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Finishing coating: immersion gold
Immersion gold (ENIG) is a chemical precipitation of a gold layer on a nickel sublayer. One of the main alternatives to HASL technology. It is a finishing coating of Ni / Au category (Immersion Gold + Electroless Nickel).

The basic thickness of the gold layer is 0.05−0.1 micron of nickel 3−7 micron. The material meets all the mandatory requirements of EU RoHS directive.
— Immersion gold is a safe, lead-free finishing coating.
— It allows an additional in-circuit testing.
— Production of perfectly flat, smooth landing areas.
— It ensures good solderability and a high degree of surface conductivity.
— The material is durable - average period of storage is 6 months. It is not subject to ionic pollution; it does not oxidize.
— It is possible to apply on components with a small pitch (0.4-0.5 mm), the use of various types of soldering (including multiple) and assembling methods (for example, splicing crystals).
The finishing coating is applied to galvanic nickel using a chemical method of producing a thin gold layer. Due to its low mechanical strength, ENIG wears out quickly. That is why it is not recommended to apply immersion gold on the knife connectors.

In addition, when making intermetallic alloys and alloying gold with copper through a nickel layer, the possibility of black areas, a defect that prevents from assembling and further use of printed circuit boards, increases.
The appearance of black areas occurs due to alloying of metals (the process of dissolving a thin layer of gold in the nickel sublayer). As a result, when nickel precipitates, phosphorus is introduced into it. Another reason for this defect is overexposure during the process of flux soldering.
This process is multiple stage. At first, printed circuit boards undergo primary processing during which the surfaces are activated. Then, a layer of chemical nickel (3−7 micron thick) is precipitated. Using an exchange reaction, the reduction of gold layer 0.05−0.2 micron takes place.

The main function of gold is to protect nickel from possible oxidation, while nickel effectively prevents the diffusion of copper/gold. All coating processes occur in a special temperature regime (<90).
Physical and mechanical indexes