Electrical and electronic components and devices are usually packed in gastight housings. The only parts exposed to the atmosphere are terminals with their electrical contacts. The basic function of electrically conductive surfaces of electrical contacts is electrical conduction. Copper and copper alloys are widely used base materials for electrical contacts.
Copper and copper alloys are materials, which are prone to corrosion. The electrical conductivity of contact materials can be largely reduced by corrosion and in order to avoid corrosion, protective surfaces must be used. There are basically two groups of materials, which can be employed for corrosion protection of electrically conductive surfaces. The first group includes noble metals such as gold, silver and palladium. The second group comprises of corrosion resistant so-called passive metals such as tin and nickel. These metals are basically ignoble and they derive their corrosion resistance from the presence of a thin oxide film on the surface, called the passive film, which acts as a protective barrier between the metal and its environment. The passive film inhibits deeper corrosion, and is usually an oxide or nitride with a thickness of nanometers [1
]. Corrosion protection of passive metals is limited to certain conditions, which are applied to many working environments of electrical contacts.
Nevertheless, not all the passive metals or alloys are suitable for corrosion protection of electrically conductive surfaces. In the case of aluminum and aluminum alloys, the passive films are too thick and too stable. They are therefore highly electrically resistant. A good electrical contact can only be realized by destroying the passive film either by high mechanical pressure or by a strong electrical field. These conditions are often not available, especially in microelectronics.
Among the plating materials gold has the highest standard electrode potential of 1.38 Volt [2
] and is therefore the noble material used for plating with the highest corrosion resistance. A perfect coating with gold is not problematic regarding corrosion. Due to the high cost of material the plating of gold is often so thin that the final coating is not free of pores. These pores are the cause of corrosion of electrically conductive surfaces with protective coatings. Therefore one of the goals of the optimization of gold coating is minimization of both the number and the size of pores of gold coating with the thinnest possible plating. This is also one of objectives of our investigation.
The passive plating materials such as tin and nickel can also have problems with the pores on the plating and the porosity can also be decreased by increasing the thickness of the plating. In the case of the ignoble metals the main limiting factors for the thickness of coating are not mainly the consumption of plating materials and processing time but other performance factors such as mating forces or ductility of the coatings. The porosity of coatings can be largely reduced by optimization of plating processes. Passivation is another additional measure which can be employed to close the pores for a limited period of time.
The corrosion behaviors of metals to different media can differ. The most common corrosive media for electronics are hydrogen sulfide (H2S), sulfur dioxide (SO2), chlorine (Cl2), nitrogen dioxide (NO2), ammonia (NH3), air (oxygen) and salt fog. Corrosion behaviors of electrically conductive surfaces are therefore tested in the atmosphere with one or several of these media, often at elevated temperature, which accelerates the corrosion process. Before, during and after the tests the electrical contact resistance is measured. The relation of contact resistance after and before the tests is usually used to describe the degree of corrosion.
Pure gold is resistant to almost all these media. In the case of chlorine it depends on the concentration of chlorine in the atmosphere and duration of the test. Corrosion can be observed at high concentration of chlorine and after an accelerated corrosion test of 21 days. The corrosion resistance of gold becomes slightly lower, when hard gold alloys such as AuNi or AuCo are used. The critical media for gold alloys are hydrogen sulfide (H2S), sulfur dioxide (SO2), chlorine (Cl2) and air (oxygen). A marked increase of corrosion due to the porosity of the coating was observed, when a very thin gold coating was used. A similar corrosion protection like gold alloy can be provided by tin, silver and palladium. These plating materials have quite different physical, chemical and technological properties and the availability and the cost of plating materials differ immensely. Tin is used because of the very slow growth of its oxide layer and due to the fact that its oxide film remains very thin under many technically relevant conditions and can be mechanically broken by a low contact force. Good electrical conduction can therefore easily take place. The critical media for tin, depending on the modification of tin, are sulfur dioxide (SO2), chlorine (Cl2) and salt fog. Silver has a high standard electrode potential of 0.8 Volt and is therefore regarded as a noble metal. Palladium has a standard electrode potential of 0.915 Volt and is regarded as a noble metal. Their corrosion resistance is almost as high as gold alloys. The critical media for silver are hydrogen sulfide (H2S), chlorine (Cl2), nitrogen dioxide (NO2) and salt fog and hydrogen sulfide (H2S), sulfur dioxide (SO2) and chlorine (Cl2) for palladium. For electrically conductive surfaces coatings of palladium nickel alloys are generally used. Nickel coating does not provide very good corrosion protection for electrically conductive surfaces. It is normally only used for high temperature cases, because of its high thermal resistance.
provides an overview of corrosion resistances of the most common used coating materials for electrically conductive surfaces and the base material copper [2
Corrosion resistance of selected metals: 1 = high; 2 = fair; 3 = poor.
|Material||H2S||SO2||Cl2/4 d||NO2||NH3||air/120 °C||Salt fog|
|Au, 3 µm||1||1||2||1||1||1||1|
|AuCo1, 3 µm||1||1||3||1||1||3||1|
|AuNi10, 3 µm||2||2||1||1||1||2||1|
|Au, 0.2 µm||1||3||3||3||1||3||2|
|Ni, 3 µm/Au, 1 µm||3||1||3||1||1||1||1|
|Ni, 3 µm/Au, 3 µm||1||1||3||1||1||1||1|
|Ag, 10 µm||2||1||2||3||1||1||2|
|Ni/Pd, 3 µm||2||2||3||1||1||1||1|
|Chem. Sn, 0.5 µm||2||2||3||3||1||3||2|
|electroplated Sn, dull, 15 µm||1||3||1||1||1||1||2|
|electroplated Sn, bright, 15 µm||1||2||3||1||1||1||2|
|PbSn 40/60, 6 µm||1||1||2||1||1||1||2|
|hot dip tin, 6 µm||1||1||3||1||1||1||2|
|Ni, 3 µm||3||3||3||2||2||2||2|
1.6. The Main Scope—Solutions for Combined Protection against Corrosion, Fretting Wear and Fretting Corrosion
The above description shows that a very high corrosion resistance of coatings is a necessary condition for good protection of electrically conductive surfaces. However this alone is not sufficient for good protection. As a result of the motion between electrical contacts, the coatings must also possess a high wear resistance, a high fretting corrosion resistance and of course a high and stable electrical conductivity. Ignoble coating materials such as tin and nickel are all affected by the fretting corrosion. Palladium is normally not subject to corrosion problems. Palladium nickel alloys are mostly used for electrically conductive surfaces. They have an excellent wear resistance, although the high level of brittleness can possibly lead to cracks, which in turn reduces the corrosion resistance. Fretting corrosion is also possible with coatings of palladium [3
]. Silver has very high corrosion resistance and is not subject to problems concerning fretting corrosion. Silver is soft and has a low wear resistance. A silver plating of 3 microns is therefore required for more than 50 mating cycles [7
]. The most substantial problem with silver is tarnishing in a sulfur sulfite environment. The tarnish film is very soft and can therefore be easily broken by the contact force. It does not show relevant influences on electrical conductivity. However, the visible brown color of the tarnish film suggests changes on surface, which are often regarded as failures. Gold alloys fundamentally have the combination of desired properties for the overall protection of electrically conductive surfaces. The critical points with gold coating are the wear resistance and the materials cost. We have discovered some measures to increase the wear resistance of gold coatings and to reduce the materials consumption of gold.
There are a variety of basic approaches to the solutions of the problems caused by fretting wear and fretting corrosion. Noble metals as final coating materials, design which reduces the wipe caused by external forces and thermal expansion, lubricants that prevent the zone of contact from exposure to the atmosphere and therefore from the fretting oxidation, are widely used measures to enhance the resistance to fretting corrosion. If using noble metals, the wear resistance of the plating, material and cost saving should be especially considered. Problems with lubricants could be the long-term stability factor and the high operating temperature. Most lubricants evaporate extremely fast at temperatures of over 100 °C.
Gold is the plating material with the best corrosion resistance and therefore one of the most commonly used noble plating materials for high performance electrical contacts with good and stable electrical behaviors. Today well over 300 t of gold are used annually in electronic components and a large proportion of this amount is used as plating material for high performance electrical contacts. Although miniaturization and cost saving efforts, such as selective gold plating, drive down the speciﬁc gold input, the booming growth in sales of electronic devices and their new features have led to a constant gold demand in this field in recent years [12
]. In addition to the material cost, the production of gold also causes environmental problems [15
]. Therefore, the reduction in the demand for gold on electrically conductive surfaces is of vital importance, from both the economic and ecological point of view. These facts are the reasons that gold alloys and other modifications of gold are the main scope of our investigation.
The basic function of gold platings in electrical contacts is the protection against corrosion and fretting corrosion. The amount of gold used for electrically conductive surface is determined by the area and the thickness of gold plating. The basic idea of selective gold plating is that gold is plated at and around the electrical contact areas and not on the whole surface of electronic components. In this way the area of gold plating can be reduced to the minimum. Depending on the contact operating conditions and contact functions the thickness of gold platings varies from 0.1 to 5 μm. Thick platings are normally used for contacts for high mating cycles or other heavy duty applications. Platings of more than 5 μm are also used. However they do not improve the surface perfections and wear resistance because of stress crack corrosion and therefore should not be used [7
]. For corrosion protection, gold platings must be free of imperfections (porosity). This feature can be only guaranteed up to a certain degree by a hard gold plating thickness of more than 0.6 μm [17
Different ways to minimize the consumption of gold for electrically conductive surface and to improve the performance of gold plating were investigated:
Increasing the hardness by means of:
Improved wear resistance and wear pattern of coatings;
Improved surface perfection with thin coatings by employing SAM (self assembled monolayer).
The high degree of hardness of hard gold is achieved by alloying elements such as cobalt, iron or nickel. However, the effect of alloying elements is limited by the electroplating process and other surface properties, such as ductility and cracking, which are also important for the final coatings.
Instead of alloying elements, nanoscale particles mostly metal oxides can be used for the modification of gold platings. The reason for using nanoscale particles is based on the fact that the hardness of pure gold is about 70 HV and the hardness of hard gold is about 170 HV (10 N/mm2
]. The hardness of nanoscale particles of metal oxides ranges from 700 to 2300 HV (10 N/mm2
]. PTFE reduces the friction between contacts. These can provide an additional lifetime improvement [20
]. However PTFE is basically an isolator therefore only nanoparticles of PTFE can be used for electrical contacts, in order to alleviate increase of contact resistance.
Another means of corrosion protection of electrically conductive surfaces is the passivation of surface. Passivation is the change process of metal surface, which creates the resistance to environmental media. Metal surfaces can be passivated either by their own gastight oxide layer or through the assembly of a non-reactive layer such as self-organization of nanoparticles, monolayers, SAM (self assembled monolayer). Besides the resistance to corrosion there are two very important requirements for electrically conductive surfaces:
The passivation layer should not increase or destabilize the electrical resistance markedly and;
For many applications the passivation layer must resist the operating temperature which is the sum of resistive heating and ambient temperature.
Self assembled molecules are bi-functional or multi-functional molecules that offer two- or more-termination groups with different functionality, Figure 6
. Typically one end is attached to a specific surface while the other end provides a specific functionality. These molecules can be attached to metals, Figure 7
. In this manner it is possible to impart or enhance the intended performance of a surface or enable completely new applications. The important intended properties for electrically conductive surfaces are high resistance to oxidation and corrosion by lowering porosity, high resistance to wear-through and good electrical conductivity. Self assembled molecules can be applied in different ways, ultrasonic assisted immersion, photo catalytic, electrochemical or gas phase evaporation. Ultrasonic assisted immersion was used for our investigation. In this case, electroplated samples are immersed in the SAM liquid for about one minute and then washed with clear water and dried, Figure 8
]. AUTRONEX™ Nano 104S was used for the investigation.
Molecules for self assembly.
Method of application—Ultrasonic assisted immersion.
Methods of Application.