Precious metal-reduced contact materials in telecom- and signal relays

Palladium is the most frequently used contact material for telecommunication and signal relays, due to its good contact resistance stability and material transfer characteristics. The significant increase in the price of palladium in recent years has made it necessary to look for alternatives. The development of gastight plastic sealed relay housings, which keep the gas filling inside the relay for a long time, e.g. more than 10 years for nitrogen and more than 100 years for sulfur-hexafluoride, allows nonprecious metal contact materials to be used, as an inert atmosphere can be kept for the entire life of the relay. Tests were performed with gold covered tungsten and ruthenium contacts mounted in a standard telecom relay filled with N₂ or SF₆. Although the contact resistance was always measured with dry circuit conditions applied, no relevant increase was observed during all electrical endurance tests. The ruthenium as well as the tungsten layer with a thickness of only 5 μm withstood endurance tests with maximum loads at 30 W. As only minimum contact erosion and material transfer occurred, no contact sticking or bridging was observed. Overall, the switching performance of tungsten and ruthenium is similar to that of PdRu10. As PdRu10 has superior performance, especially when operating in an SF₆ atmosphere, the properties of Ru and W in N₂ as well as in SF₆ are at least comparable with all contact materials in current use. Gastight plastic sealed relay housings offer the advantages of hermetically sealed housings at an affordable cost