A sensor is developed for simple, in situ characterization of dielectric thermal interface materials (TIMs) at bond line thicknesses less than 100 μm. The working principle is based on the detection of regions of contrasting electric permittivity. An array of long, parallel electrodes is flush-mounted into each opposing substrate face of a narrow gap interface, and exposed to the gap formed between the two surfaces. Electrodes are oriented such that their lengthwise dimension in one substrate runs perpendicular to those in the other. A capacitance measurement taken between opposing electrodes is used to characterize the interface region in the vicinity of their crossing point (junction). The electric field associated with each electrode junction is numerically simulated and analyzed. Criteria are developed for the design of electrode junction geometries that localize the electric fields. The capacitances between floating-ground electrodes in the electrode sensor configuration employed give rise to a nontrivial network of interacting capacitances which strongly influence the measured response at any junction. A generalized solution for analyzing the floating network response is presented. The technique is used to experimentally detect thermal grease spots of 0.2 mm to 1.8 mm diameter within a 25 μm interface gap. It is necessary to use the generalized solution to the capacitance network developed in this work to properly delineate regions of contrasting permittivity in the interface gap region using capacitance measurements.