Does ENIG go before or after solder mask?

Metal plating process

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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. 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.[1]

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, 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 component mounting), good oxidation resistance, 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.[1]

Standards

The quality and other aspects of ENIG coatings for PCBs are covered by IPC Standard A,[2] while IPC standard D, about ball array connectors, covers some ENIG problems and their remediation.[3]

See also

References

Electroless Nickel and Immersion Gold has a flat solderable surface and allows tighter placement of surface mount pads. The finish is solderable and bondable with aluminum wire.

The process line is fully automated with vibration in all process stations to ensure bubbles are removed from the holes.

Electroless Nickel (EN Plating) is applied after copper trace/feature definition (after Cu plate, image, and etch operations). It can be applied before or after the solder mask application providing a conformal coating on all exposed copper surfaces including sidewalls.

The deposit is a nominal 120 &#; 240 micro-inches of approximately 93% purity nickel over exposed copper traces and features. Intrinsic to the electroless process is a significant amount of phosphorus co-deposited with the nickel. We also have two electroless nickel choices one 6-8% phosphorous and another with 8-10% phosphorous.

The electroless nickel acts as a barrier layer between the copper and the gold, preventing unwanted (non-solderable) intermetallic formation. The electroless nickel layer also adds strength to plated through holes and vias.

Immersion Gold is applied after the electroless nickel process during ENIG plating and provides a gold coating on all exposed nickel surfaces including sidewalls.

Gold is applied by a molecular replacement process in which previously deposited nickel molecules are replaced by gold molecules in a processing tank. The gold deposited is 99.99% pure, and its thickness can build up to 2 to 5 micro-inches. By design, the immersion deposition process is self-limiting. When the underlying nickel is completely covered the reaction will stop and no more gold will deposit. Immersion gold provides excellent solderability properties, and it retards the passivation (oxidation) of the nickel layer.

The combined nickel/gold, ENIG plating, and metallization provide excellent corrosion resistance, solderability, pad coplanarity, and shelf life. Electroless Nickel and Immersion Gold is a good choice for fine-pitch SMT and BGA technologies.

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