Electroplating (or Galvanic Deposition)

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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 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.

Precious Metal Electrolytes

All precious metals can be electroplated with silver and gold by far the most widely used ones Table 1 and Table 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 1: Precious Metal Electrolytes for Technical Applications
Type of Electrolyte pH-Range Deposit Properties Hardness Areas of Application
HV Purity [kt]
Gold electrolytes
AUROMET TN 3.2 - 4.2 ca. 70 99.99% Au Base-deposits
AUROMET XPH 0.3 - 0.6 160 - 180 99.8% Au Base-deposits for stainless steel etc.
DODUREX COC 4.6 - 4.9 160 - 180 99.6% Au Printed circuit boards, connectors, contact parts, etc.;
hard gold coatings for rack and barrel plating
DODUREX HS 100 4.3 - 4.6 160 - 180 99.6% Au High speed process for connectors and PCB plating
PURAMET 202
PURAMET 402
5.5 - 6.5
7.0 - 7.5
60 - 80
60 - 80
99.99% Au
99.99% Au
High purity gold coatings for electrical and electronic parts incl. semi
conductors and PCBs; for demanding requirement on bonding properties
Platinum metal electrolytes
RHODOPLAT T strongly acidic 900 99.0% Rh Ductile rhodium deposits for thicker layers, reed contacts, sliding contacts
RUTHENIUMBAD stronglyacidic 900 99.0% Ru Crack free thick ruthenium deposits
PLATINBAD 5 stronglyacidic 240 - 260 99.9% Pt High temperature switching devices, etc
DODUPAL 3 7.0 - 8.0 220 - 250 99.9% Pd Thin palladium layers as diffusion barrier
DODUPAL 5 7.0 - 8.0 220 - 250 99.9% Pd Connectors and contact parts
DODUPAL 10 8.0 - 8.5 350 - 400 80.0% Pd Pd/Ni for connectors and contact parts
Silver electrolytes
ARGOL 30 cyanidebased a pprox. 90 99.9% Ag Contact parts, connectors
ARGOL HS 100 approx. 9.0 90 - 120 99.9% Ag
ARGOL 2000 approx. 12.0
ARGOL 400 160 - 180
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 3.

  • 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 2: Precious Metal Electrolytes for Decorative Applications
Type of Electrolyte pH-Bereich Deposit Properties Areas of Application
Hardness HV Purity [kt]
Gold electrolytes
DURAMET 1N14
DURAMET 2N18
DURAMET 3N
DURAMET 265S
DURAMET 333S
DURAMET 386S
3.4 - 3.8
3.4 - 3.8
3.4 - 3.8
3.4 - 3.8
3.2 - 3.6
3.4 - 3.8
1N
2N
3N
Hamilton
1N
Hamilton
23
23
23
23
23
23
Jewelry, watches, writing instruments, frames for glasses, fixtures
HELODOR 630 8.5 - 9.5 rose colored 22 Frames for glasses, jewelry, watches, writing instruments
DODUPLAT Y18
DODUPLAT Y18HS
9.5 - 10.5
9.5 - 11
2N
2N
18
16
Jewelry, watches, writing instruments
AUROMET TN
AUROMET 2
AUROMET 4
3.2 - 4.2
3.2 - 4.0
3.2 - 4.2
Pure Gold
2 - 3N
2 - 3N
23
23
23
Colored gold for jewelry, watches, writing instruments, fixtures, frames for glasses, etc.
Platinum metal electrolytes
RHODIOR 2
RHODIOR 20
RHODIOR 25
RHODIOR 40
< 1
< 1
< 1
< 1
white
white
white
white
99.99%Rh
99.99%Rh
99.99%Rh
99.99%Rh
Bright white rhodium coatings with high hardness for jewelry, watches, writing instruments, etc.
RUTHENIUMBAD strongly acidic grey/black 99.0%Ru Very hard and luster retaining Ruthenium coating
PLATINBAD strongly acidic white 99.9%Pt Jewelry, watches, etc.
DODUPAL 3 7.0 - 7.6 Pd - color 95%Pd Thin Pd/Zn coating as Ni-free diffusion barrier
DODUPAL 10 8.0 - 8.5 white 80%Pd Pd/Zn alloy for jewelry
DODUPAL 12 7.0 - 8.0 white 95%Pd Pd/Zn alloy for decorations
Silver electrolytes
ARGOL 2000 11.5 - 12.5 bright white 99.9%Ag Jewelry, watches, decoration
  • 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 3: Typical Electrolytes for the Deposition of Non-Precious Metals
Type of Electrolyte pH-Range Electrolyte
temperature [°C]
Current density
[A/dm²]
Yield [%]
Copper electrolytes
Cyanide copper 10 - 13 40 - 65 0 .5 - 4 70-95
Acidic copper <1 20 - 35 2 - 8 <100
Nickel electrolytes
Watts nickel
(Sulfate)
3 - 5 40 - 70 3 - 10 95-97
Sulfamate nickel 3 - 4 30 - 60 5 - 20 95-97
Tin electrolytes
Acidic tin (Sulfate) <1 18 - 25 1 - 3 <100
Alkaline tin >10 75 - 80 2 - 17 max.95
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

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

Electroplated Parts


  • Materials
Coatings
Precious metals Pure gold, hard gold (HV 150 – 250), palladium, palladium-nickel,
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,
composite materials such as aluminum – silicon carbide


  • Coating thickness
Precious metals: 0.2 – 5 μm (typical layer thicknesses; for Ag also up to 25 μm)
Non-precious metals: Up to approx. 20 μm
Tungsten 0.5 N
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


  • Quality criteria

Besides others the following layer parameters are typically monitored in-process and documented:

  • Coating thickness
  • Adhesion strength
  • Porosity
  • Solderability
  • Bonding property
  • Contact resistance

These quality tests are performed according to industry standards, internal standards, and customer specifications resp.

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.

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 Figure 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.

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

  • Immersion electroplating

Overall or selective electroplating of both sides of solid strips or pre-stamped parts in strip form

  • Stripe electroplating

Stripe electroplating on solid strips through wheel cells or using masking techniques

  • Selective electroplating

One-sided selective coating of solid, pre-stamped, or metallically belt-linked strips by brush plating

  • Spot electroplating

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)

Typical examples of electroplated semi-finished materials (overall or selectively)
  • Materials
Type of Coatings Coating Thickness Remarks
Gold electrolytes
Pure gold
Hard gold (AuCo 0.3)
0.1 - 3 μm In special cases up to 10 μm
Palladium-nickel (PdNi20) 0.1 - 5 μm Frequently with additional 0.2 μm AuCo 0.3
Silver 0.5 - 10 μm In special cases up to 40 μm
Non-precious Metals
Nickel 0.5 - 4 μm Diffusion barrier especially for gold layers
Copper 1 - 5 μm Intermediate layer used in tinning of CuZn
Tin, tin alloys 0.8 - 25 μm materials
  • Carrier Materials

Copper, copper alloys, nickel, nickel alloys, stainless steel

  • Dimensions and Tolerances
Dimensions and Tolerances


  • Tolerances
Coating thickness approx. ± 10 %
Coating thickness and position ± 0,5 mm


  • Quality Criteria

Mechanical properties and dimensional tolerances of the carrier materials follow the typical standards, i.e. DIN EN 1652 and 1654 for copper and copper alloys. 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.

References

References