Influence of Contact Resistance in Carbon-Gold Contacts in Automotive Keypad Switches

In membrane switching technology, a mobile contact, mounted on supple elastomeric parts is pressed onto a static part of the PCB by means of a knob. This technology is commonly used for keyboards in stationary applications where the contact material is made of carbon film while the PCB is coated with a gold layer. For automotive applications that require high reliability and low cost switches, further developments have been made. These are discussed in this paper that concerns the study of contact resistance under low and medium force indentation either in an industrial switch or in a specific sample. A preliminary study on a keypad has shown that the standard power law of contact resistance versus force R
_c
= K
_c
F
_c
^-n
does not reflect the real contact resistance regarding the complexity of the technologies (multi contact points, path conduction, etc.). Fortunately, the direct indentation of the carbon pill with a reference probe and two secondary probes for current and contact voltage measurement allows us to study the mechanism of conduction of the carbon pill and to perform a parametric study of materials and kinematical closure. As in previous work with metallic materials, we have obtained a similar power law which expresses the contact resistance versus contact force in the domain lcN-10 N. Polymers doped with carbon give a contact resistance value three orders of magnitude higher than in the metallic case. The power n is found to be dependent on mechanical parameters such as roughness and closing speed while the constant K
_c
reflects the resistivity of the carbon pill i.e. the carbon particle doping level in the polymer. Finally the main results are that the Young's modulus value of 10 MPa is four orders of magnitude lower than in metals (100 GPa for copper material) while the resistivity is five orders of magnitude higher than for common metallic contacts. In addition, an aging test carried out on the switches has demonstrated the stability of the contact properties.