Solder Surfaces for Printed Circuit Boards, Part 2: Electroless Nickel-Gold

The solder surface electroless nickel-gold (chem. NiAu) is also called ENIG (Electroless Nickel Immersion Gold). In a wet chemical process, a 3 to 6 µm thin layer of nickel (phosphorus-doped) is first deposited on the copper surfaces of the component connections. The nickel layer forms a barrier for the 0.05 to 0.125 µm thin gold layer, which is then also chemically applied. During the soldering process, the nickel-gold layer wets itself quickly and evenly with the solder. Between the tin of the solder alloy and the nickel of the solder surface a sequence of intermetallic nickel-tin phases with different composition is formed. The gold is completely dissolved during the soldering process. Ideally, after the solder has solidified, there is a uniform metallic zone a few micrometers that adheres firmly to the substrate and enables reliable SMD solder connections.

Solder surface electroless nickel gold
Micrograph of hole entrance with electroless nickel-gold surface

Planar surface and 12 months shelf life

Chemical NiAu offers not only a planar soldering surface but also a long shelf life. If stored properly, the PCB manufacturer provides a 12-month solderability warranty. The wetting of the solder pads is evaluated according to the standard J-STD-003. The resulting nickel-tin alloy is more brittle than a copper-tin compound. The solder surface is therefore not recommended for electronic assemblies that are exposed to strong bending or vibrations. Chemical NiAu is RoHS compliant and described in IPC-4552. The coating thicknesses are measured non-destructively with an X-ray test or in a destructive test with a microsection analysis. In the micrograph, however, only the nickel layer is visible. The gold layer is too thin to be detected and measured in the micrograph.

The nickel-gold surface is also suitable for switching contacts and wire bonding. ENIG is predestined for aluminium wire bonding according to MIL-STD 883. In our opinion, the gold layer is too thin for gold wire bonding. However, we do have customers who have validated the bonding process for themselves and in their applications the gold wire base works. Chemical NiAu is only suitable to a limited extent for press-fit technology because nickel is brittle and hard. However, some customers have also validated the process for themselves.

It is possible to combine this with electroplated nickel-gold on the connector contacts at the edge of the circuit board. The electroplated gold surface for PCB connectors (hard gold or connector gold) is significantly thicker and more robust than ENIG.

The most used soldering surface

Chemical NiAu is the most widely used solder surface in Europe. An estimated 50 % of all printed circuit boards manufactured in Europe have this end surface, and the trend is rising (source: ZVEI). At KSG, ENIG is also by far the most widely used PCB finish by customers. We estimate that with the rise of fine or ultra-fine conductor technologies, the manufacturing share of this surface will continue to increase.

Electroless nickel gold solder surface
SEM image of a surface with the typical “cauliflower” structure of the nickel deposition (source: IPC-4552)

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