2,315
edits
Changes
→Selective Electroplating
For the electroplating of metals, especially precious metals and 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 Solutions – Electrolytes====
The actual metal deposition occurs in the electrolytic solution which containsthe plating material as metal ions. Besides this basic ingredient, the electrolytescontain additional components depending on the processes used, such as forexample conduction salts, brighteners, and organic additives which are codepositedinto the coatings, influencing the final properties of the electroplatingdeposit.
===== Precious Metal Electrolytes=====
All precious metals can be electroplated with silver and gold , also they are by far the mostwidely used ones ''(Tables <xr id="tab:Precious Metal Electrolytes for Technical Applications"/><!--(Tab. 7.1 2)--> and <xr id="tab:Precious Metal Electrolytes for Decorative Applications"/>)<!--(Tab. 7.23)''-->.
The following precious metal electrolytes are the most important ones:
*Gold electrolytes <br/>For functional and decorative purposes pure gold, hard gold, low-karatgold, or colored gold coatings are deposited. Depending on therequirements, acidic, neutral, or cyanide electrolytes based onpotassium gold cyanide or cyanide free and neutral electrolytes basedon gold sulfite complexes are used.<br /> *Palladium and Platinum electrolytes <br/> 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.<br /> *Ruthenium electrolytes <br>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.<br />
*Palladium and Platinum Rhodium electrolytes <br/> Palladium is mostly deposited as a pure metal, for applications Rhodium deposits are extremely hard (HV 700 – 1000) and wear resistant. They also excel inelectrical contacts however also as palladium nickellight reflection. For higher Both properties are of valuejewelry allergy protective palladium intermediate layers are used for technical as well as adiffusion barrier over copper alloy substrate materialsdecorative applications. Platinum is mostlyused as a surface layer on While technical applications mainly require hard, stress and crack free coatings, the jewelry itemsindustry takes advantage of the light whitish deposits with high corrosion resistance.<br />
*Ruthenium Silver electrolytes <br/>Ruthenium coatings Silver electrolytes without additives generate dull soft deposits (HV ~ 80) which are mostly mainly used as contact layers on connectors with limited insertion and withdrawal cycles. Properties required for decorative purposes creating afashionable “grey” ruthenium color on , such as shiny bright surfaces and higher wear resistance, are achieved through various additives to the surface. An additional colorvariation is created by using “ruthenium-black” deposits which aremainly used in bi-color decorative articlesbasic Ag electrolyte.<br />
=====<!--7.1.1.1.2 -->Non-Precious Metal Electrolytes=====
The most important non-precious metals that are deposited by electroplatingare: Copper, nickel, tin, and zinc and as well as their alloys. The deposition is performed inthe form of pure metals with different electrolytes used ''(<xr id="tab:Typical Electrolytes for the Deposition of Non-Precious Metals"/><!--(Table 7.4)''-->).
*Copper electrolytes < br/>Copper electrolytes are used for either depositing an intermediate layer onstrips or parts, for building up a printed circuit board structure, or for thefinal strengthening during the production of printed circuit boards.<br />
*Tin electrolytes < br/>Pure tin and tin alloy deposits are used as dull or also bright surfacelayers on surfaces required for soldering. In the printed circuit boardmanufacturing , they are also utilized as an etch resist for the conductivepattern design after initial copper electroplating.<br />
*Nickel electrolytes <br>Nickel layers are mostly used as diffusion barriers during the gold plating of copper and copper alloys or as an intermediate layer for tinning<br /> *Bronze electrolytes <br>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.<br /> <figtable id="tab:Typical Electrolytes for the Deposition of Non-Precious Metals"><caption>'''<!--Table 7.24: -->Typical Electrolytes for the Deposition of Non-Precious Metals'''</caption> {| class="twocolortable" style="text-align: left; font-size: 12px"|-!Type of Electrolyte !pH-Range!Electrolyte<br />temperature [°C]!Current density<br />[A/dm²]!Yield [%]|-|colspan="5" |'''Copper electrolytes'''|-|Cyanide copper|10 - 13|40 - 65|0 .5 - 4|70-95|-|Acidic copper|<1|20 - 35|2 - 8|<100|-|colspan="5" |'''Nickel electrolytes'''|-|Watts nickel<br />(Sulfate)|3 - 5|40 - 70|3 - 10|95-97|-|Sulfamate nickel|3 - 4|30 - 60|5 - 20|95-97|-|colspan="5" |'''Tin electrolytes'''|-|Acidic tin (Sulfate)|<1|18 - 25|1 - 3|<100|-|Alkaline tin|>10|75 - 80|2 - 17|max.95|-|colspan="5" |'''Bronze electrolytes'''|-|DODUBRONCE W|Strongly alkaline|55 - 60|0.5 - 1.5||-|DODUBRONCE G|Strongly alkaline|45 - 50|2 - 3.5||-|DODUBRONCE AF|Strongly alkaline|58 - 62|0.5 - 1.5||}</figtable>
==== Electroplating of Parts====
The complete or all-around electroplating of small mass produced parts , likecontact springs, rivets, or pins is usually done as mass plating in electroplatingbarrels of different shape. During the electroplating process the parts arecontinuously moved and mixed to reach a uniform coating.
Larger parts are frequently electroplated on racks either totally or by differentmasking techniques also partially. Penetrating the coating into the interior ofdrilled holes or tubes can be achieved with the use of special fixtures.
'''Electroplated Parts'''
<div class="clear"></div>
'''Materials'''
{| class="twocolortable" style="text-align: left; font-size: 12px;width:70%"
|-
!colspan="2" style="text-align:center"|Coatings
|-
|Precious metals
|Pure gold, hard gold (HV 150 – 250), palladium, palladium-nickel,<br />rhodium, pure silver, hard silver (HV 130 – 160)
|-
|Non-precious metals
|Copper, nickel, tin, tin alloys
|-
|Carrier materials
|Copper, copper alloys, nickel, nickel alloys, iron, steel, aluminum, aluminum alloys,<br />composite materials such as aluminum – silicon carbide
|}
Besides others the following layer parameters are typically monitored in-process and documented:
*Coating thickness *Solderability*Adhesion strength *Porosity *Solderability*Bonding property*Porosity Contact *resistance These quality tests are performed according to industry standards, internalstandards, and customer specifications resp.
==== Electroplating of Semi-finished Materials====
The process for overall electroplating of strips, profiles, and wires is mostlyperformed on continuously operating reel-to-reel equipment. The processingsteps for the individual operations such as pre-cleaning, electroplatingor rinsing, rinsingare following the same principles as those employed in parts electroplating.
The overall coating is usually applied for silver plating and tin coating of stripsand wires. Compared to hard gold or palladium , these deposits are ratherductile, ensuring that during following stamping and forming operations , nocracks are generated in the electroplated layers.
==== Selective Electroplating====
Since precious metals are rather expensive , it is necessary to perform theelectroplating most economically and to coat only those areas that need the layersfor functional purposes. This leads from overall plating to selectiveelectroplating of strip material in continuous reel-to-reel processes. Dependingon the final parts design and the end application , the processes can be appliedto solid strip material , as well as pre-stamped and formed continuous strips orutilizing wire-formed or machined pins , which have been arranged as bandoliersattached to conductive metal strips.
The core part of selective precious metal electroplating is the actualelectroplating cell. In it Inside the cell, the anode is arranged closely to the cathodic polarizedmaterial strip. Cathode screens or masks may be applied between the two , tofocus the electrical field onto closely defined spots on the cathode strip.
Special high performance electrolytes are used in selective electroplating toreach short plating times and allow a high flow rate of the electrolyte for a fastelectrolyte exchange in the actual coating area.
For a closely targeted electroplating of limited precious metal coating of contactsprings , so-called brush-electroplating cells are employed ''(<xr id="fig:Brush Tampon plating cell"/><!--(Fig. 7.1)''-->). The “brush”or “tampon” consists of a roof shaped titanium metal part covered with a specialfelt-like material. The metal body has holes in defined spots , through which theelectrolyte reaches the felt. Also located In the same spots is also the anode , consisting of afine platinum net. The pre-stamped and in the contact area pre-formed contactspring part is guided under a defined pressure over the electrolyte soaked feltmaterial and gets wetted with the electrolyte. This allows the metalelectroplating in highly selective spots.
'''Summary of the processes for selective electroplating'''
*'''Immersion electroplating'''
Overall or selective electroplating of both sides of solid strips or pre-stampedparts in strip form
*'''Stripe electroplating'''
Stripe electroplating on solid strips through wheel cells or using maskingtechniques
*'''Selective electroplating'''
One-sided selective coating of solid, pre-stamped, or metallically belt-linkedstrips by brush plating
*'''Spot electroplating'''
Electroplating in spots of solid strips with guide holes or pre-stamped parts instrip form
'''Typical examples of electroplated semi-finished materials'''
(overall or selectively)
*Coating thickness *Solderability*Adhesion strength *Bonding property *Porosity *Contact resistance
These quality tests are performed according to industry industrial standards, internalstandards, and customer specifications resp.
==References==
[[Surface Coating Technologies#References|References]]
[[de:Galvanische_Beschichtung)]]