Electrical Contact Resistance: Fundamental Principles

Abstract

All solid surfaces are rough on the microscale. Surface microroughness consists of peaks and valleys whose shape, height variations, average separation and other geometrical characteristics depend on fine details of the surface generation process (1). Contact between two engineering bodies thus occurs at discrete spots produced by the mechanical contact of asperities on the two surfaces, as illustrated in Fig. 1.1. For all solid materials, the area of true contact is thus a small fraction of the nominal contact area, for a wide range of contact loads (1,2). The mode of deformation of contacting asperities is either elastic, plastic or mixed elastic-plastic depending on local mechanical contact stresses and on materials properties such as elastic modulus and hardness. In a bulk electrical interface where the mating components are metals, the contacting surfaces are often covered with oxide or other electrically insulative layers. Generally, the interface becomes electrically conductive only when metal-to-metal contact spots are produced, i.e. where electrically insulative films are ruptured or displaced at contacting surface asperities. In a typical bulk electrical junction, the area of electrical contact is thus appreciably smaller than the area of true mechanical contact.