In creating an electrical contact, designers must take many factors into account and make the best choices for their application, with the first of those being the type of material. In selecting a material type, the conductivity, strength, spring properties, and durability all affect the contact design. While a material such as pure copper may be highly conductive, it has poor spring properties. Alloys such as beryllium copper are a good option to achieve a combination of desired traits. The metal is formed while still soft and then heat treated to create the spring characteristics.
In choosing the type of spring material used in a contact, designers need to take the temper or hardness into account. A softer temper will provide less spring force but will be more flexible and less prone to fracture, while a harder temper will provide a higher spring force but is more difficult to form and is more brittle, fracturing more suddenly rather than plastically deforming.
This spring force is also called the normal force. The force is perpendicular to the contact and the surface it is interfacing. The ideal contact interface is between a perfect sphere and a flat plate while the least desirable is between two flat planes.
While the interface between two contact surfaces appears to be smooth and constant, when viewed under magnification, the surfaces are actually made up of a series of peaks and valleys. The peaks that contact each other are called asperity or A-spots. The area where the two surfaces outwardly appear to be touching is the apparent contact area while the sum of the A-spots is the effective contact area. A connection with a smaller effective contact area will have a higher resistance and will be less effective.
The goal in designing a contact is to maximize the number of A-spots. A higher number of contact points produces less constriction in the electric flow, providing
more paths for the signal to travel through the interface.
The primary way to increase the number
of A-spots is by increasing the normal force. Increasing the force causes the peaks and valleys to elastically deform and “smooth out” connecting along more surfaces and increasing the effective contact area. The interface resistance decreases as a function of the contact force until flattening out, reaching a point of diminishing returns.
Another method of increasing the effective contact area is to decrease the irregularity of the interface surfaces by applying an additional finish to the base contact material. As a base material corrodes, it develops an oxide layer and increased porosity. Protective platings can reduce this corrosion and wear. Gold plating provides a high level of conductivity and corrosion resistance as a noble metal however it has reduced solderability and high costs. Nickel plating gives excellent wear characteristics and also provides high corrosion resistance. Tin plating is one of the most popular finishes. Tin offers moderate levels of corrosion and wear resistance while being easily solderable and low cost.
Sources
-Slade, P. G. (2014). Electrical contacts principles and applications. CRC Press.
-Brush Wellman, Connector Engineering Design Guide, Cleveland Ohio, 1999 Print
