Magnetic fields can pose a problem for electronic equipment, and attempting to shield for magnetic fields is not as simple and straightforward as for electric fields. No material is actually able to block magnetic fields without itself being attracted to the magnetic force. Unlike electricity, magnetic fields cannot technically be blocked or insulated, but can only be redirected. In general, high permeability materials, or those with the ability to support the formation of a magnetic field within themselves, are used for this purpose. When using a high permeability shielding enclosure to protect electrical components in the presence of a magnetic field, the shield works by diverting the magnetic flux and drawing the magnetic field lines into the shielding material rather than them passing into the protected space. The electromagnetic energy within the material is then dissipated and converted into heat, thus creating ohmic losses; this phenomenon is known as absorption loss.
Absorption loss is the primary shielding mechanism to shield low frequency magnetic fields. The absorption loss achieved by a magnetic shield is directly proportional to the shield’s material thickness, the permeability of the material, the conductivity of the material, and the frequency of the incident wave. Therefore, the ideal material choice for magnetic shielding would be a thicker, high permeability and electrically conductive material.
There are some overlooked aspects that need to be considered when choosing a magnetic material. The permeability of a material will in fact decrease as the frequency of the incident wave increases. For example, at 100 kHz the permeability of HyMu 80 is no better than cold-rolled steel. As a general rule of thumb, high permeability materials are ideal when dealing with frequencies below 10kHz. Another important factor is that the material’s effectiveness also depends on the magnetic field strength which it is being exposed to. High magnetic strengths can cause a material to become saturated, which will vary based on the thickness and type of the material being used.
The most popular material being used today in the magnetic shielding industry based on its superior characteristics with respect to permeability and saturation is an 80 wt% nickel-iron alloy that conforms to MIL-N-14411C, Composition 1 and/or ASTM A753, Type 4 such as HyMu 80. Leader Tech maintains stock of this material in varying thicknesses, and can help assist with the design of your enclosure and offer recommendations for your specific magnetic shielding application.
We at Leader Tech understand the need for magnetic shielding, and can help provide you with an appropriate and cost-effective solution for your unique application. In a future article, we will further explore the design considerations that need to be accounted for when designing magnetic shielding enclosures.