PCB Surface Finish Processes—HASL / ENIG / OSP

August 19, 2026

Hot Air Solder Leveling (HASL) (Process & Purpose)

Process: Pre-treatment → Flux application → Solder leveling → Solder leveling → Post-treatment

Purpose:
1. Protect the copper surface
2.Provide good solderability for subsequent assembly processes

Principle: Chemical reaction

Main raw material: Tin-lead bar

HASL (Pre-treatment)

Purpose: Remove organic contaminants and oxides from the copper surface.

Main chemical: SPS

Process key points: Copper ion concentration in the bath, temperature, line speed

HASL (Flux application)

Purpose: Facilitate solder adhesion to the copper surface.

Main raw material: 818 flux

Process key points: Viscosity and acidity of the flux; ease of cleaning

HASL (Solder leveling)

Purpose: Apply solder onto the copper surface.

Main raw material: Tin-lead bar (63/37)

Process key points:
A. Equipment performance
B. Air knife structure, angle, spray pressure, hot air temperature, solder pot temperature, board passing speed through the air knives, immersion time, etc.

HASL (Post-treatment)

Purpose: Wash off residual flux and oily residues carried out from the solder pot.

Electroless Nickel Immersion Gold (ENIG) (Process & Principle)

Process: Pre-treatment → Electroless nickel plating → Immersion gold → Post-treatment

Purpose:
1. Flat solderable surface
2.Excellent electrical conductivity and oxidation resistance

Principle: Displacement reaction

Main raw material: Gold salt

ENIG (Pre-treatment)

Purpose: Remove excessive oxidation and minor impurities from the copper surface.

Main raw material: SPS (Sodium persulfate)

Process key points: A. Brush pressure
B. SPS concentration
C. Line speed

ENIG (Nickel/Gold Deposition – Purpose)

Electroless nickel/immersion gold stage

Purpose: Form a thin nickel-gold layer on the copper surface via displacement reaction (nickel thickness 3‑5µm, gold thickness 0.025~0.1µm)

Main raw material: Gold salt (Potassium Gold Cyanide, PGC)

Process key points:
A. Control of bath concentration and temperature
B. Rinsing water flow rate
C. Stability of the automatic dosing system

ENIG (Post-treatment)

Purpose: Rinse off residual chemicals from the gold surface to prevent gold oxidation

Main material: DI water

Process key points:
A. Water quality
B. Line speed
C. Drying temperature

Electroplated Nickel Gold (Purpose)

Purpose: Deposit a nickel layer between the gold and copper as a barrier to prevent atomic interdiffusion between gold and copper over long-term use, which would expose the copper and affect contact performance; the main purpose of gold plating is to protect the copper surface from oxidation in air.

Main raw material: Gold salt

Process key points:
A. Control of bath concentration and temperature
B. Line speed control
C. Metallic contamination

Gold Finger (G/F) (Process)

Purpose: Excellent electrical conductivity, oxidation resistance, and wear resistance

Principle: Redox reaction

Main raw material: Gold salt

Application Scenarios for Surface Finish Processes

1. HASL process – Suitable for through-hole insertion (THT) devices, surface-mount resistors/capacitors/inductors, and SOP packages with a pitch of 0.5mm or above, where flatness requirements are not overly stringent.





2. OSP process – Short process flow, cost-effective…





3. ENIG process – Suitable for BGA, QFN, and QFP with a pitch below 0.4mm, for components requiring high surface flatness.