Electrical contacts play a seemingly simple yet critical role in electrical systems. They act as the interface between circuit segments, allowing the segments to connect and disconnect when necessary. Although this seems straightforward, the design of this connection requires extensive planning and consideration. If designed incorrectly, electrical contacts can exhibit excessive tarnishing, welding wear, mechanical degradation, or corrosive wear, which can result in a faulty connection. Contacts must be designed to minimize the occurrence of these failure modes, all while maintaining low contact resistance, minimal electrical noise, and reliable connection/disconnection ability. There are hundreds of different contact materials available that have been successfully used in many different applications. This article highlights three different categories of contacts commonly used in electrical equipment design: (1) Laminated Button Contacts, (2) Sintered Contacts, and (3) Metallized Carbon Contacts.
Laminated Button Contacts
Laminated button contacts, sometimes referred to as “clad metal contacts”, consist of two or more metal layers, ranging in material from fine silver to complex alloys. These layers are often bonded to each other by means of a cold-welding method called cladding. Put simply, cladding is a process by which dissimilar metals are joined under pressure to form a metallurgical bond. For button contacts, this process often involves bonding two or more form-rolled strips by means of heated compaction (see Figure 1). The resulting contact tape consists of multiple metal layers, each of which has specific material properties that are beneficial to the final product. For example, highly conductive precious metals like silver are often backed by stronger ferrous metals like nickel or steel, resulting in a contact that is both strong and conductive. Practically any combination of metals can be used to optimize electrical conductivity, strength, corrosion resistance, weldability, wear resistance, etc.
Figure 1: Bimetal or trimetal overlays from which clad contact tapes are produced are often form rolled, which allows the manufacturer to provide a wide range of weld projection surface designs on the contact tape.
In addition to optimizing certain material properties, cladding also allows for cost optimization. The amount of expensive precious metal in a button contact can be minimized by decreasing the thickness of the precious metal layer as much as possible without diminishing the conductivity of the contact below an acceptable level. Through tight control of layer thicknesses and regular quality inspection, electrical contact manufacturers have developed reliable processes that result in high quality contact tape with consistent properties. Once the contact tape is formed, the final step in producing button contacts is to coin the buttons from the tape. Any unused metal in the tape can then be recycled and recovered.
Even though contact manufacturers can tailor the material properties of button contacts for a specific application, these types of contacts are not suitable for all applications. They are typically reserved for relatively low power connections. Unlike most sintered contacts, button contacts often do not have a refractory metal in their composition, so they are less resistant to heat and wear.