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EMC Question of the Week: October 8, 2018
For effective magnetic field shielding at frequencies above 100 kHz, which material property tends to be most important?
- permeability
- permittivity
- conductivity
- thickness
Answer
The best answer is "c". Incident magnetic fields induce eddy currents in conductive materials. The eddy currents generate their own magnetic fields that oppose the incident field. At higher frequencies, magnetic fields cannot penetrate conductive materials due to these eddy currents. Therefore, the best high-frequency magnetic field shields tend to be good conductors.
The circulating eddy currents experience both a resistance and an inductance. The inductance is associated with the opposing magnetic flux being generated by these currents. As long as the inductance dominates, the eddy currents are blocking most of the incident field. However, at lower frequencies (or at lower conductivities), the resistance becomes more significant and limits the ability of the eddy currents to counter the incident field.
For a given conductivity, thicker materials present a lower resistance to the eddy currents and tend to work better at low frequencies. Nevertheless, at frequencies below 100 Hz, excellent conductors like copper or aluminum are virtually invisible to magnetic fields even at thicknesses of several millimeters. On the other hand, at 100 kHz and higher, materials as thin as the half-ounce copper in a circuit board plane layer provide excellent magnetic-field shielding.
Low-frequency magnetic field shielding requires materials with a high permeability to capture and redirect the magnetic field lines. This option generally requires shields that tend to be heavy, bulky and/or expensive. For this reason, conductive materials are the preferred option at frequencies where they are effective.
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