Influence of Contact Pressure on the Specific Electrical Contact Resistance of Copper

The specific electrical contact resistance (SECR) [ $\Omega \text{m}^{2}$ ] of Cu-ETP (CW004A) contacts is characterized in dependence of contact pressure up to high pressures close to the tensile strength of the base material. Two different material states are considered, R200 (soft) and R300. Static four-wire measurements are carried out on a special experimental setup, which favors homogeneous contact states during mechanical loading. A finite-element simulation is utilized for interpretation and evaluation of the measurements, including the deformation of the samples during the test. The results show that the SECR of the harder material state R300 is higher than that of the softer material R200. The developed data show a decrease of SECR with increasing contact pressure. For contact pressures up to 0.5 of the tensile strength $\sigma _{t}$ of the base material, the SECR decreases according to a power law. For contact pressures above $0.6~\sigma _{t}$ , the decrease shifts toward a linear behavior. In addition, the measurements show that the SECR vanishes at contact pressures in region of the tensile strength of the base material.

I. Introduction

The electrical contact resistance (ECR) between two components is of relevance in many different technologies. For example, in the field of resistance welding, the ECR contributes to the joining process as a local heat source in the joining zone. In other applications, such as electrical contacting via plug-in connectors or screw connections, a low ECR is advantageous, as low ECR results in low heat generation during current flow. The ECR arises in the contact area between two parts, which are brought together via the mechanical force . Fig. 1 shows a sketch of the effect. The ECR is based on two effects. First, due to roughness and waviness of the surfaces, the size of the real contact area is significantly smaller than the macroscopically visible apparent contact area and only local contact spots exist, which restricts the current flow. Furthermore, the surface often is covered with a film of contamination, e.g., an oxide layer. Depending on the thickness and properties of these films, they are isolating or semiconductive only. The presence of the insulating film layers and contaminants in the apparent contact area can enlarge the effect of constriction resistance. In addition, the areas of the real contact area who are covered with semiconductive layers result in a locally different conductivity in the conductive spots than the metallic base material. This generates an additional film resistance, which also contributes to ECR. Altogether, the described effects form the ECR, and the detailed discussion of ECR fundamentals can be found in [1]. Fig. 1.

Development of the ECR between two rough surfaces.