Capacitive sensing of local bond layer thickness and coverage in thermal interface materials

An instrumentation technique is developed using embedded capacitive sensors to measure the thickness and evenness of coverage of a thin layer of dielectric thermal interface material (TIM) between two substrates. The technique requires an array of sensors embedded into one substrate, with an electrically conductive opposing substrate. Local capacitance measurements are sensitive to both local bond layer thickness and local voiding. We propose a means for using an array of capacitance measurements to interpret both bond layer thickness and local voiding at every sensor location. An algorithm is developed which reveals both characteristics from a single set of capacitance measurements. Experiments are conducted with thermal grease layers of different bond layer thicknesses and void distributions using a prototype system constructed on printed circuit boards. The thickness and void distribution are successfully mapped across the bond layer using the algorithm developed. The technique offers a sensing approach for in situ instrumentation of layers of thermal grease in a thermal test vehicle.