The application of a suitable lubricant to conducting surfaces at the manufacturing state or during maintenance will help overcome many of the problems associated with poor contact as well as ensuring ease of movement of all mechanical parts. This is true of all types of connectors including sliding contacts, wiping contacts, relays, pcb edge connectors, make and break contacts, i.e. plugs and sockets and potentiometers with wire wound or carbon brushes.
In order to understand the importance of lubrication in improving electrical contact, it is necessary to consider the problems affecting the passage of current between contact surfaces. These are:
No contact surface, whether it be machined or plated, is perfectly smooth. It actually consists of a series of minute peaks and troughs, and when two surfaces are brought together only the peaks make contact. Therefore, when the surface area for an electrical contact is calculated in order to provide maximum efficiency at a given voltage and current, it will not be anything like the true value achieved in practice.
The resultant reduction in contact area increases electrical resistance and results in much higher operating temperatures on contact surfaces causing inefficient operation and damage to contacts themselves, especially in power switching systems where metal transfer and the welding together of surfaces can occur.
As metal contacts move against each other friction and wear occur. Even where printed circuit boards and similar connector systems are concerned, separating the edge connector from the socket will be more difficult in these cases and will result in wear to the male and female connectors.
Moving or sliding contacts are usually designed to take this into account as much as possible, the switch life being expressed in terms of the number of operations. Factors which affect how quickly contacts wear are:
The shape of the contacts themselves will affect the way in which they wear and to some degree, the extent.
An uneven surface will wear on the contact areas. Debris from the abrasion and erosion process will collect in the cavity areas, oxidise, and further reduce contact surface area and increase the abrasion.
The choice of which metals are used for the contacts is the most important single factor in the design of the contact. Not only does it determine the performance and cost, but also the life of the contacts, as metals may be more or less resistant to wear, tarnishing and oxide formation.
The use of dissimilar metals for opposing contact surface areas aggravates the problem, as the metals will inter-react with the harder material abrading the softer one. Some metals may abrade in the form of sharp chips which will then, between the contact surfaces, grind away at the mating contacts.
If contacts are plated wear will occur on the surface metal initially and rapidly result in the expensive, usually noble, metal being stripped from the base material. The contact peaks will therefore be of the less conductive material.
Contacts expand and contract with changes in temperature and this motion can itself cause wear to the contact surfaces. This is known as thermally induced fretting. If dissimilar metals are used on mail and female connectors this fretting will become more significant as the coefficients of thermal expansion differ, and the amount of movement between the two faces therefore increases. For similar reasons, the contact will become either tighter or slacker as temperature rises. If the contact becomes slacker than contact resistance and heat generation increases. If the contact becomes tighter then any movement of one contact surface will result in greater wear.
Obviously, the higher the contact pressure, the higher the mechanical wear. Certain metals, being less conductive, require higher contact pressure in order to perform adequately.
A contact lubricant will fill the troughs in an uneven metal surface thus greatly increasing the surface of the contact and, being semi-conductive, the electrical contact surface area is similarly increased. This in turn results in lower contact resistance and less build up of hot spots lessening the risk of contacts welding together or metal transfer and providing greater efficiency of operation.
From mechanical point of view the use of a lubricant will reduce the fictional effect which causes wear.
When electrical current is switched across a pair of contacts, a miniature lightning strike takes place. As contacts open and close heat is generated by the arcing, this heat can in turn cause a reaction between dissimilar metals, melt, vaporise and disfigure contact surfaces. This will allow air-borne contaminants to react with the metal contacts creating a surface film and, by increasing contact resistance, generate even more heat.
Arching does not only occur on make and break type contacts. Air gaps exist between all contact surfaces due to the uneven surface finish. The greatest proportion of the current will be switched through the peaks, creating hot spots, but some arching across the air gap between the troughs will also occur. This is called “micro-arcing”. Even pcb edge connectors, pin and socket connections on components and other electronic connector types therefore commonly suffer from all the effects of arc erosion, especially as these are normally DC systems.
A contact lubricant will form a film, filling the air gaps between two metal surfaces. As the contact lubricant allows current to flow through it, arching across this gap cannot occur. On make and break contacts, the current will flow through this bridge of contact lubricant and the arc be suppressed. As the contacts close, the film of contact lubricant on each contact will meet just before the contacts close and allow a rapid fall in p.d. between the two faces. Any arcing which does occur will break at the contact lubricant rather than the metal, and any damage will thus be greatly reduced. By eliminating arcing, metal transfer and the deposition of corrosive nitric acid film – effects of arc erosion – are also eliminated.
Contamination of exposed metal surfaces is inevitable because of the presence of contaminant materials in the air, through handling and servicing. Unless removed, a very high contact resistance will result.
A contact lubricant forms a protective film over the metal surface which will prevent contaminants coming into contact with the metal to form tarnish or polymer film. Any contaminants deposited on the contact lubricant will combine with it to form islands of thicker material. As the contacts move, this thicker material will tend to be brushed aside.
With the exception of the noble metals, oxidation occurs rapidly on all metal contact surfaces. Obviously a contact lubricant will, by covering the metal surface, help prevent oxygen from the atmosphere combining with the metal to form metal oxides.
As metal oxides are almost always present on a contact surface before a contact lubricant is applied, the question arises of how these can be broken down. Some companies select slightly acidic or alkaline contact lubricants in order to attack oxides. This policy is however mistaken as the metal will also be attacked and corroded and other metal salts formed, which also have a high electrical resistance, long after the metal oxides have been removed. An oxide film will be broken down naturally as metal oxides are more brittle than pure metals and have different thermal coefficients of expansion. Flexing and expansion of the metal will in this way result in the oxide film cracking off and being washed away by the contact lubricant.
Gold Contact Contamination
Gold contacts do not oxidise. They may, however, suffer due to the porosity of the gold. This is quite common on edge connectors and is thought to relate to the plating method and quality of the gold used.
If the gold flash is porous, then moisture, metal sales, etc. may creep under the gold surface layer and separate it from the base metal, causing first blisters, then holes in the gold surface, leaving a crater of base metal which can corrode or oxidise.
The correct contact lubricant on gold will form reservoirs in areas where the gold is porous and prevent moisture penetrating to corrode the non-noble metal beneath. Certain contact lubricants do have an affinity to the gold surface and will not form a tenacious film. In this way, mechanical wear, stripping and cracking off of the gold flash can be minimised and even eliminated. The contact resistance of gold contacts will therefore be stable at a lower level and contact life greatly increased.
Silicone contamination can be caused only if low molecular weight silicones (silicone oligomers) are present. Silicone oligomers exist in abundance in silicone oils, greases, pastes and materials such as furniture and floor polish, and hand creams. Its use as a mould release also means it is often present on moulded plastic components. As silicones creep amazing distance, silicone contamination can occur far from the point where the silicone material was used.
When free silicones are present between moving or vibrating electrical contacts – and as that includes fretting, all electrical contacts are included – and especially where arching occurs, then the free silicones will be transformed into hard crystals as silicone compounds, notably silicone carbide. These crystals, as well as rapidly abrading the contact surface, will cause breakdown.
Silicone contamination cannot be removed easily and should be avoided wherever possible. Silicone carbide crystals in particular are extremely difficult to remove other than with a file, which will of course damage the contacts. Certain contact lubricants can prevent the damage caused by silicones. A particularly effective treatment is Electrolubes Eltinert F range of contact lubricants which will remove silicone contamination even after silicone carbide crystals have formed. Eltinert F is a modified fluorinated oil which is believed to act by reacting with silicone carbide to form volatile silicone tetraflouride gas.
Frettage and Frettage Corrosion
Frettage is a small amplitude contact movement caused by changing temperature, electromagnetically induced vibration or sonic vibration. Metal transfer and wear are the natural consequences of frettage. Further corrosion of the contact surface or the amassing of debris formed in the process is known as frettage corrosion.
Frettage corrosion will occur on the majority of types of contact whether they are made of copper, solder, tin or silver or a mixture of these metals. While contacts made from gold might be expected to suffer from fretting only, as gold is chemically inert, the effects of the debris abrading through the gold plating, coupled with the porosity of the gold, can lead to frettage corrosion of the substrate material.
This situation will be exacerbated particularly when metals of different hardness are used, as the metal transfer will be only in one direction. For example, in the use of gold/solder contacts, the solder will be transferred to the gold as it is harder material. The good electrical properties of the gold are therefore rapidly lost. Additionally, an oxide film will form over this layer of solder, further increasing contact resistance. This type of contact therefore exhibits even higher contact resistance after fretting than solder/solder contacts.
Frettage can only occur if metal surfaces vibrate one against the other. Inserting a contact lubricant between the faces will limit the actual metal contact and dampen the actual vibration between the two faces. Debris formed by frettage will be less if a contact lubricant is used and any debris that is form will tend to be flushed away from the contact areas by the contact lubricant. Frettage corrosion will also be reduced by preventing atmospheric contact, therefore preventing oxidation of debris.
Mechanical switches unfortunately do not operate by opening and closing only once each time they are operated. Most contacts bounce, arching with each bounce as well as significantly increasing the wear. This bouncing action has the effect of repeatedly heating the contact faces, subjecting them to bursts of high energy arcs and greatly enhancing the conditions for erosion and welding to take place.
The contact bounce also means that clean on and off switching of the equipment does not occur. In precision electronic measuring equipment for example, this can lead to ‘ghost’ readings and misleading results.
A contact lubricant will dampen contact bounce by providing a cushion and forming a bridge between opposing metal surfaces, suppressing or eliminating arcing. Thus a contact lubricant means cleaner switching, more accurate readings, a stable contact resistance and less erosion of contacts.
Contact Lubricant selection
There are many considerations when selecting a contact lubricant:
As suitable viscosity lubricant should be chosen for the application. Some examples are:
|Low contact pressure||✔|
|High contact pressure||✔|
The viscosity chosen should remain constant. Grease must not melt when temperature rises. Oil must not solidify as it cools, causing the contact to stick.
The lubricant must be compatible with the contact metal and be non-corrosive. It also must be compatible with any plastics in close proximity to the contact.
The lubricant must be thermally stable over the specified range, and must perform consistently over this range.
The contact resistance must be low enough to allow the passage of current between closed contacts, but high enough so that tracking or leakage does not occur if the contact bridges electrical contacts that are close.
The lubricant must protect the contact from the environmental factors that could contribute to corrosion, for example the electrolyte in electroplating or salt/humidity in external applications.
Low molecular weight silicones (silicone oligomers) must not be present in a contact lubricant.
The lubricant must have a high flash point. Arching or sudden rises in temperature could otherwise result in fire.
Non hazardous lubricants should be chosen to avoid health hazards.
Whatever contact lubricant is chosen, it is critically important that the chosen method of application applies the material in the right place and in the right quantity in order that it can perform the task for which it was designed.
For assistance in selecting the most appropriate lubricant for a specified application, please contact the Electrolube technical team.
Tel: +44(0) 1530 419600 E-mail: firstname.lastname@example.org
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