Motivation
General coatings on nickel
Benefits of the nickel - phosphorus electrical contact industry
Advantage of nickel - phosphorus for the semiconductor industry
Corrosion resistance
Conclusion

General coatings on nickel :
There are mainly three types of electrodeposited nickel coatings used in industry :

• deposits of dull nickel,

• deposits of bright nickel,

• deposits of nickel - phosphorus .

These three deposits differ in their chemical composition and structure .

The matte nickel deposits are deposits of pure nickel . The grain size is about one micrometer and properties are those of nickel metal . Their hardness is about 200 HV and they are ductile .

The bright nickel is a nickel deposit containing traces of carbon and sulfur optionally . These elements come from organic chemical additives used in the electrolysis bath . The grain size is nanometric and deposits are hard ( HV 550-700 ) , brittle.

The nickel - phosphorous is a nickel deposit containing phosphorus in the form of intermetallic Ni 3 P. The phosphorus content can vary between 2 and 15 % by mass . Increasing the phosphorus content leads to increase resistance to corrosion . The hardness of the deposits is comparable to that of bright nickel deposits and the electroplates are brittle.

Nickel in these three forms is generally used as an underlayer to limit the diffusion phenomena between the substrate and the surface layer usually consists of tin , silver or gold . This layer is made necessary because the electrical contact properties ( static ) and solderability of nickel layers are poor .

Benefits of the nickel - phosphorus in electrical contact industry

The advantages of NiP underlayer for golden contacts have been known for over twenty years . With the tribological properties of NiP , gold contacts have a shelf life more than doubled for the same thickness of gold . Inversely , compared to the situation on a classical nickel underlayer (matte or bright) can reduce the thickness of gold (by a factor 2) , while maintaining the same contact properties .

The following figures , taken from an internal study of PEM represent the evolution of the coefficient of friction and contact resistance based on the number of cycles in the case of a fretting test simulating the vibration which is subject to a connector in an electrical system on board ( in a car for example) .

The conditions of this test are:

• ball-plane geometry ,

• Displacement : 50 microns ( + / - 25 microns )

• Displacement Frequency : 1 Hz

• Current : 20 mA

• Force ( normal ) applied : 2.5 N

Benefit of nickel - phosphorus for the semiconductor industry

During the phase of " wire bonding , the use of nickel - phosphorus increase the cohesion of the welded joint between aluminum and nickel .

Corrosion resistance

The corrosion resistance of NiP coatings is higher than that of nickel (matte or bright) and it is even as better as the phosphorus level is high .

Conclusion

The use of a NiP underlayer for gold ( said connector, that is to say, allied with a few tenths of % Ni or Co ) for making electrical contacts can increase very significantly the duration of life of connectors because the wear of surfaces in contact is smaller. This remains true even if the NiP layer cracks under the effect of stress ( shaping of the connector ).

The corrosion resistance is also improved from the time the connector is formed before surface treatment . Otherwise , cracking of the NiP layer can expose the substrate .

The NiP ( final layer ) is a coating that improves welding with aluminum wire called " wire bonding " which is one step in the assembly of many components for electronics .

The NiP surface is not efficient in terms of solderability .