Contact Performance in Relays

5.8 Evaluation of Contact Materials

Perhaps the most troublesome aspect of relay evaluation and application is in choosing the contact material and predicting performance under a multiplicity of operating conditions. To a large extent the difficulty results from a lack of familiarity with the parameters affecting contact performance. Much of the trouble, however, is caused by reluctance to accept the limitations inherent in a design or in choice of contact material. There seems to be a lack of appreciation that higher reliability is attainable, although sometimes at an initial cost penalty. Often overlooked are the more severe economic penalties of increased maintenance and replacement and the consequences of catastrophic failure. Although the state of the art has not advanced sufficiently to eliminate contact troubles, a high degree of reliability can be achieved by adhering to a few basic principles and applying acquired knowledge.

Palladium/Platinum: Palladium and platinum are used extensively as contact materials due to their high melting and boiling points that provide good resistance to transfer. Their high resistance to oxidation or chemical attack provides low, relatively stable, contact resistance. Palladium is more popular due to the high cost of platinum; however, palladium is somewhat inferior to platinum in resistance to oxidation and chemical attack. Both palladium and platinum are low in thermal and electrical conductivity and are susceptible to formation of insulating polymers, in the presence of organic vapors when "wipe" or sliding contact action occurs. In pure form, these materials are also low in hardness making them susceptible to deformation and mechanical wear. They are therefore generally alloyed with other materials (silver, gold, nickel, copper, etc.) to increase hardness and resistance to polymer formation (silver) but with a resultant reduction in conductivity. The low conductivity of the platinum-palladium materials restricts their use to relatively low current ( 1-2 A) switching applications.

Gold: Contacts containing a high percentage of gold are excellent for use in low energy or dry circuits switching where low, stable contact-resistance is essential because of gold's very high resistance to surface film formation. Contact materials high in gold content are high in cost however and how relatively poor resistance to material transfer especially at higher current levels (1/2 A and up) with poor resistance to sticking or welding and mechanical deformation or wear. Alloying other materials such as nickel, silver, palladium, etc., with gold improves resistance to transfer, welding, and mechanical wear but at a considerably reduced conductivity. For these reasons, contact materials high in gold content are seldom used at currents above 1/2 A. Recent developments in pressure bonding techniques have allowed the use of thin layers of alloys high in gold content over silver, palladium, platinum, nickel, etc. The alloys provide, at moderate cost, excellent low-level and dry-circuit switching characteristics while allowing use of the contact at higher currents, up to 2 or 3 A or more, without the serious transfer and mechanical wear normally associated with gold. Adequate contact breakaway force must be provided, however, as sticking or welding can still be a problem depending on the thickness of gold and its alloy composition. Gold-plated contacts exhibit lower initial contact resistance than some unplated contacts, such as silver. However, the plating technique should be controlled to avoid porosity and creepage of the underlying material onto the contact surface. Gold plating has a short life under loads that cause electrical erosion. It is useful to the extent that it inhibits formation of sulfides and oxides during storage or in applications having relatively short operating life requirements. Gold plated contacts, if thoroughly clean, may also tend to stick.

Silver: Silver is probably the most widely used contact material available. It has the highest electrical and thermal conductivity of any know metal and shows good resistance to oxidation and tarnishing except in the presence of sulfur. Sulfur-containing atmospheres will produce silver sulfide that increases contact resistance. Silver sulfide, however, is quite soft and easily displaced with adequate contact pressure and wipe or slide. Shelf life protection and low initial contact resistance will be provided by a pore-free (80 to 100 microinches) gold plate. As a result of silver's high electrical and thermal conductivity, contacts of fine silver work well at currents in the light to medium range (1-20 A) where light to moderate contact pressure is available and low contact resistance is a requirement. Silver has the lowest cost of all precious metal contact materials and it is readily formed into various contact shapes (rivets, buttons, etc.) due to its ductility.

Silver's tendency to erode and weld at medium current levels and its somewhat low hardness can be overcome by combining the silver with materials such as cadmium, palladium, nickel, copper, etc. A combination of 60% silver and 40% palladium will provide a contact with the best characteristics of each material: 1) excellent resistance to oxidation and tarnishing, including sulfur compounds, 2) high resistance to frictional polymerization normally characteristic of materials of high palladium content, 3) significantly improved resistance to pitting and material transfer, 4) medium cost (higher than fine silver, but much lower than alloys of high hold, platinum, or palladium content necessary to produce equivalent contact resistance levels under low pressure). The electrical and thermal conductivity of 60/40 silver-palladium alloys is, however, somewhat low compared to silver so switching current capability will also be lower than that of silver where contact temperature rise is of importance (e.g., UL specifications).

Silver Nickel: Silver nickel (10% to 40% nickel) is advantageous, when a silver contact is needed but is not hard enough. Silver nickel (at 30% nickel) is about 15% harder, but about 30% less conductive than silver. The above material is now very popular, in low signal relay applications, as an alternative to silver palladium.

Silver Indium Tin Oxide: Silver indium tin oxide (AgInSnO), as well as silver tin oxide (AgSnO) has become good alternatives to the AgCdO contacts, described below. Over the past years, the use of cadmium in contacts as well as batteries has been restricted in many areas of the world; therefore, tin oxide contacts (10%), which are about 15% harder than AgCdO, are good alternative. Also, the above contacts are good for high inrush loads, like tungsten lamps, where the steady state current is low. Although more weld resistant, AgInSn and AgSn contacts have a higher bulk resistance (less conductivity) than Ag and AgCdO contacts.
Due to their resistance to welding, the above contacts are very popular in automotive applications, where heavy 12VDC inductive loads tend to cause material transfer.

Silver Cadmium Oxide: Silver cadmium oxide (AgCdO) has become very popular as a general-purpose contact material in medium to high current switching applications because of its excellent resistance to erosion and welding and its very high electrical and thermal conductivity. AgCdO is produced by mixing silver and cadmium oxide using powder metallurgy techniques. The result is a material with a conductivity and contact resistance (using somewhat higher contact pressures) that is close to that of silver but with superior erosion and welding resistance due to the inherent welding resistance and arc-quenching characteristics of cadmium oxide. Usual AgCdO contact materials contain 10-15% cadmium oxide. Resistance to sticking or welding improves as the cadmium oxide content increases; however, electrical conductivity decreases and cold working characteristics degrade due to decreases ductility.

Silver Cadmium oxide contacts are available either post-oxidized or pre-oxidized. Pre-oxidized material has been internally oxidized prior to forming into contacts and contains more uniformly distributed cadmium oxide than does post-oxidizing which tends to bring the cadmium oxide closer to the contact surface. Post-oxidized contacts may pose problems with surface cracking if the contact shape must be changed appreciably subsequent to oxidization as is the case with double-headed, moving-blade, form-C contact rivets.

Tungsten: Tungsten is used in contacts when there is need for high resistance to mechanical wear, electrical erosion and welding taking advantage of tungsten's high melting point and boiling point. The current carrying capacity of tungsten, however, is limited to the order of 3-5 A due to its low electrical conductivity and its high contact resistance even with very high contact forces (a result of its tendency to form thick oxide surface films). Tungsten cannot be cold worked due to its low ductility and is usually provided in the form of a disc which is secured to copper, nickel or nickel-plated steel backing-material in form of a screw or rivet.