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| − | Besides manufacturing contact materials from the solid phase, i.e. by melt or powder metallurgy, the production starting in the liquid or gaseous phase is generally preferred when thin layers within the μm range are required, which cannot be obtained economically by conventional cladding methods (<xr id="tab:Overview_of_Important_Properties_of_Electroplated_Coatings_and_their_Applications"/><!--(Tab. 7.1)-->). Such coatings fulfill different requirements depending on their composition and thickness. | + | Besides manufacturing contact materials from the solid phase, i.e. by melt or |
| − | They can serve as corrosion or wear protection or can fulfill the need for thin contact layers for certain technical applications. In addition they serve for decorative purposes as a pleasing and wear resistant surface coating. | + | powder metallurgy, the production starting in the liquid or gaseous phase is |
| | + | generally preferred when thin layers in the μm range are required which cannot |
| | + | be obtained economically by conventional cladding methods. Such coatings |
| | + | fulfill different requirements depending on their composition and thickness. |
| | + | They can serve as corrosion or wear protection or can fulfill the need for thin |
| | + | contact layers for certain technical applications. In addition they serve for |
| | + | decorative purposes as a pleasing and wear resistant surface coating. |
| | | | |
| − | <figtable id="tab:Overview_of_Important_Properties_of_Electroplated_Coatings_and_their_Applications">
| + | Table 7.1: Overview of Important Properties of Electroplated Coatings |
| − | <caption>'''<!--Table 7.1:-->Overview of Important Properties of Electroplated Coatings and their Applications'''</caption>
| + | and their Applications |
| | | | |
| − | {| class="twocolortable" style="text-align: left; font-size: 12px"
| + | To reduce the mechanical wear of thin surface layers on sliding and connector |
| − | |-
| + | contacts additional lubricants in liquid form are often used. On silver contacts |
| − | !Properties
| + | passivation coatings are applied as protection against silver sulfide formation. |
| − | !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 connector contacts, additional lubricants in liquid form are often used. On silver contacts, passivation coatings are applied as protection against silver sulfide formation.
| + | ===7.1 Coatings from the Liquid Phase=== |
| | + | For thin coatings starting from the liquid phase two processes are used |
| | + | differentiated by the metallic deposition being performed either with or without |
| | + | the use of an external electrical current source. The first one is electroplating |
| | + | while the second one is a chemical deposition process. |
| | | | |
| − | ==Coatings from the Liquid Phase== | + | ===7.1.1 Electroplating (or Galvanic Deposition)=== |
| − | For thin coatings starting from the liquid phase, two processes are used differentiated by the metallic deposition being performed either with or without the use of an external electrical current source. The first one is electroplating, while 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 pieces |
| | + | as the cathode forces the positively charged metal ions to move to the cathode |
| | + | where they give up their charge and deposit themselves as metal on the surface |
| | + | of the work piece. |
| | + | Depending on the application, for electric and electronic or decorative end use, |
| | + | different electrolytic bath solutions (electrolytes) are used. The electroplating |
| | + | equipment 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 important |
| | + | parameter for creating strongly adhering deposits is the surface of the goods to |
| | + | be metallic clean without oily or oxide film residues. This is achieved through |
| | + | various pre-treatment processes specifically developed for the types of material |
| | + | and surface conditions of the goods to be plated. |
| | + | In the following segments electrolytes – both precious and non-precious – as |
| | + | well as the most widely used electroplating processes are described. |
| | | | |
| − | === Electroplating (or Galvanic Deposition)=== | + | ===7.1.1.1 Electroplating Solutions – Electrolytes=== |
| − | 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 pieces as the cathode, forces the positively charged metal ions to move to the cathode where they give up their charge and deposit themselves as metal on the surface of the work piece.
| + | The actual metal deposition occurs in the electrolytic solution which contains |
| − | Depending on the application, for electric and electronic or decorative end use, different electrolytic bath solutions (electrolytes) are used. The electroplating equipment used for precious metal plating and its complexity varies widely, depending on the process technologies employed.
| + | the plating material as metal ions. Besides this basic ingredient, the electrolytes |
| − | Electroplating processes are encompassing, besides the pure metal deposition, also preparative and post treatments of the goods to be coated. An important parameter for creating strongly adhering deposits is that the surface of the goods has to be metallic clean without oily or oxide film residues. This is achieved through various pre-treatment processes, specifically developed for the types of material and surface conditions of the goods to be plated.
| + | contain additional components depending on the processes used, such as for |
| − | In the following segments, electrolytes – both precious and non-precious – as well as the most widely used electroplating processes are described.
| + | example conduction salts, brighteners, and organic additives which are codeposited |
| | + | into the coatings, influencing the final properties of the electroplating |
| | + | deposit. |
| | | | |
| − | Main Articel: [[Electroplating (or Galvanic Deposition)| Electroplating (or Galvanic Deposition)]]
| + | ===7.1.1.1.1 Precious Metal Electrolytes=== |
| | + | All precious metals can be electroplated with silver and gold by far the most |
| | + | widely used ones ''(Tables 7.1 and 7.2)''. |
| | + | The following precious metal electrolytes are the most important ones: |
| | | | |
| − | ===<!--7.1.2-->Electroless Plating===
| + | *'''Gold electrolytes''' For functional and decorative purposes pure gold, hard gold, low-karat gold, or colored gold coatings are deposited. Depending on the requirements, acidic, neutral, or cyanide electrolytes based on potassium gold cyanide or cyanide free and neutral electrolytes based on gold sulfite complexes are used. |
| | | | |
| − | Electroless plating is defined as a coating process which is performed without the 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 the very good dispersion capability of the used electrolytes, also cavities and the inside of drilled holes in parts can be coated for example.
| + | *'''Palladium and Platinum electrolytes''' Palladium is mostly deposited as a pure metal, for applications in electrical contacts however also as palladium nickel. For higher value jewelry allergy protective palladium intermediate layers are used as a diffusion barrier over copper alloy substrate materials. Platinum is mostly used as a surface layer on jewelry items. |
| − | In principal, two different mechanisms are employed for electroless plating: processes in which the carrier material serves as a reduction agent (Immersion processes) and those in which a reduction agent is added to the electrolyte (Electroless processes).
| |
| | | | |
| − | Main Articel: [[Electroless Plating| Electroless Plating]]
| + | *'''Ruthenium electrolytes''' Ruthenium coatings are mostly used for decorative purposes creating a fashionable “grey” ruthenium color on the surface. An additional color variation is created by using “ruthenium-black” deposits which are mainly used in bi-color decorative articles. |
| | | | |
| − | ==<!--7.2-->Coatings from the Gaseous Phase (Vacuum Deposition)==
| + | *'''Rhodium electrolytes''' Rhodium deposits are extremely hard (HV 700 – 1000) and wear resistant. They also excel in light reflection. Both properties are of value for technical as well as decorative applications. While technical applications mainly require hard, stress and crack free coatings, the jewelry industry takes advantage of the light whitish deposits with high corrosion resistance. |
| − | The term PVD (physical vapor deposition) defines processes of metal, metal alloys and chemical compounds deposition in a vacuum by adding thermal and kinetic energy by particle bombardment. The main processes are the following four coating variations (<xr id="tab:Characteristics of the Most Important PVD Processes"/><!--(Table 7.6-->):
| |
| | | | |
| − | *Vapor deposition | + | *'''Silver electrolytes''' Silver electrolytes without additives generate dull soft deposits (HV ~ 80) which are mainly used as contact layers on connectors with limited insertion and withdrawal cycles. Properties required for decorative purposes such as shiny bright surfaces and higher wear resistance are achieved through various additives to the basic Ag electrolyte. |
| − | *Sputtering (Cathode atomization)
| |
| − | *Arc vaporizing
| |
| − | *Ion implantation
| |
| | | | |
| − | In all four processes, the coating material is transported in its atomic form to the substrate and deposited on it as a thin layer (a few nm to approx. 10 μm)
| + | Table 7.2: Precious Metal Electrolytes for Technical Applications |
| | | | |
| | + | ===7.1.1.1.2 Non-Precious Metal Electrolytes=== |
| | + | The most important non-precious metals that are deposited by electroplating |
| | + | are: Copper, nickel, tin, and zinc and their alloys. The deposition is performed in |
| | + | the form of pure metals with different electrolytes used ''(Table 7.4).'' |
| | | | |
| − | <figtable id="tab:Characteristics of the Most Important PVD Processes">
| + | *'''Copper electrolytes''' Copper electrolytes are used for either depositing an intermediate layer on strips or parts, for building up a printed circuit board structure, or for the final strengthening during the production of printed circuit boards. |
| − | <caption>'''<!--Table 7.6:-->Characteristics of the Most Important PVD Processes'''</caption>
| |
| | | | |
| − | {| class="twocolortable" style="text-align: left; font-size: 12px"
| + | *'''Tin electrolytes''' Pure tin and tin alloy deposits are used as dull or also bright surface layers on surfaces required for soldering. In the printed circuit board manufacturing they are also utilized as an etch resist for the conductive pattern design after initial copper electroplating. |
| − | |-
| |
| − | !Process
| |
| − | !Principle
| |
| − | !Process Gas Pressure
| |
| − | !Particle Energy
| |
| − | !Remarks
| |
| − | |-
| |
| − | |Vapor deposition
| |
| − | |Vaporizing in a crucible <br />(electron beam or resistance heating)
| |
| − | |10<sup>-3</sup> Pa
| |
| − | |< 2eV
| |
| − | |Separation of alloy components may occur
| |
| − | |-
| |
| − | |Arc vaporizing
| |
| − | |Vaporizing of the target <br />plate in an electrical arc
| |
| − | |10<sup>-1</sup> Pa-1Pa
| |
| − | |80eV-300eV
| |
| − | |Very good adhesion due to ion bombardement
| |
| − | |-
| |
| − | |Sputtering
| |
| − | |Atomizing of the target plate<br />(cathode) in a gas discharge
| |
| − | |10<sup>-1</sup> Pa-1Pa
| |
| − | |10eV-100eV
| |
| − | |Sputtering of non-conductive materials possible through RF operation
| |
| − | |-
| |
| − | |Ion implantation
| |
| − | |Combination of vapor <br />deposition and sputtering
| |
| − | |10<sup>-1</sup> Pa-1Pa
| |
| − | |80eV-300eV
| |
| − | |Very good adhesion from ion bombardment but also heating of the substrate material
| |
| − | |}
| |
| − | </figtable>
| |
| | | | |
| | + | Table 7.3: Precious Metal Electrolytes for Decorative Applications |
| | | | |
| − | The sputtering process has gained the economically most significant usage. Its process principle is illustrated in (<xr id="fig:Principle of sputtering"/><!--(Fig. 7.5)-->).
| + | *'''Nickel electrolytes''' Nickel layers are mostly used as diffusion barriers during the gold plating of copper and copper alloys or as an intermediate layer for tinning |
| | | | |
| − | <figure id="fig:Principle of sputtering">
| + | *'''Bronze electrolytes''' Bronze coatings – in white or yellow color tones – are used either as an allergy free nickel replacement or as a surface layer for decorative purposes. For technical applications the bronze layers are utilized for their good corrosion resistance and good brazing and soldering properties. |
| − | [[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 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 oppositely arranged anode (the substrate) and 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.
| |
| | | | |
| − | The advantages of the PVD processes and especially sputtering for electrical contact applications are:
| + | Table 7.2: Typical Electrolytes for the Deposition of Non-Precious Metals |
| | | | |
| − | *High purity of the deposit layers
| + | ===7.1.1.2 Electroplating of Parts=== |
| − | *Low thermal impact on the substrate
| + | The complete or all-around electroplating of small mass produced parts like |
| − | *Almost unlimited coating materials
| + | contact springs, rivets, or pins is usually done as mass plating in electroplating |
| − | *Low coating thickness tolerance
| + | barrels of different shape. During the electroplating process the parts are |
| − | *Excellent adhesion (also by using additional intermediate layers)
| + | continuously moved and mixed to reach a uniform coating. |
| | | | |
| − | Coatings produced by PVD processes are used for contact applications, for example on miniature-profiles, in electrical engineering and for electronic components, for solderability in joining processes, for metalizing of nonconductive materials, as well as in semiconductors, opto-electronics, optics and medical technology applications.
| + | Larger parts are frequently electroplated on racks either totally or by different |
| | + | masking techniques also partially. Penetrating the coating into the interior of |
| | + | drilled holes or tubes can be achieved with the use of special fixtures. |
| | | | |
| − | There are few limitations regarding the geometrical shape of substrate parts. Only the interior coating of drilled holes and small diameter tubing can be more problematic (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 coated selectively by using masking fixtures that at the same time serve as holding fixtures (<xr id="fig:Examples of vacuum coated semi finished materials and parts"/>).
| + | ===Electroplated Parts=== |
| | + | bild |
| | | | |
| − | <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 class="twocolortable">
| |
| − | <tr><th rowspan="2"><p class="s8">Substrate Materials</p></th><th colspan="12"><p class="s8">Coating Materials</p></th></tr>
| |
| − | <tr><th><p><span>Ag</span></p></th><th><p><span>Au</span></p></th><th><p><span>Pt</span></p></th><th><p><span>Pd</span></p></th><th><p><span>Cu</span></p></th><th><p><span>Ni</span></p></th><th><p><span>Ti</span></p></th><th><p><span>Cr</span></p></th><th><p><span>Mo</span></p></th><th><p><span>W</span></p></th><th><p><span>Ai</span></p></th><th><p><span>Si</span></p></th></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 few restrictions. 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, internal standards and customer specifications resp.
| |
| − |
| |
| − | ==<!--7.3-->Comparison of Deposition Processes==
| |
| − | The individual deposition processes have in part different performance characteristics. For each end application, the optimal process has to be chosen, considering all technical and economical factors. The main selection criteria should be based on the electrical and mechanical requirements for the contact layer 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 coating processes.
| |
| − |
| |
| − | The electroless metal coating is not covered here because of the low thickness of deposits, which makes them in most cases not suitable for contact
| |
| − | applications.
| |
| − |
| |
| − |
| |
| − | <figtable id="tab:Comparison of different coating processes">
| |
| − | <caption>'''<!--Table 7.7:-->Comparison of different coating processes'''</caption>
| |
| − |
| |
| − | {| class="twocolortable" style="text-align: left; font-size: 12px"
| |
| − | |-
| |
| − | !Process / Coating Properties
| |
| − | !Mechanical Processes (Cladding)
| |
| − | !Electroplating
| |
| − | !Vaccum Deposition (Sputtering)
| |
| − | |-
| |
| − | |Coating material
| |
| − | |formabe metal and alloys
| |
| − | |metals, alloys only limited
| |
| − | |metals and alloys
| |
| − | |-
| |
| − | |Coating thickness
| |
| − | |> 1μm
| |
| − | |0.1 - approx. 10 μm <br />(in special cases up to 100 μm)
| |
| − | |0.1 approx. 10 μm
| |
| − | |-
| |
| − | |Coating configuration
| |
| − | |selectively, stamping edges not coated
| |
| − | |all around and selectively<br />stamping edges coated
| |
| − | |mostly selectivity
| |
| − | |-
| |
| − | |Adhesion
| |
| − | |good
| |
| − | |good
| |
| − | |very good
| |
| − | |-
| |
| − | |Ductility
| |
| − | |good
| |
| − | |limited
| |
| − | |good
| |
| − | |-
| |
| − | |Purity
| |
| − | |good
| |
| − | |inclusions of foreign materials
| |
| − | |very good
| |
| − | |-
| |
| − | |Porosity
| |
| − | |good
| |
| − | |good for > approx. 1μm
| |
| − | |good
| |
| − | |-
| |
| − | |Temperature stability
| |
| − | |goodvery good
| |
| − | |good
| |
| − | |very good
| |
| − | |-
| |
| − | |Mechanical wear
| |
| − | |little
| |
| − | |very little
| |
| − | |little
| |
| − | |-
| |
| − | |Environmental impact
| |
| − | |little
| |
| − | |significant
| |
| − | |none
| |
| − | |}
| |
| − | </figtable>
| |
| − |
| |
| − | The main differences between the coating processes are found in the coating materials and thickness. While mechanical cladding and sputtering allow the use of almost any alloy material, electroplating processes are limited to metals and selected alloys, such as for example high-carat gold alloys with up to .3 wt% Co or Ni. Electroplated and sputtered surface layers have a technological and economical upper thickness limit of about 10μm. While mechanical cladding has a minimum thickness of approx. 1 μm, electroplating and sputtering can 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 their alloys, which in the case of gold and palladium based materials, are vacuum melted and therefore exhibit a very high purity. During electroplating, depending on the type of electrolytes and the deposition parameters, some electrolyte components such as carbon and organic compounds are incorporated into the precious metal coating. Layers deposited from the gaseous phase however are very pure.
| |
| − |
| |
| − | ==<!--7.4-->Hot (-Dipped) Tin Coated Strip Materials==
| |
| − | During hot-dip tinning, pre-treated strip materials are coated with pure tin or tin alloys from a liquid solder metal. During overall (or all-around) tinning the strips through a liquid metal melt. For strip tinning, rotating rolls are partially immersed into a liquid tin melt and transport the liquid onto the strip, which is guided above them. Through special wiping and gas blowing procedures, the deposited tin layer can be held within tight tolerances. Hot tinning is performed directly onto the base substrate material without any pre-coating with either copper or nickel. Special cast-on processes or the melting of solder foils onto the carrier strip, also allows the production of thicker solder layers ( > 15 μm).
| |
| − |
| |
| − | The main advantage of hot tinning of copper and copper alloys, compared to tin electroplating, is the formation of an inter-metallic copper-tin phase (Cu<sub>3</sub>Sn, Cu<sub>6</sub>Sn<sub>5</sub>) at the boundary between the carrier material and the tin layer. This thin (0.3 – 0.5 μm) intermediate layer, which is formed during the thermal tinning process, is rather hard and reduces the frictional force and mechanical wear in connectors. Tin coatings produced by hot tinning have a good adhesion to the substrate material and do not tend to tin whisker formation.
| |
| − |
| |
| − | A special process of hot tinning is the “Reflow” process. After depositing a tin coating by electroplating, the layer is short-time melted in a continuous process.
| |
| − | The properties of these reflow tin coatings are comparable to those created by conventional hot tinning.
| |
| − |
| |
| − | 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''' | | *'''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 closely related to the standard for Cu and Cu alloy strips according to DIN EN 1652 and DIN 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 of sliding and connector contacts can be substantially reduced. In the electrical contact technology, solid as well as high and low viscosity liquid lubricants are used.
| |
| − |
| |
| − | Contact lubricants have to fulfill a multitude of technical requirements:
| |
| − |
| |
| − | *They must wet the contact surface well; after the sliding operation the lubrication film must close itself again, i.e. mechanical interruptions to heal
| |
| − | *They should not transform into resins, not evaporate, and not act as dust collectors
| |
| − | *The lubricants should not dissolve plastics, they should not be corrosive to non-precious metals or initiate cracking through stress corrosion of plastic components
| |
| − | *The specific electrical resistance of the lubricants cannot be so low that wetted plastic surfaces lose their isolating properties
| |
| − | *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, which are added as surface layers on top of the actual contact material.
| |
| − |
| |
| − | Among the various contact lubricants offered on the market, contact lubrication oils have shown performance advantages. They are mostly synthetic, chemically inert and silicone-free oils which differ in their chemical composition and viscosity.
| |
| − |
| |
| − | 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.
| |
| − |
| |
| − |
| |
| − | ==<!--7.6-->Passivation of Silver Surfaces==
| |
| − | The formation of silver sulfide during the shelf life of components with silver surface in sulfur containing environments, can be significantly eliminated by coating them with an additional protective film layer (Passivation layer). For electrical contact use, such thin layers should be chemically inert and sufficiently conductive, otherwise they are easily broken by the applied contact force.
| |
| − | <figure id="fig:Typical process flow for the SILVERBRITE W ATPS process">
| |
| − | [[File:Typical process flow for the SILVERBRITE W ATPS process.jpg|right|thumb|Figure 4: Typical process flow for the SILVERBRITE W ATPS process]]
| |
| − | </figure>
| |
| − | The passivation process SILVERBRITE W ATPS is a water-based tarnish preventer for silver (<xr id="fig:Typical process flow for the SILVERBRITE W ATPS process"/>). It is free of chromium(VI) compounds and solvents. The passivating layer is applied by immersion, which creates a transparent organic protective film which barely changes the appearance and only slightly
| |
| − | increases the good electrical properties such as for example the contact resistance. The good solderability and bond properties of silver are not
| |
| − | negatively affected. Because of its chemical composition, this protective layer has some lubricating properties which reduce the insertion and withdrawal forces of connectors noticeably.
| |
| − |
| |
| − | ==References==
| |
| − |
| |
| − | Vinaricky, E. (Hrsg.): Elektrische Kontakte, Werkstoffe und Anwendungen.
| |
| − | Springer-Verlag, Heidelberg 2002
| |
| − |
| |
| − | Ganz, J.; Heber, J.; Macht, W.; Marka, E.: Galvanisch erzeugte
| |
| − | Edelmetallschichten für elektrische Kontakte. Metall 61 (2007) H.6, 394-398
| |
| − |
| |
| − | Song, J.: Edelmetalle in Steckverbindungen - Funktionen und Einsparpotential.
| |
| − | VDE - Fachbericht 67 (2011) 13-22
| |
| − |
| |
| − | Heber, J.: Galvanisch abgeschiedene Rhodiumschichten für den dekorativen
| |
| − | Bereich. Galvanotechnik, 98 (2007) H.12, 2931-2935
| |
| − |
| |
| − | Johler, W.; Pöffel, K.; Weik, G.; Westphal, W.: High Temperature Resistance
| |
| − | th Galvanically Deposited Gold Layers for Switching Contacts. Proc. 15 Holm
| |
| − | Conf. on Electrical Contacts, Chicago (2005) 48-54
| |
| − |
| |
| − | Grossmann, H. Schaudt, G.: Untersuchung über die Verwendbarkeit von
| |
| − | Überzügen der Platinmetallgruppe auf elektrotechnischen Verbindungselementen.
| |
| − | Galvanotechnik 67 (1976) 292-297
| |
| − |
| |
| − | Grossmann, H.; Vinaricky, E.: Edelmetalleinsparung in der Elektrotechnik durch
| |
| − | selektives Galvanisieren. In: Handbuch der Galvanotechnik. München, Hanser-
| |
| − | Verlag, 37 (1981) 132-141
| |
| − |
| |
| − | Grossmann, H.; Schaudt, G.: Hochgeschwindigkeitsabscheidung von Edelmetallen
| |
| − | auf Kontaktwerkstoffen. Galvanotechnik 84 (1993) H.5, 1541-1547
| |
| − |
| |
| − | Bocking, C.; Cameron, B.: The Use of High Speed Selective Jet
| |
| − | Electrodeposition of Gold for the Plating of Connectors. Trans. IMF. 72 (1994)
| |
| − | 33-40
| |
| | | | |
| − | Endres, B.: Selektive Beschichtungen von Kontaktmaterial im
| + | *'''Coating thickness''' |
| − | Durchzugsverfahren. Metalloberfläche 39 (1985) H.11, 400-404
| |
| | | | |
| − | Kaspar, F.; Marka, E.; Normann, N.: Eigenschaften von chemisch Nickel
| + | Precious metals: 0.2 – 5 μm (typical layer thicknesses; for Ag also up to 25 μm) |
| − | Goldschichten für Baugruppen der Elektrotechnik.
| + | Non-precious metals: Up to approx. 20 μm |
| − | VDE Fachbericht 47 (1995) 19-27
| + | Tolerances: Strongly varying depending on the geometrical shape of |
| | + | parts (up to 50% at a defined measuring spot). |
| | + | It is recommended to specify a minimum value for the |
| | + | coating thickness at a defined measuring spot |
| | | | |
| − | Schmitt; W.; Kißling, S.; Behrens, V.: Elektrochemisch hergestellte
| + | als Bild? |
| − | Schichtsysteme auf Aluminium für Kontaktanwendungen.
| |
| − | VDE - Fachbericht 67 (2011) 136-141
| |
| | | | |
| − | Freller, H.: Moderne PVD-Technologien zum Aufbringen dünner
| + | *'''Quality criteria''' |
| − | Kontaktschichten. VDE-Fachbericht 40 (1989) 33-39
| + | Besides others the following layer parameters are typically monitored in-process and documented: |
| | | | |
| − | Ganz, J.: PVD-Verfahren als Ergänzung der Galvanik. Metalloberfläche 45 (1991)
| + | *Coating thickness *Solderability |
| | + | *Adhesion strength *Bonding property |
| | + | *Porosity Contact *resistance |
| | | | |
| − | Schmitt, W.; Franz, S.; Heber, J.; Lutz, O.; Behrens, V.: Formation of Silver
| + | These quality tests are performed according to industry standards, internal |
| − | Sulfide Layers and their Influence on the Electrical Characteristics of Contacts in
| + | standards, and customer specifications resp. |
| − | th the Field of Information Technology. Proc. 24 Int. Conf.on Electr. Contacts,
| |
| − | Saint Malo, France (2008) 489-494
| |
| | | | |
| − | Buresch, I; Ganz, J.; Kaspar, F.: PVD-Beschichtungen und ihre Anwendungen
| + | ===7.1.1.3 Electroplating of Semi-finished Materials=== |
| − | für Steckverbinder. VDE-Fachbericht 59 (2003) 73-80
| + | The process for overall electroplating of strips, profiles, and wires is mostly |
| | + | performed on continuously operating reel-to-reel equipment. The processing |
| | + | steps for the individual operations such as pre-cleaning, electroplating, rinsing |
| | + | are following the same principles as those employed in parts electroplating. |
| | | | |
| − | Gehlert, B.: Edelmetalllegierungen für elektrische Kontakte.
| + | The overall coating is usually applied for silver plating and tin coating of strips |
| − | Metall 61 (2007) H.6, 374-379
| + | and wires. Compared to hard gold or palladium these deposits are rather |
| | + | ductile, ensuring that during following stamping and forming operations no |
| | + | cracks are generated in the electroplated layers. |
| | | | |
| − | Ganz, J.: Einsatz von Sputterverfahren bei komplexen
| + | ===7.1.1.4 Selective Electroplating=== |
| − | Beschichtungsaufgaben. JOT 11 (1997)
| + | Since precious metals are rather expensive it is necessary to perform the |
| | + | electroplating most economically and coat only those areas that need the layers |
| | + | for functional purposes. This leads from overall plating to selective |
| | + | electroplating of strip material in continuous reel-to-reel processes. Depending |
| | + | on the final parts design and the end application the processes can be applied |
| | + | to solid strip material as well as pre-stamped and formed continuous strips or |
| | + | utilizing wire-formed or machined pins which have been arranged as bandoliers |
| | + | attached to conductive metal strips. |
| | | | |
| − | Buresch, I.; Bögel, A.; Dürrschnabel, W.: Tin Coating for Electrical Components.
| + | The core part of selective precious metal electroplating is the actual |
| − | Metall 48 (1994) H.1, 11-14
| + | electroplating cell. In it the anode is arranged closely to the cathodic polarized |
| | + | material strip. Cathode screens or masks may be applied between the two to |
| | + | focus the electrical field onto closely defined spots on the cathode strip. |
| | | | |
| − | Buresch, I.; Horn, J.: Bleifreie Zinnoberflächen.
| + | Special high performance electrolytes are used in selective electroplating to |
| − | VDE-Fachbericht 61 (2005) 89-94
| + | reach short plating times and allow a high flow rate of the electrolyte for a fast |
| | + | electrolyte exchange in the actual coating area. |
| | | | |
| − | Adler, U.; Buresch, I.; Riepe, U.; Tietz, V.: Charakteristische Eigenschaften der
| + | For a closely targeted electroplating of limited precious metal coating of contact |
| − | schmelzflüssigen Verzinnung von Kupferwerkstoffen.
| + | springs so-called brush-electroplating cells are employed ''(Fig. 7.1)''. The “brush” |
| − | VDE-Fachbericht 63 (2007) 175-180
| + | or “tampon” consists of a roof shaped titanium metal part covered with a special |
| | + | felt-like material. The metal body has holes in defined spots through which the |
| | + | electrolyte reaches the felt. In the same spots is also the anode consisting of a |
| | + | fine platinum net. The pre-stamped and in the contact area pre-formed contact |
| | + | spring part is guided under a defined pressure over the electrolyte soaked felt |
| | + | material and gets wetted with the electrolyte. This allows the metal |
| | + | electroplating in highly selective spots. |
| | | | |
| − | Huck, M.: Einsatz von Schmiermitteln auf Gleit- und Steckkontakten.
| + | Fig. 7.1: |
| − | Metalloberfläche (1982) 429-435
| + | Brush (or “Tampon”) plating cell; |
| | + | 1 Strip; 2 Anode; 3 Electrolyte feed; |
| | + | 4 Felt covered cell |
| | | | |
| − | Abbott, W.,H.: Field and Laboratory Studies of Corrosion Inhibiting Lubricants
| + | For special applications, such as for example electronic component substrates, |
| − | for Gold-Plated Connectors. Proc. HOLM Conf.on Electrical Contacts, Chicago | + | a dot shaped precious metal coating is required. This is achieved with two belt |
| − | (1996) 414-428
| + | masks running synchronous to the carrier material. One of these two masks has |
| | + | windows which are open to the spot areas targeted for precious metal plating |
| | + | coverage. |
| | | | |
| − | Noel, S.; Alarmaguy, D.; Correia, S.; Gendre, P.: Study of Thin Underlayers to
| + | ===Summary of the processes for selective electroplating=== |
| − | Hinder Contact resistance Increase Due to Intermetallic Compound Formation.
| |
| − | th Proc. 55 IEEE Holm Conf. on Electrical Contacts, Vancouver, BC,
| |
| − | Canada (2009) 153 – 159
| |
| | | | |
| − | Weik, G.; Johler, W.; Schrank, C.: Zuverlässigkeit und Eigenschaften von Gold –
| + | *'''Immersion electroplating''' |
| − | Schichten bei hohen Einsatztemperaturen.
| + | Overall or selective electroplating of both sides of solid strips or pre-stamped |
| − | VDE – Fachbericht 65, (2009) 13 – 21
| + | parts in strip form |
| | | | |
| − | Buresch, I.; Hack, M.: Eigenschaften von Zinnschichten für elektromechanische
| + | *'''Stripe electroplating''' |
| − | Bauelemente – Einflussfaktoren und ihre Auswirkungen. VDE – Fachbericht 65,
| + | Stripe electroplating on solid strips through wheel cells or using masking |
| − | (2009) 23 – 30
| + | techniques |
| | | | |
| − | Buresch, I.: Effekte intermetallischer Phasen auf die Eigenschaften von
| + | *'''Selective electroplating''' |
| − | Zinnoberflächen auf Kupferlegierungen. VDE – Fachbericht 67 (2011) 38-46
| + | One-sided selective coating of solid, pre-stamped, or metallically belt-linked |
| | + | strips by brush plating |
| | | | |
| − | Schmitt, W.; Heber, J.; Lutz, O.; Behrens, V.: Einfluss des Herstellverfahrens auf
| + | *'''Spot electroplating''' |
| − | das Korrosions- und Kontaktverhalten von Ag – Beschichtungen in
| + | Electroplating in spots of solid strips with guide holes or pre-stamped parts in |
| − | schwefelhaltiger Umgebung. VDE – Fachbericht 65 (2009) 51 – 58
| + | strip form |
| | | | |
| − | [[de:Beschichtungsverfahren]]
| + | ===Typical examples of electroplated semi-finished materials=== |
| | + | (overall or selectively) |
| | + | bild |
Besides manufacturing contact materials from the solid phase, i.e. by melt or
powder metallurgy, the production starting in the liquid or gaseous phase is
generally preferred when thin layers in the μm range are required which cannot
be obtained economically by conventional cladding methods. Such coatings
fulfill different requirements depending on their composition and thickness.
They can serve as corrosion or wear protection or can fulfill the need for thin
contact layers for certain technical applications. In addition they serve for
decorative purposes as a pleasing and wear resistant surface coating.
Table 7.1: Overview of Important Properties of Electroplated Coatings
and their Applications
To reduce the mechanical wear of thin surface layers on sliding and connector
contacts additional lubricants in liquid form are often used. On silver contacts
passivation coatings are applied as protection against silver sulfide formation.
7.1 Coatings from the Liquid Phase
For thin coatings starting from the liquid phase two processes are used
differentiated by the metallic deposition being performed either with or without
the use of an external electrical current source. The first one is electroplating
while the second one is a chemical deposition process.
7.1.1 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 pieces
as the cathode forces the positively charged metal ions to move to the cathode
where they give up their charge and deposit themselves as metal on the surface
of the work piece.
Depending on the application, for electric and electronic or decorative end use,
different electrolytic bath solutions (electrolytes) are used. The electroplating
equipment 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 important
parameter for creating strongly adhering deposits is the surface of the goods to
be metallic clean without oily or oxide film residues. This is achieved through
various pre-treatment processes specifically developed for the types of material
and surface conditions of the goods to be plated.
In the following segments electrolytes – both precious and non-precious – as
well as the most widely used electroplating processes are described.
7.1.1.1 Electroplating Solutions – Electrolytes
The actual metal deposition occurs in the electrolytic solution which contains
the plating material as metal ions. Besides this basic ingredient, the electrolytes
contain additional components depending on the processes used, such as for
example conduction salts, brighteners, and organic additives which are codeposited
into the coatings, influencing the final properties of the electroplating
deposit.
7.1.1.1.1 Precious Metal Electrolytes
All precious metals can be electroplated with silver and gold by far the most
widely used ones (Tables 7.1 and 7.2).
The following precious metal electrolytes are the most important ones:
- Gold electrolytes For functional and decorative purposes pure gold, hard gold, low-karat gold, or colored gold coatings are deposited. Depending on the requirements, acidic, neutral, or cyanide electrolytes based on potassium gold cyanide or cyanide free and neutral electrolytes based on gold sulfite complexes are used.
- Palladium and Platinum electrolytes Palladium is mostly deposited as a pure metal, for applications in electrical contacts however also as palladium nickel. For higher value jewelry allergy protective palladium intermediate layers are used as a diffusion barrier over copper alloy substrate materials. Platinum is mostly used as a surface layer on jewelry items.
- Ruthenium electrolytes Ruthenium coatings are mostly used for decorative purposes creating a fashionable “grey” ruthenium color on the surface. An additional color variation is created by using “ruthenium-black” deposits which are mainly used in bi-color decorative articles.
- Rhodium electrolytes Rhodium deposits are extremely hard (HV 700 – 1000) and wear resistant. They also excel in light reflection. Both properties are of value for technical as well as decorative applications. While technical applications mainly require hard, stress and crack free coatings, the jewelry industry takes advantage of the light whitish deposits with high corrosion resistance.
- Silver electrolytes Silver electrolytes without additives generate dull soft deposits (HV ~ 80) which are mainly used as contact layers on connectors with limited insertion and withdrawal cycles. Properties required for decorative purposes such as shiny bright surfaces and higher wear resistance are achieved through various additives to the basic Ag electrolyte.
Table 7.2: Precious Metal Electrolytes for Technical Applications
7.1.1.1.2 Non-Precious Metal Electrolytes
The most important non-precious metals that are deposited by electroplating
are: Copper, nickel, tin, and zinc and their alloys. The deposition is performed in
the form of pure metals with different electrolytes used (Table 7.4).
- Copper electrolytes Copper electrolytes are used for either depositing an intermediate layer on strips or parts, for building up a printed circuit board structure, or for the final strengthening during the production of printed circuit boards.
- Tin electrolytes Pure tin and tin alloy deposits are used as dull or also bright surface layers on surfaces required for soldering. In the printed circuit board manufacturing they are also utilized as an etch resist for the conductive pattern design after initial copper electroplating.
Table 7.3: Precious Metal Electrolytes for Decorative Applications
- Nickel electrolytes Nickel layers are mostly used as diffusion barriers during the gold plating of copper and copper alloys or as an intermediate layer for tinning
- Bronze electrolytes Bronze coatings – in white or yellow color tones – are used either as an allergy free nickel replacement or as a surface layer for decorative purposes. For technical applications the bronze layers are utilized for their good corrosion resistance and good brazing and soldering properties.
Table 7.2: Typical Electrolytes for the Deposition of Non-Precious Metals
7.1.1.2 Electroplating of Parts
The complete or all-around electroplating of small mass produced parts like
contact springs, rivets, or pins is usually done as mass plating in electroplating
barrels of different shape. During the electroplating process the parts are
continuously moved and mixed to reach a uniform coating.
Larger parts are frequently electroplated on racks either totally or by different
masking techniques also partially. Penetrating the coating into the interior of
drilled holes or tubes can be achieved with the use of special fixtures.
Electroplated Parts
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Precious metals: 0.2 – 5 μm (typical layer thicknesses; for Ag also up to 25 μm)
Non-precious metals: Up to approx. 20 μm
Tolerances: Strongly varying depending on the geometrical shape of
parts (up to 50% at a defined measuring spot).
It is recommended to specify a minimum value for the
coating thickness at a defined measuring spot
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Besides others the following layer parameters are typically monitored in-process and documented:
- Coating thickness *Solderability
- Adhesion strength *Bonding property
- Porosity Contact *resistance
These quality tests are performed according to industry standards, internal
standards, and customer specifications resp.
7.1.1.3 Electroplating of Semi-finished Materials
The process for overall electroplating of strips, profiles, and wires is mostly
performed on continuously operating reel-to-reel equipment. The processing
steps for the individual operations such as pre-cleaning, electroplating, rinsing
are following the same principles as those employed in parts electroplating.
The overall coating is usually applied for silver plating and tin coating of strips
and wires. Compared to hard gold or palladium these deposits are rather
ductile, ensuring that during following stamping and forming operations no
cracks are generated in the electroplated layers.
7.1.1.4 Selective Electroplating
Since precious metals are rather expensive it is necessary to perform the
electroplating most economically and coat only those areas that need the layers
for functional purposes. This leads from overall plating to selective
electroplating of strip material in continuous reel-to-reel processes. Depending
on the final parts design and the end application the processes can be applied
to solid strip material as well as pre-stamped and formed continuous strips or
utilizing wire-formed or machined pins which have been arranged as bandoliers
attached to conductive metal strips.
The core part of selective precious metal electroplating is the actual
electroplating cell. In it the anode is arranged closely to the cathodic polarized
material strip. Cathode screens or masks may be applied between the two to
focus the electrical field onto closely defined spots on the cathode strip.
Special high performance electrolytes are used in selective electroplating to
reach short plating times and allow a high flow rate of the electrolyte for a fast
electrolyte exchange in the actual coating area.
For a closely targeted electroplating of limited precious metal coating of contact
springs so-called brush-electroplating cells are employed (Fig. 7.1). The “brush”
or “tampon” consists of a roof shaped titanium metal part covered with a special
felt-like material. The metal body has holes in defined spots through which the
electrolyte reaches the felt. In the same spots is also the anode consisting of a
fine platinum net. The pre-stamped and in the contact area pre-formed contact
spring part is guided under a defined pressure over the electrolyte soaked felt
material and gets wetted with the electrolyte. This allows the metal
electroplating in highly selective spots.
Fig. 7.1:
Brush (or “Tampon”) plating cell;
1 Strip; 2 Anode; 3 Electrolyte feed;
4 Felt covered cell
For special applications, such as for example electronic component substrates,
a dot shaped precious metal coating is required. This is achieved with two belt
masks running synchronous to the carrier material. One of these two masks has
windows which are open to the spot areas targeted for precious metal plating
coverage.
Summary of the processes for selective electroplating
Overall or selective electroplating of both sides of solid strips or pre-stamped
parts in strip form
Stripe electroplating on solid strips through wheel cells or using masking
techniques
One-sided selective coating of solid, pre-stamped, or metallically belt-linked
strips by brush plating
Electroplating in spots of solid strips with guide holes or pre-stamped parts in
strip form
Typical examples of electroplated semi-finished materials
(overall or selectively)
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