ENIG is short for Electroless nickel immersion gold. It is one of the popular PCB surface finish types. ENIG is made of electroless nickel plating and a thin layer of immersion gold, protecting the nickel from oxidation.
ENIG has several advantages. Firstly, it has excellent surface planarity, particularly helpful for PCBs with large BGA packages. Secondly, oxidation resistance is very good. Besides, its usability is suitable for untreated contact surfaces such as membrane switches and contact points. ENIG also does not wet as evenly or easily as HASL.ENIG is a expensive finish, but offers the best characteristics for printed circuit boards. The process requires the most steps compared to other common finishing types. The ENIG also has its other good characteristics, such as excellent flatness for fine pitch components, robust for multiple reflow cycles, supports alternate connections, good for in-circuit testing and press fit connectors.
The Gold readily dissolves into solder and does not tarnish or oxidize. While the Nickel strengthens the PTH and increases thermal cycle life, the Nickel barrier prevents Copper dissolution during wave solder & rework.
The electroless nickel step is an auto-catalytic process, involving depositing nickel on the palladium-catalyzed copper surface. The reducing agent with nickel ions must be filled for proper concentration, temperature and pH levels, creating a consistent coating. During the immersion gold step, the gold adheres to the nickel-plated areas through molecular exchange. This protects the nickel until the soldering process. The gold thickness needs to meet certain tolerances to ensure the nickel maintains its solderability.
Immersion Gold is replacement chemistry. In other words, it attaches to the Nickel by replacing atoms of Nickel with atoms of Gold. Immersion Gold layer is to protect the Nickel surface. The recommended Gold thickness is 2-4 µin. Its primary purpose is to maintain the solderability of the Nickel surface. Nickel, as a barrier layer to Copper, can eventually diffuse to the surface of Gold and cause the same solderability issue, which just takes place at a slower rate than Copper.
Typical ENIG specifications are defined by IPC- 4552 Specification for Electroless Nickel/Immersion Gold. The Nickel thickness must be in the range of 3-6 µm. It is sufficient to prevent porosity through the base Copper.
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