Novel Conductivity Measurement of Thin Metallic Materials Using Crossover Frequency Feature From Triple-Frequency Eddy Current Signals

Non-ferromagnetic thin metallic materials find extensive applications in the fields of electronics, aerospace, and instrumentation, wherein the conductivity of these metal materials serves as a crucial indicator of material quality. However, the established methods are subject to computational burdens like model-based inversion or invariant thickness. In this work, a novel simple but accurate method is proposed to determine the conductivity of thin films using the crossover frequency feature from eddy current impedance. First, the crossover phenomenon of resistance and reactance from swept-frequency eddy current impedance was investigated with a transformer model. It is found that the impedances of only triple frequencies are enough to accurately determine the crossover frequency point. Second, the crossover frequency is found to have an inverse proportionality to the conductivity of metal materials after mathematical manipulations, and it would change accordingly when the liftoff distance and sample thickness change. Afterward, a mathematical map was derived by fitting work between sample thickness and slope rate. In this case, the proposed method is suitable for samples with thickness change without recalibration. Last, a PCB single-coil eddy current sensor was designed to validate the presented method through simulations and experiments. The results demonstrate that the developed crossover feature based method from triple frequency impedances, compared with a reported single frequency method, achieves better accuracy and more stable measurement results, with a maximum relative error of 2.26%. This new method only needs to gauge sample thickness without recalibration work when the thickness of samples changes, and it does not need large computational resource.