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Lift-off can cause significant errors in the measurement of metallic ball thickness using the eddy current method. In this paper, a triple-coil sensor, which is composed of two receiving coils and one excitation coil, is applied to address the problem of lift-off variation. There exists a peak frequency feature, which is the frequency where the peak value of the imaginary part of the inductance change occurs. The difference in peak frequencies of two receiving coils is linearly proportional to the thickness of the metal balls, with the advantage of reducing the effect of the lift-off during the measurements. The simulation results agree well with the experimental results and therefore a look-up table can be built for different thicknesses by simulation and the actual thickness can be found through comparing the measured results with the look-up table. This thickness measurement approach is effective and the error of the measurement is within 6.1% based on the experimental results. [ABSTRACT FROM AUTHOR]