Electroless nickel immersion gold (ENIG) is a type of surface plating used for printed circuit boards. It consists of an electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidation.
ENIG has several advantages over more conventional (and cheaper) surface platings such as HASL (solder), including excellent surface planarity (particularly helpful for PCB's with large BGA packages), good oxidation resistance, and usability for untreated contact surfaces such as membrane switches and contact points.
Early ENIG processes suffered from reliability issues which caused the ENIG finish to separate from the copper pads, taking the parts with them. ENIG also does not wet as evenly or easily as HASL.
IPC Standard IPC-4552 covers the quality and other aspects of ENIG finish on printed circuit boards.