Besides manufacturing contact materials from the solid phase, i.e. by melt orpowder metallurgy, the production starting in the liquid or gaseous phase isgenerally preferred when thin layers in the μm range are required which cannotbe obtained economically by conventional cladding methods. Such coatingsfulfill different requirements depending on their composition and thickness.They can serve as corrosion or wear protection or can fulfill the need for thincontact layers for certain technical applications. In addition they serve fordecorative purposes as a pleasing and wear resistant surface coating.

To reduce the mechanical wear of thin surface layers on sliding and connectorcontacts additional lubricants in liquid form are often used. On silver contactspassivation coatings are applied as protection against silver sulfide formation.

For thin coatings starting from the liquid phase two processes are useddifferentiated by the metallic deposition being performed either with or withoutthe use of an external electrical current source. The first one is electroplatingwhile the second one is a chemical deposition process.

For electroplating of metals, especially precious metals, water based solutions(electrolytes) are used which contain the metals to be deposited as ions (i.e.dissolved metal salts). An electric field between the anode and the work piecesas the cathode forces the positively charged metal ions to move to the cathodewhere they give up their charge and deposit themselves as metal on the surfaceof the work piece.Depending on the application, for electric and electronic or decorative end use,different electrolytic bath solutions (electrolytes) are used. The electroplatingequipment used for precious metal plating and its complexity varies widelydepending on the process technologies employed.Electroplating processes are encompassing besides the pure metal depositionalso preparative and post treatments of the goods to be coated. An importantparameter for creating strongly adhering deposits is the surface of the goods tobe metallic clean without oily or oxide film residues. This is achieved throughvarious pre-treatment processes specifically developed for the types of materialand surface conditions of the goods to be plated.In the following segments electrolytes – both precious and non-precious – aswell as the most widely used electroplating processes are described.

Electroless plating is defined as a coating process which is performed withoutthe use of an external current source. It allows a uniform metal coatingindependent of the geometrical shape of the parts to be coated. Because of thevery good dispersion capability of the used electrolytes also cavities and theinside of drilled holes in parts can be coated for example.In principal two different mechanisms are employed for electroless plating:processes in which the carrier material serves as a reduction agent (Immersionprocesses) and those in which a reduction agent is added to the electrolyte(Electroless processes).

The term PVD (physical vapor deposition) defines processes of metal, metalalloys, and chemical compounds deposition in a vacuum by adding thermal andkinetic energy through particle bombardment. The main processes are thefollowing four coating variations ''(Table 7.6)'':

In all four processes the coating material is transported in its atomic form to thesubstrate and deposited on it as a thin layer (a few nm to approx. 10 μm)

The sputtering process has gained the economically most significant usage. Itsprocess principle is illustrated in ''(Fig. 7.5)''.

Initially a gas discharge is ignited in a low pressure (10 – 1 Pa) argonatmosphere. The argon ions generated are accelerated in an electric field andimpact the target of material to be deposited with high energy. Caused by thisenergy atoms are released from the target material which condensate on theoppositely arranged anode (the substrate) and form a layer with high adhesionstrength. Through an overlapping magnetic field at the target location the

The advantages of the PVD processes and especially sputtering for electricalcontact applications are:

Coatings produced by PVD processes are used for contact applications, forexample on miniature-profiles, in electrical engineering and for electroniccomponents, for solderability in joining processes, for metalizing of nonconductivematerials, as well as in semiconductors, opto-electronics, optics,

There are few limitations regarding the geometrical shape of substrate parts.Only the interior coating of drilled holes and small diameter tubing can be moreproblematic (ratio of depth to diameter should be < 2:1). Profile wires, strips,and foils can be coated from one side or both; formed parts can be coatedselectively by using masking fixtures that at the same time serve as holdingfixtures.

The individual deposition processes have in part different performancecharacteristics. For each end application the optimal process has to be chosenconsidering all technical and economical factors. The main selection criteriashould be based on the electrical and mechanical requirements for the contactlayer and on the design characteristics of the contact component. Table 7.7gives some indications for a comparative evaluation of the different coatingprocesses.

The electroless metal coating is not covered here because of the low thicknessof deposits which makes them in most cases not suitable for contact

The main differences between the coating processes are found in the coatingmaterials and thickness. While mechanical cladding and sputtering allow theuse of almost any alloy material, electroplating processes are limited to metalsand selected alloys such as for example high-carat gold alloys with up to .3wt% Co or Ni. Electroplated and sputtered surface layers have a technologicaland economical upper thickness limit of about 10μm. While mechanical claddinghas a minimum thickness of approx. 1 μm, electroplating and sputteringcan also be easily applied in very thin layers down to the range of 0.1 μm.

The properties of the coatings are closely related to the coating process.Starting materials for cladding and sputtering targets precious metals and theiralloys which in the case of gold and palladium based materials are vacuummelted and therefore exhibit a very high purity. During electroplating, dependingon the type of electrolytes and the deposition parameters, some electrolytecomponents such as carbon and organic compounds are incorporated intothe precious metal coating. Layers deposited from the gaseous phase howeverare very pure.