PCBs SURFACE FINISHING PROTECTION
The surface finish protects the exposed copper traces from oxidation and ensures reliable solderability for component assembly.
Common Surface Finishes & Processes:
1. HASL (Hot Air Solder Leveling) – Leaded or Lead-Free
◦ The PCB is dipped in molten solder, then excess solder is removed using hot air knives.
◦ Pros: Cost-effective, good shelf life.
◦ Cons: Uneven surface, not ideal for fine-pitch components.
2. ENIG (Electroless Nickel, Immersion Gold)
◦ A two-layer finish: nickel (barrier layer) and gold (prevents oxidation).
◦ Pros: Flat surface, excellent for fine-pitch components, good durability.
◦ Cons: More expensive than HASL, risk of "black pad" defects.
3. OSP (Organic Solderability Preservative)
◦ A thin, organic coating that prevents copper oxidation.
◦ Pros: Low cost, flat surface, eco-friendly.
◦ Cons: Limited shelf life, requires careful handling.
4. Immersion Tin
◦ A thin layer of tin chemically deposited on copper traces.
◦ Pros: Flat surface, lead-free, good for fine-pitch components.
◦ Cons: Tin whiskering risk, short shelf life.
5. Immersion Silver
◦ A thin layer of silver applied via a chemical process.
◦ Pros: Good conductivity, flat surface, better oxidation resistance than OSP.
◦ Cons: Tarnishes over time, needs careful storage.
6. Soft gold for wire bonding
◦ A thick layer (1-5 µm) of pure gold is applied with an electroplating process.
◦ Pros: Good conductivity, flat surface, possibility to avoid Nickel for a signal integrity in high frequency.
◦ Cons: underetching problems in case of fineline patterns.
7. ENEPIG (Electroless Nickel, Palladium, Immersion Gold)
◦ A three-layer finish: nickel, palladium (barrier layer) and gold (prevents oxidation).
◦ Pros: Flat surface, excellent for fine-pitch components, good durability, good performance for wire bonding process.
◦ Cons: Expensive
This phase ensures long-term reliability, solderability, and performance of the PCB before final assembly.
