Phase-sensitive eddy current method

Measure metallic coatings reliably with eddy current.

The phase-sensitive eddy current method is a modification of the amplitude-sensitive eddy current method for coating thickness measurement. The phase-sensitive eddy current method can be used to test electrically conductive coatings on any substrate in accordance with ISO 21968, such as copper on PCBs or nickel on steel or insulating material. The phase-sensitive eddy current method is not very sensitive to many external influences. For example, curvature of the test part or the roughness of the surface hardly influence the measuring – a great advantage compared to magnetic induction or the amplitude-sensitive method. For this reason, phase-sensitive probes are very well suited for testing the zinc thickness on small parts in electroplating without additional calibration.

This is how the phase-sensitive eddy current method works.

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

By means of the second coil – the measuring coil – the AC resistance (impedance) of the probe is measured. This probe impedance is modified by the eddy currents in the sample and is out of phase (phase angle φ) compared to the excitation current (probe without sample).

The phase angle φ depends on the layer thickness and the electrical conductivity of the material. If the conductivity is known, the phase angle is compared in the instrument with a stored characteristic curve and converted into a coating thickness value.

What happens during the lift-off effect?

For coating thickness measurement, the phase-sensitive eddy current method has a great advantage. As described above, the actual measurement signal is generated directly in the coating. This distinguishes the method significantly from magnetic induction and the amplitude-sensitive method, where the attenuation of the signal from the base material is measured.

For this reason, the probe does not have to be placed directly on the metallic layer, but metal layers below a coating can also be measured, such as in duplex measuring.

Where is this process used?

  • Coating thickness measurement on electroplated surfaces and printed circuit boards, e.g.
    • Ni on Fe
    • Zn or Cu on Fe
    • Cu on brass or bronze
    • Thermally sprayed aluminum coatings (TSA) on Fe
    • Cu on printed circuit boards
    • Cu in holes in printed circuit boards

What factors can influence the measurement?

All electromagnetic measuring methods are comparative. This means that the measured signal is compared with a characteristic curve stored in the device. To ensure that the result is correct, the characteristic curve must be adapted to the current conditions. This is done by calibrating the measuring device for coating thickness measurement.

  • The right calibration makes the difference

      Factors that can strongly influence coating thickness measurement using the phase-sensitive eddy current method are primarily the electrical conductivity and the permeability of the materials. The thickness of the test part is also critical. In addition, the operator should always ensure the correct probe position for all measurements.

  • Electrical conductivity

      The conductivity of the coating and the base material determine the density of the induced eddy currents and thus have a direct influence on the coating thickness measurement. Therefore, the instrument must be calibrated with the correct combination of base material and coating, namely with the materials on which real measurements are to be made later.

  • Thickness of the test piece

      With metallic samples, eddy currents are generated not only in the coating material, but also in the base material. If the base material is very thin (such as flat sheet metal), care must be taken to ensure that the measurement frequency and material-dependent minimum thickness is guaranteed.

  • Operating the measuring device

      Last but not least, how the coating thickness gauge is operated also plays a major role in determining the coating thickness. Always make sure the probe is held level above the coating surface and applied without pressure. The smaller the probe pole, the less the influence due to tilting. If the probe pole is large or flat, the influence is correspondingly greater. For better precision, a tripod can also be used to automatically lower the probe onto the test part. In addition, we offer placement aids for various probes, for example prisms for curved surfaces.

      Principle: Calibration is always performed on the uncoated part on the measuring surface on which the coating thickness is also measured later.

To counteract erroneous measurement results, the following influences must also be taken into account:

  • Indentation errors with particularly soft coatings (such as phosphate coatings).
  • Scatter increases due to wear of the probe pole. We recommend carrying out regular checks.

Which standard is applied here?

Phase-sensitive eddy current method according to ISO 21968