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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 within the μm range are required , which cannotbe obtained economically by conventional cladding methods(<xr id="tab:Overview_of_Important_Properties_of_Electroplated_Coatings_and_their_Applications"/><!--(Tab. 7.1)-->). 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.
<figtable id="tab:Overview_of_Important_Properties_of_Electroplated_Coatings_and_their_Applications"><caption>'''<!--Table 7.1: -->Overview of Important Properties of Electroplated Coatingsand their Applications'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Properties!Applications!Examples|-|Color|Pleasing appearance|Brass plated lamps and furniture hardware|-|Luster|Decorative appearance, Light reflection|Chrome plated fixtures, silver coated mirrors|-|Hardness / Wear Resistance|Prolonging of mechanical wear life|Hard chrome plated tools|-|Sliding properties|Improvement of dry sliding wear|Lead-tin-copper alloys for slide bearings|-|Chemical stability|Protection against chemical effects|Lead-Tin coatings as etch resist on PC boards|-|Corrosion resistance|Protection against environmental corrosion|Zinc coatings on steel parts|-|Electrical conductivity|Surface conduction of electrical current|Conductive path on PC boards|-|Thermal conductivity|Improved heat conduction on the surface|Copper plated bottoms for cookware|-|Machining capability|Shaping through machining|Copper coatings on low pressure cylinders|-|Magnetic properties|Increase of coercive force [[#text-reference|<sup>*)</sup>]] |Cobalt-nickel layers on magnetic storage media|-|Brazing and soldering|Brazing without aggressive fluxes|Tin-Lead coatings on PC board paths|-|Adhesion strength|Improvement of adhesion|Brass coating on reinforcement steel wires in tires|-|Lubricating properties|Improvement of formability|Copper plating for wire drawing|}</figtable><div id="text-reference">*) Coercive force= force to retaim the adopted magnetisation</div> To reduce the mechanical wear of thin surface layers on sliding and connectorcontacts , additional lubricants in liquid form are often used. On silver contacts, passivation coatings are applied as protection against silver sulfide formation.
==Coatings from the Liquid Phase==
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 electroplating, while the second one is a chemical deposition process.
=== Electroplating (or Galvanic Deposition)===
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 widely, depending on the process technologies employed.Electroplating processes are encompassing , besides the pure metal deposition, also preparative and post treatments of the goods to be coated. An importantparameter for creating strongly adhering deposits is that the surface of the goods has 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.
Main Articel: [[Electroplating (or Galvanic Deposition)| Electroplating (or Galvanic Deposition)]]
===<!--7.1.2 -->Electroless Plating===
Main Articel: [[Electroless Plating| Electroless Plating]]
====<!--7.1.2.4 Electroless -->Coatings from the Gaseous Phase (Vacuum Deposition of Nickel/Gold==)== Electroless deposited nickel coatings with an additional immersion layer of goldare seeing increased importance in the coating of printed circuit boards (PCBs).The process sequence is shown in ''term PVD (Fig. 7.2physical vapor deposition)'' using the example defines processes of theDODUCHEM process. Tabelle After the pre-cleaning (degreasing metal, metal alloys and etching) a palladium sulfate activator isused which activates the exposed copper surfaces on the printed circuit boardand thus facilitates the nickel chemical compounds deposition. The electroless working chemicalnickel electrolyte contains – besides other ingredients – Sodium-hypophosphite,which is reduced to phosphorus in a parallel occurring process andincorporated into the nickel deposit. At the temperature of 87 – 89°C a veryhomogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is depositedwith layer thicknesses > 5 μm possible. During a consecutive processing stepa very thin and uniform layer (< 0.1 μm) of gold is added in an immersionelectrolyte. This protects the electroless nickel layer against corrosion achievinga solderable and well bondable surface for thick or fine aluminum bond wires. It is possible to enhance this layer combination further vacuum by adding a immersionpalladium layer between the electroless nickel thermal and the gold coating(DODUBOND process)kinetic energy by particle bombardment. This Pd layer acts as a diffusion barrier and allows theusage of this surface combination also for gold wire bonding. As an alternative, for gold wire bonding applications a thicker gold layer of 0.2 –0.5 μm can be applied using an electroless process. Typical electrolytes work ata temperature of approx. 80°C with deposition rates of 0.3 – 0.4 μm per 30minutes. There The main processes are however limitations with these electroless electrolytesconcerning their stability and the robustness of the process compared to otherelectroplating processes which reduces their wider usage ''following four coating variations (Fig. 7.3)''. Fig. 7.3<xr id="tab:Coating compositionof a printed circuit board withreductively enhanced gold ====7.1.2.5 Immersion Deposition Characteristics of Tin====A tin coating by ion exchange is usually not possible since copper is the moreprecious metal. By adding thioMost Important PVD Processes"/><!-urea the electro-chemical potential of copper isreduced to a level (approx. 450 mV, significantly lower than tin) that allows theexchange reaction. Using a suitable electrolyte composition and enhancersolutions like with the DODUSTAN process ''(Fig. Table 7.46-->)'' tin coatings can beproduced that, even under usually unfavorable conditions of copperconcentrations of 7 g/l in the electrolyte, are well solderable. Fig. 7.4: Process flow for electroless tin deposition using the DODUSTAN process
The sputtering process has gained the economically most significant usage. Itsprocess principle is illustrated in ''(<xr id="fig:Principle of sputtering"/><!--(Fig. 7.5)''-->).
<br style="clear:both;"/>
*'''Materials'''
Selection of possible combinations of coating and substrate materials
<table border="1" cellspacing="0" styleclass="border-collapse:collapsetwocolortable"><tr><tdth rowspan="2"><p class="s8">Substrate Materials</p></tdth><tdth colspan="12"><p class="s8">Coating Materials</p></tdth></tr><tr><tdth><p class="s8">Substrate Materials<span>Ag</span></p></tdth><tdth><p><span>AgAu</span></p></tdth><td><p><span>Au</span></p></td><tdth><p><span>Pt</span></p></tdth><tdth><p><span>Pd</span></p></tdth><tdth><p><span>Cu</span></p></tdth><tdth><p><span>Ni</span></p></tdth><tdth><p><span>Ti</span></p></tdth><tdth><p><span>Cr</span></p></tdth><tdth><p><span>Mo</span></p></tdth><tdth><p><span>W</span></p></tdth><tdth><p><span>Ai</span></p></tdth><tdth><p><span>Si</span></p></tdth></tr><tr><td><p class="s8">Precious metal / alloys</p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">NF metals / alloys</p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Fe alloys / stainless steel</p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Special metals (Ti, Mo, W)</p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Carbide steels (WC-Co)</p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Ceramics (Al<span class="s16">2</span>O<span class="s16">3</span>, AlN)</p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Glasses (SiO<span class="s16">2</span>, CaF)</p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-leer.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr><tr><td><p class="s8">Plastics (PA, PPS)</p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td><td><p><span>[[File:K7-gef.png]]</span></p></td></tr></table> [[File:K7-gef.png]] can be produced[[File:K7-leer.png]] can be produced with intermediate layer
*'''Dimensions'''
{| class="twocolortable" style="text-align: left; font-size: 12px;width:40%"|-!colspan="2" style="text-align:center"|'''Dimensions'''|-|Coating thickness: |10 nm - 15 μm|-|Coating thicknesses for contact applications: |0.1 - 10 μm|}
For the geometry of semi-finished products to be coated , there are fewrestrictions. Only the coating of the inside of machined holes and tubing has
limitations.
*'''Tolerances'''
Coating thickness +±10 - 30 %, depending on the thickness
*'''Quality criteria'''
Depending on the application , the following parameters are tested and recorded(see also: Electroplating of parts):
*Coating thickness *Solderability *Adhesion strength *Bonding property*Porosity *Contact resistance
These quality tests are performed according to industry standards, internalstandards, and customer specifications resp.
==<!--7.3 -->Comparison of Deposition Processes==The individual deposition processes have in part different performancecharacteristics. For each end application , the optimal process has to be chosen, considering 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. <xr id="tab:Comparison of different coating processes"/><!--Table 7.7--> gives 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
applications.
The main advantage properties of hot tinning of copper the coatings are closely related to the coating process. Starting materials for cladding and sputtering targets precious metals and copper their alloys as compared totin electroplating is , which in the formation case of an inter-metallic copper-tin phase (Cu<sub>3</sub>Sngold and palladium based materials,Cu<sub>6</sub> Sn<sub>5</sub>) at the boundary between the carrier material are vacuum melted and the tin layertherefore exhibit a very high purity. This thin(0.3 – 0.5 μm) intermediate layerDuring electroplating, which is formed during depending on the type of electrolytes and the thermal tinningprocessdeposition parameters, is rather hard some electrolyte components such as carbon and reduces in connectors organic compounds are incorporated into the frictional force andmechanical wearprecious metal coating. Tin coatings produced by hot tinning have a good adhesion toLayers deposited from the substrate material and do not tend to tin whisker formationgaseous phase however are very pure.
Besides overall tin coating of strip material, the hot tinning can also be applied in the form of single or multiple stripes on both sides of a continuous substrate strip (<xr id="fig:Typical examples of hot tinned strip materials"/>).
<figure id="fig:Typical examples of hot tinned strip materials">
[[File:Typical examples of hot tinned strip materials.jpg|left|Figure 3: Typical examples of hot tinned strip materials]]
<br style="clear:both;"/>
</figure>
<br style="clear:both;"/>
*'''Materials'''
Coating materials: Pure tin, tin alloys<br>Substrate materials: Cu, CuZn, CuNiZn, CuSn, CuBe and others<br />
*'''Dimensions and Tolerances'''
{| class="twocolortable" style="text-align: left; font-size: 12px;width:40%"|-|Width of tinning: > |≥ 3 + ± 1 mm|-|Thickness of tinning: |1 - 15 μm|-|Tolerances (thickness): + |± 1 - +± 3 μm depending on tin thickness|}
*'''Quality Criteria'''
Mechanical strength and dimensional tolerances of hot tinned strips are closelyrelated to the standard for Cu and Cu alloy strips according to DIN EN 1652 andDIN EN 1654.
Quality criteria for the actual tin coatings are usually agreed upon separately.
==<!--7.5 -->Contact Lubricants==By using suitable lubricants , the mechanical wear and frictional oxidation ofsliding and connector contacts can be substantially reduced. In the electricalcontact technology , solid, as well as high and low viscosity liquid lubricants areused.
Contact lubricants have to fulfill a multitude of technical requirements:
*The lubricant layer should not increase the contact resistance; the wear reducing properties of the lubricant film should keep the contact resistance low and consistent over the longest possible operation time
Solid lubricants include for example 0.05 – 0.2 μm thin hard gold layers , whichare added as surface layers on top of the actual contact material. Among the various contact lubricants offered on the market contact lubricationoils have shown performance advantages. They are mostly synthetic, chemically inert, and silicone-free oils such as for example the DODUCONTAcontact lubricants which differ in their chemical composition and viscosity. For sliding contact systems with contact forces < 50 cN and higher slidingspeeds oils with a lower viscosity (<50 mPa·s) are preferential. For applicationswith higher contact forces and operating at higher temperatures contact oilswith a higher viscosity are advantageous. Contact oils are mainly suited forapplications at low current loads. At higher loads and in situations wherecontact separation occurs during the sliding operation thermal decompositionmay be initiated which causes the lubricating properties to be lost. Most compatible with plastics are the contact oil varieties B5, B12K, and B25,which also over longer operating times do not lead to tension stress corrosion.
For sliding contact systems with contact forces < 50 cN and higher sliding speeds, oils with a lower viscosity (< 50 mPa·s) are preferential. For applications with higher contact forces and operating at higher temperatures, contact oils with a higher viscosity are advantageous. Contact oils are mainly suited for applications at low current loads. At higher loads and in situations where contact separation occurs during the sliding operation, thermal decomposition may be initiated, which causes the lubricating properties to be lost.
==References==
das Korrosions- und Kontaktverhalten von Ag – Beschichtungen in
schwefelhaltiger Umgebung. VDE – Fachbericht 65 (2009) 51 – 58
[[de:Beschichtungsverfahren]]