Electroless nickel immersion gold

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 (Ni) phosphorus (P) could form during the electroless nickel plating step, more specifically due to the solder mask sulfur substances leaching into the plating bath, greatly reducing solder joint reliability. To remedy this, a layer of electroless palladium can be plated onto the nickel to create ENEPIG surface finish.[1]

ENEPIG is a costlier finish, but offers the best characteristics for printed circuit boards. The process requires more steps, compared to other common finishing types.