Hasl: the default for decades
Hot-air solder levelling floods the board with molten solder and blows it flat with air knives. It is cheap, forgiving of rework and easy to inspect. Its limit is coplanarity: the meniscus is never perfectly flat, which hurts fine-pitch printing.
Lead-free leveling
The move to lead-free solder raised the process temperature and changed the alloy. HASL remained viable for through-hole and coarse SMT work, but the flatness problem did not go away.
ENIG: flat and stable
Electroless nickel with an immersion gold layer gives a flat surface with good shelf life and a nickel barrier against copper diffusion. It became the default for fine-pitch SMT and for boards that must survive multiple assembly passes.
Silver and OSP: the specialists
Immersion silver offers excellent solderability and flatness at lower cost than ENIG, with a shorter shelf life and sensitivity to handling. OSP is an organic protective layer that disappears in the first reflow: the cheapest option for a single-sided assembly that ships promptly.
How we choose
We select the finish from pitch, assembly cycle count, storage and budget, then confirm it with the assembly team before tooling.
