ENIG, or Electroless Nickel Immersion Gold, is a two-layer ENIG plating process where a thin layer of electroless nickel is applied to a printed circuit board surface (the layer of nickel is about 120-240 μin thick), followed by a thin layer of immersion gold plating (the gold thickness is about 2-8 μin thick). The nickel protects the copper on the board, and the purpose of the gold plating is to extend the shelf-life of the package by ensuring the nickel is corrosion-resistant during storage and before the assembly of the board.
Other common types of surface finishes are:
HASL (Hot Air Solder Leveling)
Immersion Tin
Immersion Silver
OSP (Organic Solderability Preservative)
Hard Gold
All of these solve the basic problem of copper wiring vulnerability to ensure high-performance conductivity. The surface finishes of each require proper handling, surface treatment, and cleanliness verification to ensure the successful bonding of the components. ENIG, however, has been established as the preferable solder mount method for high-reliability electronics applications with complex circuitry design.