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 printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich stru ...

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

electroless nickel plating Electroless nickel-phosphorus plating is a chemical process that deposits an even layer of nickel- phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution conta ...

, covered with a thin layer of

gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a Brightness, bright, slightly orange-yellow, dense, s ...

, which protects the nickel from oxidation. The gold is typically applied by quick immersion in a solution containing gold salts. 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 Solder mask, solder stop mask or solder resist is a thin lacquer-like layer of polymer that is usually applied to the copper traces of a printed circuit board (PCB) for protection against oxidation and to prevent solder bridges from formi ...

, 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 (; NA: ) is a 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 after cooling. Metals or alloys suitable ...

, 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, and suitability for movable contacts such as

membrane switch A membrane switch is a custom switch assembly that can open or close the conducting path in an electrical circuit and requires at least one contact made of or attached to a flexible substrate. Its assembly differs from traditional mechanical switch ...

es 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 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