Common PCB Surface Treatment Processes and Their Application Scenarios

1. HASL (Hot Air Solder Leveling)

Process Characteristics: The PCB is immersed in molten tin - lead (or lead - free tin), and then hot air is used to scrape the surface to form a uniform tin layer.

• Low - cost applications: Consumer electronics (such as home appliances, toys), ordinary coMPUter motherboards.
• Through - hole Technology (THT): Suitable for through - hole components that require good solder wettability.
• Non - high - density designs: The requirement for surface flatness is not high, and it is not suitable for fine - pitch BGA or QFN packages.

2. ENIG (Electroless Nickel Immersion Gold)

Process Characteristics: After electroless nickel plating (5 - 8 μm) on the copper surface, gold is immersed (0.05 - 0.2 μm) to form a flat nickel - gold layer.

• High - Density Interconnect (HDI): Smartphones, tablets, server motherboards.
• Multiple reflow soldering: Suitable for SMT processes that require multiple assembly operations.
• Gold - finger connectors: Provide low contact resistance and wear resistance.
• High - frequency/high - speed signals: The nickel layer can shield electromagnetic interference (EMI).

3. OSP (Organic Solderability Preservative)

Process Characteristics: A layer of organic protective film is coated on the copper surface to prevent oxidation.

• Short - cycle production: Fast - turnaround consumer electronics products (such as routers, LED lights).
• Lead - free soldering: Scenarios with strict environmental protection requirements (compliant with RoHS).
• Low - cost SMT: Simple designs where the surface does not require complex treatment.

• Immersion Silver: Suitable for high - frequency circuits (such as RF modules), but it is prone to sulfidation and blackening.
• ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold): A high - reliability process to replace ENIG, suitable for aerospace or automotive electronics.

1. Corrosion Mechanism of the Nickel Layer

• Uneven phosphorus content: During electroless nickel plating, the distribution of phosphorus in the nickel - phosphorus alloy is uneven (normal content: 7 - 10%). High - phosphorus regions are prone to forming an amorphous structure, while low - phosphorus regions are crystalline. Crystalline nickel is preferentially corroded in the gold - immersion solution (strong oxidizing property), leading to micro - cracks.
• Over - corrosion: The gold - immersion solution (usually containing thiourea or cyanide) overly attacks the nickel layer, forming a porous and loose phosphorus - rich layer (black nickel).

2. Failure of Process Control

• Plating solution contamination: Contamination of copper ions (Cu²⁺) in the nickel tank leads to a displacement reaction, generating a loose copper - nickel layer.
• Temperature/pH fluctuations: Unstable temperature or pH value of the nickel plating solution affects the compactness of the coating.
• Too thin gold layer: Insufficient gold - immersion time results in a gold layer (<0.05 μm) that cannot completely cover the nickel layer, and the exposed nickel oxidizes during soldering.

3. Interface Diffusion Problems

• Nickel - gold interface diffusion: During high - temperature soldering, nickel atoms diffuse into the gold layer, causing the formation of brittle intermetallic compounds (such as Ni₃Au) at the interface, reducing the bonding strength.

• Optimize the plating solution formula: Control the phosphorus content in the nickel tank (7 - 10%) and avoid copper - ion contamination.
• Strictly control process parameters: Stabilize the plating solution temperature (85 - 90°C), pH value (4.5 - 5.2), and gold - immersion time (5 - 8 minutes).
• Strengthen gold - layer protection: Ensure that the gold - layer thickness is ≥0.1 μm to reduce the risk of nickel exposure.
• Post - treatment inspection: Analyze the compactness of the nickel layer through cross - section analysis or SEM/EDS detection.