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 PCBs 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. In addition, a "black-pad" of nickel oxide would form during the immersion gold plating step, greatly reducing solder joint reliability. To remedy this, a layer of electroless palladium can be plated onto the nickel to create ENEPIG surface finish.
ENIG is a costlier finish, but offers the best characteristics for printed circuit boards. The process requires more steps, compared to other common finishing types.
The electroless nickel step is an auto-catalytic process that involves depositing nickel on the palladium-catalyzed copper surface. The reducing agent containing nickel ions must be replenished in order to provide proper concentration, temperature and pH levels necessary to create a consistent coating. During the immersion gold step, the gold adheres to the nickel-plated areas through molecular exchange, which will protect the nickel until the soldering process. The gold thickness needs to meet certain tolerances to ensure that the nickel maintains its solderability.
IPC Standard IPC-4552 covers the quality and other aspects of ENIG finish on printed circuit boards.