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. It is also the last pcb manufacturing decision that is still cheap to change, which is why it is worth settling at quotation rather than after the panels are released.
