Shielding vs. Absorbing: What's the Difference?
By Mike Benson, 3M Applications Engineering Specialist

For those engineers and  buyers who don’t often dabble in the world of electromagnetic compatibility, considering the differences between EMI shields and EMI absorbers may be overlooked. 3M^TM has been kind enough to provide a primer for those interested in learning more.

EMI Shielding can be defined as a conductive material used to reflect electromagnetic waves to either A) contain them near the source or B) keep them from coming into contact with a sensitive receiver. Shields are typically incorporated into enclosures, cables, PCB cans/covers and component cans/covers.

The best shield completely covers the components that are radiating. In order for a shield to be effective, one important consideration is to make sure any apertures, slots and seams in an enclosure have lengths less than threshold length. The threshold length is equal to the wavelength of concern divided by 20 (Max. Slot Length < λ/20). Determining the shielding effectiveness of a material depends on material conductivity and thickness.  Thicker and more conductive materials offer better shielding effectiveness. Shielding effectiveness of material is generally presented in dB. Shielding cans, conductive gaskets, and foil & conductive fabric tapes are widely used for commercial products.

EMI Absorbers can be defined as a high magnetic permeability material that reduces the transmitted energy by converting a portion of the electromagnetic wave energy to thermal energy.  The temperature rise in the material is negligible from this conversion of typical low power electromagnetic waves.  These materials are effective to either A) reduce the transmitted electromagnetic noise or B) reduce the reflected electromagnetic noise when used with a shield. The sheet-type absorbing product is made of composite material, which is a soft magnetic particle embedded in a polymer. The materials will attenuate electromagnetic noise from about 50 MHz to several GHz range.  These products already have been used in many electronic systems as final phase solutions. Because absorbers can solve EMI problems that occur towards the end of the design cycle, they are incorporated into many products such as mobile phones, laptops, GPS devices, sonar devices, and many others.

What is the bottom line? Think twice when you're looking for a shielding material. An absorber could be more efficient and a quicker fix.