The microresistivity method is suitable for measuring the thickness of electrically conductive coatings on insulating substrates according to ISO 14571. It is frequently used to inspect copper coatings on printed circuit boards and multilayer PCBs. The advantages of the microresistivity method? Because the circuit board’s other layers exert no influence on the measurement, the thickness of just the uppermost layer can be determined with precision.
For the micro-resistance method, probes are used that have 4 pins at the tip; the pins are arranged in a row. When the probe is placed on a surface, a current flows between the two outermost pins. The coating exerts an electrical resistance, so a voltage drop can be measured at the two inner pins. The resistance – and thus the voltage drop – is inversely proportional to the thickness of the layer.
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.
Factors that can strongly influence the measurement include the electrical resistance of the layer, the shape of the sample and the roughness of the surface. Furthermore, the operator can also influence the result.
Besides the thickness of the coating, the specific resistivity of the copper can also influence the drop in voltage between the measuring pins. The resistance can vary depending on the specific alloy and how the metal was processed, and different temperatures can also cause variations. This may necessitate temperature compensation or calibration under the same ambient conditions of the measurement.
With very narrow samples, the field lines run differently than they do in wide objects. This deviation from the theoretical field line leads to systematic errors in the coating thickness measurement. For this reason, there are probe-specific specifications for the minimum sample size and/or the minimum distance from the sample edge.
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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