PCB Surface Finishes
Essential PCB Surface Finishes
Printed Circuit Board (PCB) surface finishes protect copper traces from oxidation and ensure reliable solder-ability during assembly. This guide covers key finishes, technical specifications, and industry applications—critical for design engineers and PCB manufacturers.
Hot Air Solder Leveling (HASL)
Process: PCBs are immersed in molten solder (lead or lead-free), then leveled with hot air knives.
Thickness: 1–40 μm 48
Pros:
Excellent solderability
Low cost
Shelf life: 12+ months
Cons:
Poor planarity (uneven surfaces)
Unsuitable for fine-pitch components (<20 mil spacing) or HDI designs 46
Applications: Consumer electronics, low-density boards.
Organic Solderability Preservative (OSP)
Process: Forms a thin organic layer (benzimidazole-based) on copper to prevent oxidation.
Thickness: 0.15–0.65 μm 48
Pros:
Flat surfaces ideal for BGA/micro-pitch components
Eco-friendly, low-cost process
Cons:
Short shelf life (6 months max)
Degrades after 2–3 reflow cycles 610
Applications: High-density ICs, smartphones, TVs.
Electroless Nickel Immersion Gold (ENIG)
Process: Deposits a nickel-phosphorus (Ni-P) barrier layer (3–6 μm), topped with immersion gold (0.05–0.125 μm) .
Critical Parameter: Phosphorus content (7–10%) prevents “Black Pad” defects.
Pros:
Superior planarity for BGA/LGA
Long shelf life, wire-bondable
Cons:
High cost
Risk of brittle solder joints if Au >0.3 μm 1
Applications: Connectors, switches, RF devices.
Immersion Silver (IAg)
Process: A sub-micron silver layer deposited via displacement reaction.
Thickness: 0.12–0.40 μm
Pros:
Excellent high-frequency signal integrity
Moderate cost
Cons:
Tarnishes easily (requires glove handling)
Limited shelf life after packaging opened 8
Applications: Automotive, high-speed PCBs, communication hardware.
Immersion Tin (ISn)
Process: Forms a copper-tin intermetallic compound (IMC) for direct soldering.
Thickness: 1–40 μm 4
Pros: Compatible with all solder types; Flat surfaces
Cons: Prone to tin whiskers (mitigated by organic additives) 16
Short storage window
Applications: Backplanes, telecom infrastructure.
Specialized Finishes
Electrolytic Ni/Au (Hard Gold)：Hard Au (with cobalt): 0.5–1.5 μm for connectors/fingers 37.
Soft Au：>1 μm for wire bonding.
ENEPIG (Ni-Pd-Au): Pd layer prevents corrosion, used in aerospace 7.
