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 board A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...

s (PCBs), to avoid oxidation and improve the solderability of copper contacts and plated through-holes. It consists of an

electroless nickel plating Electroless Deposition, Electroless nickel-phosphorus plating, also referred to as ''E-nickel'', is a chemical engineering, chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal o ...

, covered with a thin layer of

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

, 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 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.

Advantages and disadvantages

ENIG and ENEPIG are meant to replace the more conventional coatings of

solder Solder (; North American English, NA: ) is a fusible alloy, fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces aft ...

, such as hot air solder leveling (HASL/HAL). While more expensive and require more processing steps, they have several advantages, including excellent surface planarity (important for

ball grid array A ball grid array (BGA) is a type of surface-mount packaging (a chip carrier) used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be pu ...

component mounting), good oxidation resistance, prevents 'copper migration', and suitability for movable contacts such as membrane switches and plug-in connectors. Early ENIG processes had poor adhesion to copper and lower solderability than HASL. In addition, a non-conductive layer containing nickel and phosphorus, known as "black pad", could form over the coating due to sulfur-containing compounds from the solder mask leaching into the plating bath.

Standards

The quality and other aspects of ENIG coatings for PCBs are covered by

IPC IPC may refer to: Businesses and organizations Arts and media * Intellectual Property Committee, a coalition of US corporations with intellectual property interests * International Panorama Council, an international network of specialists in ...

Standard 4552A, while IPC standard 7095D, about ball array connectors, covers some ENIG problems and their remediation.

References

Printed circuit board manufacturing Metal plating {{metalworking-stub

Advantages and disadvantages

Standards

See also

References