Mechanism of AC current transmission across rough contact interfaces

The transmission mechanism of high-frequency alternating current (AC) across rough contact interfaces is a key issue in the design and performance evaluation of connectors, relays, current-collection systems, and advanced electronic and energy devices. Under AC excitation, conductive transport occurs through real contact regions, while capacitance forms naturally across non-contact gaps. These two effects jointly govern the broadband impedance response of the interface. However, this mechanism is affected by surface roughness, normal load, skin effect, electromagnetic diffusion in the substrate, and the electrical boundary condition at the backside of the substrate. This study establishes a load-dependent resistance-capacitance model to quantify the dependence of contact resistance and gap capacitance on load, and reveals how load and frequency affect the competition between conductive transport through contact spots and displacement-current transport across non-contact gaps. The conductive formulation is quantitatively verified against Holm’s DC solution and Bowler’s finite-frequency analytical solution, with maximum deviations below 3.115% and 0.081%, respectively. The effects of surface roughness, skin effect, conductor thickness, and backside electrical boundary conditions on the interfacial impedance response are further examined. The broadband complex impedance is also investigated as a load-sensitive electrical signature of the rough contact state under measurement uncertainty. This study provides a numerical framework for interpreting the load- and frequency-dependent AC impedance response of rough electrical contact interfaces.