Contact plating material options for electronic connectors: Part 3January 18, 2019 REDWIRE is news you can use from leading suppliers. Powered by FRASERS.
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Plug-in connectors are used in a wide range of different environments, like heat or damp heat. They are also used for unfavorable environmental conditions in corrosive atmospheres. Thus, the contact surface of an electrical contact has to meet various requirements. Its purpose is to upgrade the contact substrate’s properties, especially the corrosion resistance of the base material. Corrosion of the substrate leads to changes of the electrical properties due to formation of foreign layers, which increases the film resistance. In order to limit the influence of these foreign layers, the substrate is coated. Compared to uncoated base material, the coating allows reducing the contact normal force, which is needed to break the foreign layers.
The coating also has to have a high electrical and thermal conductivity with low friction and wear. However, high wear resistance is always a compromise between low insertion and withdrawal forces and a high contact force for a large and stable contact area and, therefore, a low and stable contact resistance.
There are two main wear mechanisms. On the one hand, the mechanical wear of the contact surface due to plug-in operations, which leads after a certain time to an exposure of the none-noble intermediate layer or even the base material, which is more prone to corrosion. On the other hand, the fretting wear of the contact surface caused by temperature changes or vibrations resulting in small oscillatory movements (fretting motions < 125 μm). This typically results in fretting corrosion by building highly resistive corrosive products like oxides in the contact interface. In the case of noble metal plated contacts, the fretting corrosion starts when the contact surface is worn through and the nickel intermediate layer or the copper alloy has been exposed. Non-noble metal contact surfaces like tin are always very prone to fretting corrosion.
Common galvanic contact surfaces
There are multiple contact surfaces available on the market to meet these requirements. Although the focus of this paper is the comparison between hard gold and hard gold flashed palladium-nickel (80/20), other common contact surface will be briefly presented later.
Contact surfaces can be distinguished into noble and non-noble contact surfaces. Non-noble surfaces are mainly used in the standard consumer market. In the industrial market and for machines or equipment, where 100 per cent security and quality over the whole lifetime is needed, noble surfaces are preferred.
Non-noble contact surface: Tin
Tin belongs to the non-noble metals. The main advantage of this metal is its low price. Consequently, it is used as a surface coating mainly in the automotive industry, if not-too-severe vibration or temperature changes are expected. It forms a hard tin oxide layer on its surface and is susceptible to fretting. To achieve a good and stable effective contact area and, thus, a low and stable contact resistance, this oxide layer has to be plastically deformed and broken by a high normal force, or more precisely, by a high surface pressure. The fact that this high normal force is needed and that tin has a relatively high coefficient of friction leads to high insertion and withdrawal forces and, therefore, to increased wear. This limits the number of mating cycles to approximately 20. Thus, it is ideal for high volume applications, where multiple mating cycles are not required and for the termination area for press-in or soldered electrical contacts.
Coming up in Part 4: The types of noble contact surfaces
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