Electroplating (or Galvanic Deposition)

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 (Tab. 7.1) and (Tab. 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 < br/>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 < br/>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 < br/>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 < 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

• 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.

Table 7.2: Typical Electrolytes for the Deposition of Non-Precious Metals

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 bild

• 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 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

als Bild?

• Quality criteria

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.

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 (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

• 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) bild

• Materials

Type of Coatings	Coating Thickness	Remarks
Precious Metals
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

Bild

Dimensions Carrier thickness d= 0.1 - 1 mm Carrier width B= 6 - 130 mm Distance b > 2 mm Coating width a= 2 - 30mm Coating thickness s = 0.2 - 5 μm (typical range) Distance from edge b > 0.5 mm depending on the carrier thickness and the plating process

• 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