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).

7.1.2.2 Immersion Processes

The immersion processes are usually applied in the plating of the metals gold, silver, and tin. If the material to be coated is less precious, i.e. exhibits a negative standard potential against the metal ions in the surrounding solution, it goes into solution releasing electrons while the more precious metal ions are reduced by absorbing electrons and being deposited on the electrode. This process can continue until the complete surface of the substrate is covered with a thin layer of the more precious metal. This limits the maximum achievable layer thickness to approx. 0.1 μm (Table 7.5).

Table 7.5: Immersion Gold Electrolytes

Type of Electrolyte	pH-Range	Coating Properties	Application Ranges
Type of Electrolyte	pH-Range	Hardness HV 0.025	Punity	Application Ranges
Immersion Gold electrolytes
AUROL 4 AUROL 16 AUROL 20	3.8 - 4.2 5.8 - 6.2 5.8 - 6.2 5.8 - 6.2	60 - 80 60 - 80 60 - 80 60 - 80	99.99% Au 99.99% Au 99.99% Au 99.99% Au	Thin gold layers on Ni, Ni alloys, Fe and Fe alloys for PCB technology and technical applications

7.1.2.3 Electroless Processes

The electroless metal plating with adding reduction agents to the electrolyte is based on the oxidation of the reducing agent with release of electrons which then in turn reduce the metal ions. To achieve a controlled deposition from such solutions the metal deposition has to happen through the catalytic influence of the substrate surface.

Otherwise a “wild” uncontrollable deposition would occur. In most cases palladium containing solutions are used for the activation which seed the surfaces with palladium and act as catalysts in the copper and nickel electrolytes.

The electrolytes contain besides the complex ion compounds of the metals to be deposited also stabilizers, buffer and accelerator chemicals, and a suitable reduction agent.

These electrolytes are usually operating at elevated temperatures (50° – 90°C). The deposits contain besides the metals also process related foreign inclusions such as for example decomposition products of the reduction agents. The electroless processes are used mainly for copper, nickel, and gold deposits.

7.1.2.4 Electroless Deposition of Nickel/Gold

Electroless deposited nickel coatings with an additional immersion layer of gold are seeing increased importance in the coating of printed circuit boards (PCBs). The process sequence is shown in (Fig. 7.2) using the example of the DODUCHEM process.

Tabelle

After the pre-cleaning (degreasing and etching) a palladium sulfate activator is used which activates the exposed copper surfaces on the printed circuit board and thus facilitates the nickel deposition. The electroless working chemical nickel electrolyte contains – besides other ingredients – Sodium-hypophosphite, which is reduced to phosphorus in a parallel occurring process and incorporated into the nickel deposit. At the temperature of 87 – 89°C a very homogeneous nickel-phosphorus alloy layer with approx. 9 wt% P is deposited with layer thicknesses > 5 μm possible. During a consecutive processing step a very thin and uniform layer (< 0.1 μm) of gold is added in an immersion electrolyte. This protects the electroless nickel layer against corrosion achieving a solderable and well bondable surface for thick or fine aluminum bond wires.

It is possible to enhance this layer combination further by adding a immersion palladium layer between the electroless nickel and the gold coating (DODUBOND process). This Pd layer acts as a diffusion barrier and allows the usage of this surface combination also for gold wire bonding.

As an alternative, for gold wire bonding applications a thicker gold layer of 0.2 – 0.5 μm can be applied using an electroless process. Typical electrolytes work at a temperature of approx. 80°C with deposition rates of 0.3 – 0.4 μm per 30 minutes. There are however limitations with these electroless electrolytes concerning their stability and the robustness of the process compared to other electroplating processes which reduces their wider usage (Fig. 7.3).

Fig. 7.3: Coating composition of a printed circuit board with reductively enhanced gold

7.1.2.5 Immersion Deposition of Tin

A tin coating by ion exchange is usually not possible since copper is the more precious metal. By adding thio-urea the electro-chemical potential of copper is reduced to a level (approx. 450 mV, significantly lower than tin) that allows the exchange reaction. Using a suitable electrolyte composition and enhancer solutions like with the DODUSTAN process (Fig. 7.4) tin coatings can be produced that, even under usually unfavorable conditions of copper concentrations of 7 g/l in the electrolyte, are well solderable.

Fig. 7.4: Process flow for electroless tin deposition using the DODUSTAN process

The immersion tin deposition is suitable for the production of a well solderable surface on printed circuit boards and electronic components. It is also used as an etch resist against ammonia based solutions or as corrosion and oxidation protection of copper surfaces.

7.2 Coatings from the Gaseous Phase (Vacuum Deposition)

The term PVD (physical vapor deposition) defines processes of metal, metal alloys, and chemical compounds deposition in a vacuum by adding thermal and kinetic energy through particle bombardment. The main processes are the following four coating variations (Table 7.6):

• Vapor deposition *Sputtering (Cathode atomization)
• Arc vaporizing *Ion implantation

In all four processes the coating material is transported in its atomic form to the substrate and deposited on it as a thin layer (a few nm to approx. 10 μm)

Table 7.6: Characteristics of the Most Important PVD Processes

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The sputtering process has gained the economically most significant usage. Its process principle is illustrated in (Fig. 7.5).

Fig. 7.5: Principle of sputtering Ar = Argon atoms; e = Electrons; M = Metal atoms

Initially a gas discharge is ignited in a low pressure (10 – 1 Pa) argon atmosphere. The argon ions generated are accelerated in an electric field and impact the target of material to be deposited with high energy. Caused by this energy atoms are released from the target material which condensate on the oppositely arranged anode (the substrate) and form a layer with high adhesion strength. Through an overlapping magnetic field at the target location the deposition rate can be increased, making the process more economical.

The advantages of the PVD processes and especially sputtering for electrical contact applications are:

• High purity of the deposit layers *Low thermal impact on the
• Almost unlimited coating materials substrate
• Low coating thickness tolerance *Excellent adhesion (also by using additional intermediate layers)

Coatings produced by PVD processes are used for contact applications, for example on miniature-profiles, in electrical engineering and for electronic components, for solderability in joining processes, for metalizing of nonconductive materials, as well as in semiconductors, opto-electronics, optics, and medical technology applications.

There are few limitations regarding the geometrical shape of substrate parts. Only the interior coating of drilled holes and small diameter tubing can be more problematic (ratio of depth to diameter should be < 2:1). Profile wires, strips, and foils can be coated from one side or both; formed parts can be coated selectively by using masking fixtures that at the same time serve as holding fixtures.

• Examples of vacuum coated semi-finished materials and parts

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• Materials

Selection of possible combinations of coating and substrate materials

Substrate Materials

Coating Materials

Substrate Materials

Ag

Au

Pt

Pd

Cu

Ni

Ti

Cr

Mo

W

Ai

Si

Precious metal / alloys

NF metals / alloys

Fe alloys / stainless steel

Special metals (Ti, Mo, W)

Carbide steels (WC-Co)

Ceramics (Al2O3, AlN)

Glasses (SiO2, CaF)

Plastics (PA, PPS)

• Dimensions

Dimensions Coating thickness: 10 nm - 15 μm Coating thicknesses for contact applications: 0.1 - 10 μm

For the geometry of semi-finished products to be coated there are few restrictions. Only the coating of the inside of machined holes and tubing has limitations.

• Tolerances

Coating thickness +10 - 30 %, depending on the thickness

• Quality criteria

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.

7.3 Comparison of Deposition Processes

The individual deposition processes have in part different performance characteristics. For each end application the optimal process has to be chosen considering all technical and economical factors. The main selection criteria should be based on the electrical and mechanical requirements for the contact layer and on the design characteristics of the contact component. Table 7.7 gives some indications for a comparative evaluation of the different coating processes.

The electroless metal coating is not covered here because of the low thickness of deposits which makes them in most cases not suitable for contact applications.

Table 7.7: Comparison of different coating processes

The main differences between the coating processes are found in the coating materials and thickness. While mechanical cladding and sputtering allow the use of almost any alloy material, electroplating processes are limited to metals and selected alloys such as for example high-carat gold alloys with up to .3 wt% Co or Ni. Electroplated and sputtered surface layers have a technological and economical upper thickness limit of about 10μm. While mechanical cladding has a minimum thickness of approx. 1 μm, electroplating and sputtering can also be easily applied in very thin layers down to the range of 0.1 μm.

The properties of the coatings are closely related to the coating process. Starting materials for cladding and sputtering targets precious metals and their alloys which in the case of gold and palladium based materials are vacuum melted and therefore exhibit a very high purity. During electroplating, depending on the type of electrolytes and the deposition parameters, some electrolyte components such as carbon and organic compounds are incorporated into the precious metal coating. Layers deposited from the gaseous phase however are very pure.

7.4 Hot (-Dipped) Tin Coated Strip Materials

During hot-dip tinning pre-treated strip materials are coated with pure tin or tin alloys from a liquid solder metal. During overall (or all-around) tinning the stripsthrough a liquid metal melt. For strip tinning rotating rolls are partially immersed into a liquid tin melt and transport the liquid onto the strip which is guided above them. Through special wiping and gas blowing procedures the deposited tin layer can be held within tight tolerances. Hot tinning is performed directly onto the base substrate material without any pre-coating with either copper or nickel. Special cast-on processes or the melting of solder foils onto the carrier strip allow also the production of thicker solder layers (> 15 μm).

The main advantage of hot tinning of copper and copper alloys as compared to tin electroplating is the formation of an inter-metallic copper-tin phase (Cu₃Sn, Cu₆ Sn₅) at the boundary between the carrier material and the tin layer. This thin (0.3 – 0.5 μm) intermediate layer, which is formed during the thermal tinning process, is rather hard and reduces in connectors the frictional force and mechanical wear. Tin coatings produced by hot tinning have a good adhesion to the substrate material and do not tend to tin whisker formation.

A special process of hot tinning is the “Reflow” process. After depositing a tin coating by electroplating the layer is short-time melted in a continuous process. The properties of these reflow tin coatings are comparable to those created by conventional hot tinning.

Besides overall tin coating of strip material the hot tinning can also be applied in the form of single or multiple stripes on both sides of a continuous substrate strip.

• Typical examples of hot tinned strip materials

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• Materials

Coating materials: Pure tin, tin alloys Substrate materials: Cu, CuZn, CuNiZn, CuSn, CuBe and others

• Dimensions and Tolerances

Width of tinning: > 3 + 1 mm Thickness of tinning: 1 - 15 μm Tolerances (thickness): + 1 - +3 μm depending on tin thickness

• Quality Criteria

Mechanical strength and dimensional tolerances of hot tinned strips are closely related to the standard for Cu and Cu alloy strips according to DIN EN 1652 and DIN EN 1654. Quality criteria for the actual tin coatings are usually agreed upon separately.

7.5 Contact Lubricants By using suitable lubricants the mechanical wear and frictional oxidation of sliding and connector contacts can be substantially reduced. In the electrical contact technology solid, as well as high and low viscosity liquid lubricants are used.

Contact lubricants have to fulfill a multitude of technical requirements:

£ They must wet the contact surface well; after the sliding operation the lubrication film must close itself again, i.e. mechanical interruptions to heal

£ They should not transform into resins, not evaporate, and not act as dust collectors

£ The lubricants should not dissolve plastics, they should not

be corrosive to non-precious metals or initiate cracking through stress corrosion of plastic components

£ The specific electrical resistance of the lubricants cannot be so low that wetted plastic surfaces lose their isolating properties

£ The lubricant layer should not increase the contact resistance; the wear reducing properties of the lubricant film should keep the contact resistance low and consistent over the longest possible operation time