Surface finish
A surface finish is the protective coating applied to the exposed copper pads on a PCB to prevent oxidation and provide a solderable surface for component attachment. The choice affects cost, assembly compatibility, shelf life, and reliability.
What it is
Bare copper oxidises rapidly when exposed to air, becoming unsolderable within days. A surface finish solves this by depositing a thin protective layer over the copper pads after solder mask application but before final inspection. Different finishes use different metals or chemistries to balance cost, flatness, shelf life, and assembly compatibility.
The most widely used finishes are HASL (Hot Air Solder Levelling) and ENIG (Electroless Nickel Immersion Gold). HASL is cheap and robust but uneven, making it unsuitable for fine-pitch components. ENIG is flat, lead-free, and supports fine-pitch BGAs but costs more and has a small risk of "black pad" failure.
Other common options include OSP (Organic Solderability Preservative), a thin organic coating that is the lowest-cost ENIG alternative but with shorter shelf life; immersion silver, flat and lead-free but prone to tarnishing if mishandled; and immersion tin, used in specific applications such as press-fit assemblies.
When it matters
Surface finish affects nearly every aspect of assembly. Fine-pitch BGAs and QFNs require a flat finish (ENIG, immersion silver, or immersion tin). Multiple reflow cycles favour ENIG over HASL. Long-term storage favours ENIG over OSP. Cost-sensitive prototypes with through-hole and larger SMT components are well served by HASL. RoHS-compliant production rules out tin-lead HASL. The finish should be chosen alongside the component mix, not as an afterthought.
Comparison
Shelf life depends on storage conditions and packaging. Values shown are typical industry ranges from supplier specifications such as Würth Elektronik.
At Nordic PCB
All major surface finishes are available across our certified suppliers. If you're unsure which to specify, request a quote with the two most relevant options for your board — we'll return the cost delta so you can choose based on your specific design and budget. Our DFM review flags any incompatibility between the selected finish and your component footprints.
Related terms
- ENIG vs HASL
ENIG and HASL are two of the most common PCB surface finishes. HASL (Hot Air Solder Levelling) is cheaper and well-suited to through-hole work; ENIG (Electroless Nickel Immersion Gold) is flatter, finer-pitch capable, and lead-free by default — at a higher cost.
- OSP
OSP (Organic Solderability Preservative) is a water-based organic coating applied directly to exposed copper pads as a low-cost surface finish. It is flat, RoHS-compliant, and ideal for fine-pitch components, but has a short shelf life of 3-6 months and is sensitive to handling.
- Immersion silver
Immersion silver is a thin silver coating (typically 0.1-0.4 µm) chemically deposited on exposed copper pads as a flat, lead-free, RoHS-compliant surface finish. It supports fine-pitch components and multiple reflow cycles but is prone to tarnishing if mishandled and requires careful storage.
- RoHS compliance
RoHS compliance means the PCB and all its components meet the EU's Restriction of Hazardous Substances Directive (2011/65/EU), which limits 10 specified substances including lead, mercury, cadmium, and four phthalates. RoHS compliance is mandatory for electronic equipment placed on the EU market.