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The Basics of Phase-sensitive Eddy Current Method

Physical principles of coating thickness measurement

The phase-sensitive eddy current method is a modification of the conventional amplitude-sensitive eddy current method for coating thickness measurement. According to ISO 21968, the phase-sensitive eddy current method can be used to test electrically conductive coatings on any substrate: for example, copper on printed circuit boards or nickel on steel or insulating material. 

The phase-sensitive eddy current method is not very susceptible to many kinds of external influences. For example, the curvature of a sample or the roughness of a surface will hardly impact the measurement – a big advantage over magnetic induction or the amplitude-sensitive method. For this reason, phase-sensitive probes are ideally suited for checking the zinc thickness on small parts in the electroplating process – without requiring additional calibration. 

How does the phase-sensitive eddy current method work?

Phase-sensitive eddy current probes consist of a ferrite core around which two coils are wound. First, a current in the exciter coil generates a high-frequency magnetic field (in the kHz-MHz range). This creates eddy currents in the sample. 

The probe’s second coil, the measuring coil, measures the alternating current resistance (impedance). The probe’s impedance is modified by the eddy currents in the sample and – as compared to the excitation current (probe without sample) – subsequently phase-shifted (phase angle Ο†). 

The phase Ο† depends on the layer thickness and the electrical conductivity of the material. If the conductivity is known, the device compares the phase with a stored characteristic curve and converts it into a coating thickness value.

Lift-off effect

The phase-sensitive eddy current method offers great advantages for measuring coating thickness. As described above, the actual measurement signal is generated directly in the coating. This distinguishes the method substantially from the magnetic induction and amplitude-sensitive methods, which measure the attenuation of the signal from the substrate material. 

This is why the probe doesn’t have to directly touch the metallic layer; it can even measure metal layers underneath a coating, e.g. in duplex measurements. 

Here’s what you need to pay attention to during the measurement

All electro-magnetic test methods are comparative. This means that the measured signal is compared with a characteristic curve that’s stored in the device. In order for the result to be correct, the characteristic curve must be adapted to the current conditions. This is achieved through calibration. 

Correct calibration makes all the difference!

Factors that can strongly influence coating thickness measurements using the phase-sensitive eddy current method are mainly the electrical conductivity and the permeability of the materials. The size of the specimen is also critical. In addition, for all measurements the operator should always ensure that the probe is correctly positioned.

Electrical conductivity

The conductivity of the coating and the substrate material determine the density of the induced eddy currents and thus exert a direct influence on the coating thickness measurement. Therefore, the instrument must be calibrated for the right combination of substrate material and coating, namely with the same materials on which the real measurements will be taken later.

Thickness of the sample

With metallic base materials, eddy currents are generated not only in the coating material but also in the substrate. If the substrate is very thin (e.g. flat sheet metal), a minimum thickness – which depends on the measuring frequency and the material – is required. 

User influence

Last but not least, the way the measuring device is operated also plays a major role. Always make sure that the probe is set vertically on the surface and without pressure. For better accuracy, a stand can be used to automatically lower the probe onto the sample.

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