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 (Tables 7.1 and 7.2). The following precious metal electrolytes are the most important ones:

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

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• 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

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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.