.
• Using leaded HASL for a product that will be sold in the EU, Australia (where AS/NZS 4268 and derivative requirements apply), or other RoHS markets.
• Ordering OSP boards and then storing them unassembled for several months, resulting in poor solderability on reassembly.
• Not accounting for the HASL surface unevenness when placing fine-pitch ICs — the coplanarity variation from HASL can prevent fine-pitch leads from contacting their pads uniformly.