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.

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.

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.

Main Articel: Electroplating (or Galvanic Deposition)

7.1.2 Electroless Plating

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).

Main Articel: Other Naturally Hard Copper Alloys

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

tabelle fehlt!

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

bild

• 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

bild

• 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

Solid lubricants include for example 0.05 – 0.2 μm thin hard gold layers which are added as surface layers on top of the actual contact material.

Among the various contact lubricants offered on the market contact lubrication oils have shown performance advantages. They are mostly synthetic, chemically inert, and silicone-free oils such as for example the DODUCONTA contact lubricants which differ in their chemical composition and viscosity.

For sliding contact systems with contact forces < 50 cN and higher sliding speeds oils with a lower viscosity (<50 mPa·s) are preferential. For applications with higher contact forces and operating at higher temperatures contact oils with a higher viscosity are advantageous. Contact oils are mainly suited for applications at low current loads. At higher loads and in situations where contact separation occurs during the sliding operation thermal decomposition may be initiated which causes the lubricating properties to be lost.

Most compatible with plastics are the contact oil varieties B5, B12K, and B25, which also over longer operating times do not lead to tension stress corrosion.

For the optimum lubrication only a very thin layer of contact oil is required. Therefore it is for example recommended to dilute the oil in iso-propylenealcohol during the application to contact parts. After evaporation of the alcohol a thin and uniform layer of lubricant is retained on the contact surfaces.

• Properties of the Synthetic DODUCONTA Contact Lubricants

Lubricant	DODUCONTA
Lubricant	B5	B9	B10	B12K	B25
Contact force	>1N	0.1 - 2N	< 0.2N	0.2 - 5N	<1N
Density (20°C) [g/cm³]	1.9	1.0	0.92	1.0	1.0
Specificel. Resis- tance [S · cm]		2 x 1010	1010	6 x 109	5 x 108
Viscosity (20°C) [mPa·s]	325	47	21	235	405
Congeal temp.[°C]		-55	-60	-40	-35
Flash point[°C]		247	220	238	230
	220

• Applications of the Synthetic DODUCONTA Contact Lubricants

Lubricant	Applications
DODUCONTA B5	Current collectors, connectors, slider switches
DODUCONTA B9	Wire potentiometers, slip rings, slider switches, measuring range selectors, miniature connectors
DODUCONTA B10	Precision wire potentiometers, miniature slip rings
DODUCONTA B12K	Wire potentiometers, slider switches, miniature slip rings, connectors
DODUCONTA B25	Current collectors, measuring range selectors, connectors

7.6 Passivation of Silver Surfaces

The formation of silver sulfide during the shelf life of components with silver surface in sulfur containing environments can be significantly eliminated by coating them with an additional protective film layer (Passivation layer). For electrical contact use such thin layers should be chemically inert and sufficiently conductive, or be easily broken by the applied contact force.

The passivation process SILVERBRITE W ATPS is a water-based tarnish preventer for silver. It is free of chromium(VI) compounds and solvents. The passivating layer is applied by immersion which creates a transparent organic protective film which barely changes the appearance and only slightly increases the good electrical properties such as for example the contact resistance. The good solderability and bond properties of silver are not negatively affected. Because of its chemical composition this protective layer has some lubricating properties which reduce the insertion and withdrawal forces of connectors noticeably.

Fig. 7.7: Typical process flow for the SILVERBRITE W ATPS process

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