Surface Finishes in PCB Manufacturing: Types, Functions, and Selection Guide

In PCB manufacturing, surface finish is one of the most important steps after bare board fabrication. It ensures that exposed copper pads remain solderable, protected, and reliable during component assembly. Without proper surface finish, copper quickly oxidizes, making soldering difficult and affecting long-term performance.

This article explains the functions of surface finishes, compares common types, and provides practical tips for selecting the right finish for your project.

What Is a Surface Finish in PCB?

A surface finish is a protective coating applied to exposed copper pads on a PCB. Its main purposes are:

• Prevent Copper Oxidation
  Bare copper oxidizes easily in air and humidity.
  A surface finish protects pads until assembly.

• Ensure Solderability
  Provides a solderable surface for reliable component mounting.

• Improve Shelf Life
  Some finishes allow boards to be stored for months without quality loss.

• Enhance Electrical and Mechanical Properties
  Affects contact resistance, wear resistance, and durability in harsh environments.

Common Types of PCB Surface Finishes

1. HASL (Hot Air Solder Leveling)

• Description: Molten solder is applied to exposed copper, then leveled with hot air.

• Advantages: Low cost, widely available; good shelf life.

• Limitations: Uneven surface, not suitable for fine-pitch components; higher thermal stress during process.

2. Lead-Free HASL

• Similar to HASL but uses lead-free solder alloys.

• RoHS compliant, widely used for environmental regulations.

• Slightly higher cost than traditional HASL.

3. ENIG (Electroless Nickel Immersion Gold)

• Structure: Nickel layer as a barrier, thin gold layer for oxidation protection.

• Advantages: Flat surface, ideal for fine-pitch and BGA components; excellent solderability and long shelf life.

• Limitations: Higher cost; risk of “black pad” defect if process not controlled well.

4. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

• Structure: Nickel + Palladium + Gold layers.

• Advantages: Prevents nickel corrosion (black pad); ideal for wire bonding and high-reliability applications.

• Limitations: More expensive than ENIG.

5. OSP (Organic Solderability Preservative)

• Description: Thin organic layer applied to protect copper.

• Advantages: Low cost; very flat surface, excellent for SMT.

• Limitations: Shorter shelf life; sensitive to handling (must be assembled quickly).

6. Immersion Tin

• Description: Thin tin layer chemically deposited onto copper.

• Advantages: Good planarity; lead-free and RoHS compliant.

• Limitations: Tin whisker growth risk; limited shelf life.

7. Immersion Silver

• Description: Thin silver layer on copper surface.

• Advantages: Flat surface; good electrical conductivity.

• Limitations: Tarnishes when exposed to air; requires careful packaging and storage.

Comparison Table of Surface Finishes

How to Select the Right Surface Finish?

• Assembly Requirements
  Fine-pitch BGAs → ENIG or ENEPIG
  Simple through-hole → HASL

• Shelf Life
  Long storage needed → ENIG or ENEPIG
  Quick assembly → OSP

• Cost Consideration
  Budget-sensitive → HASL or OSP
  High reliability → ENEPIG

• Application Environment
  Harsh environments (aerospace, medical) → ENEPIG
  High-speed signals → Immersion Silver

Conclusion

Surface finishes are essential in PCB manufacturing, directly impacting solderability, reliability, and product lifespan. Each type has unique benefits and limitations, so the choice must align with assembly requirements, budget, and application environment.

At suntech PCB, we provide a full range of surface finish options, from cost-effective OSP and HASL to high-reliability ENIG and ENEPIG. With precise process control and strict quality standards, we ensure your PCBs achieve the perfect balance of performance, durability, and cost efficiency.