Definition

Immersion gold is a thin (0.05-0.1 µm) gold layer deposited by a chemical displacement reaction where gold ions replace surface atoms of an underlying metal (usually nickel in ENIG). Unlike electrolytic plating, immersion is self-limiting: deposition stops when the underlying metal is covered. Immersion gold protects the nickel from oxidation and provides a solderable surface. The thin layer dissolves into the solder during reflow, exposing the nickel for intermetallic formation. Immersion gold alone (without nickel) is not used due to gold embrittlement concerns.