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
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 containi ...
, covered with a thin layer of
gold
Gold is a chemical element with the 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 bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
, 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 forming b ...
, 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 {{unreferenced, date=November 2012
HASL or HAL (for hot air (solder) leveling) is a type of finish used on printed circuit boards (PCBs).
The PCB is typically dipped into a bath of molten solder so that all exposed copper surfaces are covered by s ...
(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 switc ...
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.
See also
*
Immersion silver plating
Immersion silver plating (or IAg plating) is a surface plating process that creates a thin layer of silver over copper objects. It consists in dipping the object briefly into a solution containing silver ions.
Immersion silver plating is used by ...
(IAg)
*
Immersion tin plating
Immersion may refer to:
The arts
* "Immersion", a 2012 story by Aliette de Bodard
* ''Immersion'', a French comic book series by Léo Quievreux
* ''Immersion'' (album), the third album by Australian group Pendulum
* ''Immersion'' (film), a 2021 ...
(ISn)
*
Organic solderability preservative
Organic solderability preservative or OSP is a method for coating of printed circuit boards. It uses a water-based organic compound that selectively bonds to copper and protects the copper until soldering.
The compounds typically used are from the ...
(OSP)
*
Reflow soldering
Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjec ...
*
Wave soldering
Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the ci ...
References
Printed circuit board manufacturing
Metal plating
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