Electro-less Gold Plating, or Electroless Gold Plating, is Electroless Nickel Immersion Gold (ENIG or Eni/IAu), also known as PCB surface finish 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 (PTHs). It consists of an electroless nickel plating, covered with a thin layer of gold, which protects the nickel from oxidation, ensuring the stability.
The gold is typically applied by quick immersion in a solution containing gold salts. Some of the nickel is oxidization 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. During the electroless nickel plating process, nickel phosphorous can sometimes form. This reduces the reliability of solder joints, potentially affecting performance. When nickel phosphorous presents a concern, some companies choose electroless nickel electroless palladium immersion gold (ENEPIG). While more complicated, ENEPIG coating may be preferable for some applications.
Like regular electroless nickel, this plating type does not require an external power source to deposit the metallic coating on the copper features of PCB base material. ENIG is applied through a chemical reaction. 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 of Electro-less Gold Plating
Electro-less gold plating ENIG and ENEPIG are meant to replace the more conventional coatings of solder, 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 BGA component’s surface mounting), good oxidation resistance, and suitability for movable contacts such as membrane switches and plug-n 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.
ENIG Standards
It’s important to consider the gold layer’s thickness. Gold can begin to corrode the nickel layer if left in the bath too long, limiting the potential gold thickness to around 5 µin or less.
The quality and other aspects of ENIG coatings for PCBs are covered by IPC standard IPC-4552A. This standard outlines ENIG quality requirements for the electronics industry. While, IPC standard IPC-7095D, about BGA connectors, covers some ENIG problems and their remediation.