Planar Immersion Gold PCB for reliable surface‑mount contacts

Electroless Nickel Immersion gold (Immersion Gold or ENIG) is a widely used surface finish process for printed circuit boards. It is a process that deposits a thin metal layer onto the copper pads of a PCB through a chemical displacement reaction.

Core Principle

The entire process primarily involves two steps:
◈ 1.Electroless Nickel Plating: A layer of nickel-phosphorus alloy (typically 3-6 microns thick) is deposited onto the cleaned copper surface via a chemical reaction. This nickel layer primarily serves as:
● A Barrier Layer: Prevents copper atoms from diffusing to the surface and reacting with gold to form brittle intermetallic compounds, which can compromise solder joint reliability.
● A Solderable Base: Provides a firm foundation for the subsequent gold layer and the final solder joint.

◈ 2.Immersion Gold Plating: The PCB with the nickel layer is immersed in a solution containing gold ions. Through a displacement reaction, the gold ions in the solution replace the nickel atoms on the nickel surface, depositing a very thin layer of pure gold (typically 0.05-0.15 microns thick). This gold layer primarily functions as:
● An Oxidation Barrier: Gold is chemically very stable, providing long-term protection for the underlying nickel layer against oxidation, ensuring excellent solderability.
● A High-Quality Contact Surface: Gold offers excellent conductivity and low contact resistance, making it ideal for connector contacts or switch points (an alternative for gold fingers).
● Preparation for Soldering: During soldering, this thin gold layer quickly dissolves into the solder, exposing the fresh, non-oxidized nickel surface, thereby achieving a good metallurgical bond.

Key Characteristics

◈ Flat Surface: Ideal for high-density, fine-pitch components (e.g., BGA, QFN) due to its very planar surface.
◈ Good Solderability and Shelf Life: The gold layer prevents oxidation, granting PCBs a long storage life.
◈ Suitable for Press-Fit and Contact Applications: The surface is relatively hard and wear-resistant, suitable for connectors and test points requiring multiple insertions.
◈ Lead-Free Process: Compliant with environmental regulations.
◈ Relatively Complex Process: Involves more steps than processes like Hot Air Solder Leveling (HASL), resulting in higher cost.

Main Applications

◈ High-Density Interconnect Boards: Consumer electronics like smartphones, tablets, and laptops.
◈ Boards Requiring Frequent Insertion/Contact: Communication equipment, server motherboards.
◈ RF/Wireless Circuits: Gold's good conductivity benefits signal transmission.
◈ Chip-on-Board Packaging.