Contact plating material options for electronic connectors: Part 4January 25, 2019 REDWIRE is news you can use from leading suppliers. Powered by FRASERS.
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In this six-part article, the HARTING Technology Group discusses plug-in connectors and options available for contact plating. This part deals with the different types of noble contact surfaces. Read Part 1, Part 2, and Part 3.
Silver belongs to the noble metals. It has the highest electrical and thermal conductivity of all metals and noble contact surfaces. Silver shows a high affinity with sulfur-containing gases. Thus, traces of hydrogen sulfide are sufficient to colour silver by forming silver sulfide (tarnishing), resulting in a smooth and very thin layer. It is removed when the contacts are mated and unmated or can be electrically broken down through by higher voltages and currents, the so-called “fritting”. This and a passivation on top of the silver surface guarantee a very low contact resistance for a long time. In case of very low currents or voltages, the silver sulfide layer cannot be electrically broken down, so that small changes to the transmitted signal will be encountered. As a result, silver is mainly used for high-current contacts.
Gold is the most noble metal, and therefore, it has the highest resistance to corrosion, which is its main advantage among noble contact surfaces. Pure gold is the softest noble metal, so that gold alloys with cobalt, nickel or iron are used as contact surfaces, the so-called “hard gold”. There is an absorption layer built on top of the hard gold surface, which can be broken very easily with very low normal forces. Similarly, the coefficients of friction and, thus, the insertion and withdrawal forces of gold surfaces are rather low. This leads into very low wear and, therefore, to a high number of mating cycles. Thus, gold is also suitable for low voltages and corrosive environments and also ideal for coatings of connectors in the data and signal range. The main disadvantage of hard gold is its high cost and its price dependence on the stock market.
Hard-gold-flashed palladium-nickel (80/20)
Palladium is a noble metal. Due to its catalytic effect, it tends to form an insulating frictional brown polymer in atmospheres containing traces of organic compounds when palladium contacts are subjected to motion relative to each other (fretting). This effect is called “brown powder” effect. Therefore, often a palladium-nickel alloy is used as contact surface, with all the advantages of palladium, but the nickel prevents the “brown powder” effect. The hard-gold flash on top of the palladium-nickel surface serves as a solid lubricant and as a corrosion protection. This coating system has a hard palladium-nickel undercoat with low coefficient of friction. This leads into very low wear and, therefore, to a high number of mating cycles. The hard-gold-flashed palladium-nickel coating is particularly suitable for low voltages typically used for data and signal transmission and in a corrosive environment.
Coming up in Part 5: Selection of the right surface, and more
For more information, contact HARTING.