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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 Coatings and their Applications'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
|-
!Material Properties!Hardness ConditionApplications!Tensile Strength Rm [MPa] min.!Elongation A<sub>10</sub> [%] min.!Vickers Hardness HVExamples
|-
|AuColor|R 140<br />R 170<br />R 200<br />R 240Pleasing appearance|140<br />170<br />200<br />240|30<br />3<br />2<br />1|20<br />50<br />60<br />70|- |AuAg20|R 190<br />R 250<br />R 320<br />R 390|190<br />250<br />320<br />390|25<br />2<br />1<br />1|38<br />70<br />95<br />115Brass plated lamps and furniture hardware
|-
|AuAg30Luster|R 220<br />R 260<br />R 320<br />R 380Decorative appearance, Light reflection|220<br />260<br />320<br />380|25<br />3<br />1<br />1|45<br />75<br />95<br />110Chrome plated fixtures, silver coated mirrors
|-
|AuAg25Cu5|R 400<br />R 470<br />R 570<br Hardness />R 700Wear Resistance|400<br />470<br />570<br />700Prolonging of mechanical wear life|25<br />4<br />2<br />2|90<br />120<br />160<br />185Hard chrome plated tools
|-
|AuAg20Cu10Sliding properties|R 480<br />R 560<br />R 720<br />R 820Improvement of dry sliding wear|480<br />560<br />720<br />820|20<br />3<br />1<br />1|125<br />145<br />190<br />230Lead-tin-copper alloys for slide bearings
|-
|AuAg26Ni3Chemical stability|R 350Protection 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|<br /sup>R 420*)<br /sup>R 500<br />R 570]] |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|-|350<br />420<br />500<br />570Lubricating properties|20<br />2<br />1<br />1Improvement of formability|85<br />110<br />135<br />155Copper 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===
Electroless plating is defined as a coating process which is performed withoutthe use of an external current source. It allows a uniform metal coating, independent 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).
Main Articel: [[Electroless Plating| Electroless Plating]]
==<!--7.2 -->Coatings from the Gaseous Phase (Vacuum Deposition)==The term PVD (physical vapor deposition) defines processes of metal, metalalloys, and chemical compounds deposition in a vacuum by adding thermal andkinetic energy through by particle bombardment. The main processes are thefollowing four coating variations ''(<xr id="tab:Characteristics of the Most Important PVD Processes"/><!--(Table 7.6-->)'':
*Vapor deposition *Sputtering (Cathode atomization)*Arc vaporizing *Ion implantation
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)
<figure id="fig:Principle of sputtering">[[File:Principle of sputtering.jpg|right|thumb|Figure 1: Principle of sputtering Ar = Argon atoms; e = Electrons; M = Metal atoms]]</figure>Initially, a gas discharge is ignited in a low pressure (10<sup>-1</sup> -1 Pa) argon atmosphere. The advantages argon ions generated, are accelerated in an electric field and impact the target of material to be deposited with high energy. Caused by this energy, atoms are released from the target material which condensate on the PVD processes oppositely arranged anode (the substrate) and especially sputtering for electricalcontact applications are:form a layer with high adhesion strength. Through an overlapping magnetic field at the target location, the deposition rate can be increased, making the process more economical.
<figure id="fig:Examples of vacuum coated semi finished materials and parts">
[[File:Examples of vacuum coated semi finished materials and parts.jpg|left|Figure 2: Examples of vacuum coated semi finished materials and parts]]
</figure>
<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.
==References==
das Korrosions- und Kontaktverhalten von Ag – Beschichtungen in
schwefelhaltiger Umgebung. VDE – Fachbericht 65 (2009) 51 – 58
[[de:Beschichtungsverfahren]]