Amazing Eddy Currents

Apply Them to Thin Metal Thickness Measurement

- George R. Steber

Eddy currents are swirling electrical currents created when a magnetic field is moved over a conducting metal, or the metal is moved through a magnetic field. George writes about his experiments using eddy currents to determine the layer thickness of copper on PCBs and aluminum and copper foils. He also describes a simple eddy current apparatus that he built using a PC, a single resistor, and a USB sound card to measure thickness.

Eddy currents are generated in a metallic object when a magnetizing coil with alternating current in its windings is brought close to the object. These remarkable eddy currents are one of the most extensively utilized electromagnetic concepts with applications including thickness measurement of metal plates or insulating coatings, detection of surface flaws or discontinuities, conductivity testing, and identification of metal alloys.

These methods provide low-cost, high-speed testing of metallic materials, without requiring direct coupling.

Over the years, many different techniques and devices have been used in these various applications. In this study I will focus on the ideas related to finding the thickness of a thin metal sheet or plate. Research has shown that there may be a direct way to achieve this goal. These efforts have led to the method employed here, which uses experimental data to determine how the terminal impedance of an eddy current (EC) sensor is related to a physical property of the material-in this case, the thickness of thin films. Here I focus on copper layer thickness on printed circuit boards (PCBs) and aluminum foil.

EC sensors can be very simple or complex depending on the job to be done.

For this project, single coil EC sensors were constructed. It was also found that lowcost, off-the-shelf devices could be easily repurposed for use as sensors.