Definition

Surface finish protects exposed copper pads and holes from oxidation while providing a solderable surface. Common options: HASL (hot air solder leveling) is cheapest but has uneven surface; ENIG (electroless nickel/immersion gold) provides flat surface for fine-pitch; OSP (organic solderability preservative) is low-cost but short shelf life; immersion silver and immersion tin offer alternatives. Choice affects solderability, shelf life, wire bondability, contact resistance, and cost. ENIG is the default for fine-pitch BGAs; HASL works for standard through-hole and larger SMT.