Open screens for local magnetic shielding at low frequencies

We discuss an approach to magnetic shielding of components mounted on a printed board, typically transducers, sensitive to electromagnetic interference in the frequency range between a few hundred Hz and a few kHz. While outside the frequency domain usually contemplated in electromagnetic compatibility studies, which is normally above 9 kHz, the here considered magnetic noise can generate artifacts in many types of transducers mounted on the PCB and can frustrate many filtering techniques, especially when the signal to be detected and the noise have similar spectral profile. In this work, the shielding properties of two types of screens, open and semi-closed, and the role of three materials (mu-metal, low-carbon steel, and copper) are investigated. The circumstance where the component to be shielded is endowed with his own magnetic casing is also considered. Modeling, performed by a 3D finite element method based on a vector potential is associated with measurements of the shielding factor. It is shown that low-carbon steels can comply with practical shielding requirements, with an acceptable reduction of performances compared to the high-permeability mu-metal alloy. On the other hand, purely conductive screens prove quite inferior under the present specific geometric and frequency constraints.