Choosing a surface finish: HASL, ENIG, OSP and immersion tin compared

The surface finish is the only part of the board that the solder touches, so it decides solderability, planarity, contact resistance and, in high-frequency designs, insertion loss. It is also one of the cheaper line items on the quotation, which is why it is often chosen by default rather than by decision.

HASL — hot air solder levelling, in leaded or lead-free form — is the economical option and remains appropriate for through-hole boards and coarse-pitch assemblies. It is not flat: the solder forms a meniscus on the pad, so it is unsuitable for fine-pitch land patterns and for large exposed thermal pads, where the meniscus reduces contact area. The lead-free version also carries a thermal-shock consideration at reflow, because the higher alloy temperature interacts with the thermal expansion difference between the board and the joint.

ENIG — immersion nickel followed by immersion gold — is flat, solderable and a good choice for fine pitch and for pressure contacts or wire bonding, provided the gold thickness is specified. It also has two well-understood weaknesses: nickel-phosphorus reacting with the immersion bath, which is managed by controlling the chemistry rather than by avoiding the process, and voiding at the interface when a board with large thermal pads is exposed to repeated high-temperature cycles.

Immersion tin, with or without silver, sits between HASL and ENIG on price. It is flatter than HASL and cheaper than ENIG, but tin grows whiskers over time, which matters in long-life, high-reliability products, and it needs controlled storage and has a limited shelf life before use.

OSP — an organic surface protectant — is cheap, flat, and good for fine pitch, with no whisker risk. Its limitations are rework: the coating degrades with repeated thermal cycling, it is not suited to wire bonding or high-temperature processing, and it also requires controlled storage and use within a shelf life.

So the decision usually follows from one question: what is the smallest land pattern and the largest thermal pad on this board? Fine pitch plus exposed pads points to ENIG or an immersion silver finish with a stated gold thickness. A lead-free, cost-sensitive assembly with no fine pitch is well served by OSP. Through-hole work that will be reworked repeatedly is still HASL. For radio and high-frequency designs, add surface roughness to the decision, because it contributes to insertion loss.

• Decide from the smallest pitchThe tightest land pattern on the board sets the planarity requirement, whatever the rest of the design can afford.
• Specify gold thicknessWhere ENIG is chosen, the gold thickness decides whether bonding or pressure contacts are supported.
• Match the finish to the assemblyThe finish and the solder paste should agree on the same alloy system, particularly on a lead-free build.
• Count the reflow cyclesOSP and immersion tin degrade with thermal cycling, so the assembly plan is part of the finish decision.
• Ask about storage and shelf lifeFinishes that need controlled conditions must have those conditions specified on the shipment.