Electroless nickel immersion gold

Electroless nickel immersion gold (ENIG or ENi/IAu), also known as immersion gold (Au), chemical Ni/Au or soft gold, is a metal plating process used in the manufacture of printed circuit boards (PCBs), to avoid oxidation and improve the solderability of copper contacts and plated through-holes. It consists of an electroless nickel plating, covered with a thin layer of gold, which protects the nickel from oxidation. The gold is typically applied by quick immersion in a solution containing gold salts such as gold sulfites with an example being Na3Au(SO3)2 or potassium gold cyanide (KAu(CN)2), with a complexing agent and a stabilizer. The bath needs to be replaced after a certain number of uses and the composition of the gold bath may be patented. Some of the nickel is oxidized to Ni2+ while the gold is reduced to metallic state. A variant of this process adds a thin layer of electroless palladium over the nickel, a process known by the acronym ENEPIG.

ENIG can be applied before or after the solder mask, also known as "overall" or "selective chemical Ni/Au," respectively. The latter type is more common and significantly cheaper as less gold is needed to cover only the solder pads.

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