Surface Coating Technologies

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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• Materials

• 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.

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

• 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

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• 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.

7.1.2.1 Introduction

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. 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).