The Importance of EMI Shielding

Published: 20th October 2009
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The most common type of EMI occurs in the radio frequency (RF) range of the electromagnetic (EM) spectrum, from 104 to 1012 Hertz. This energy can be radiated by computer circuits, radio transmitters, fluorescent lamps, electric motors, overhead power lines, lightning, and many other sources.





Device failures caused by interference-or "noise"-from electromagnetic energy are increasing due to the growing number of products that contain sensitive electronic components. The smaller size and faster operating speeds of these components make it more difficult to manage the EM pollution they create. Increased device frequencies (applications over 10 GHz are now common) cause proportionally decreased wavelengths that can penetrate very small openings in housings and containers.





Increasingly strict regulations address a product's emissions. At the same time, a product's immunity to external EMI determines its commercial success or failure. To comply with regulations on both emissions and immunity (or susceptibility), designers and manufacturers integrate shielding in their product designs through a working knowledge of EMI behavior and shielding techniques.





There are many alternative processes competing with


EMI shielding compounds such as foils and conductive fabrics, metallic inner shields, and plastics with a conductive coating. Conductive paints are the most common method of applying a conductive surface coating to plastics, but vacuum metallization is gaining popularity.





While it is rather straightforward to compare the advantages and disadvantages of coatings versus compounds, it is more complicated to compare the shielding capabilities of the two methods. Compounds shield primarily by absorption, while coatings shield primarily by reflection.





The coating industry utilizes surface resistivity as a mechanism to denote shielding capability. However, for absorption shielding (where electrical conductivity is mostly within part walls), there is little correlation between surface resistivity and shielding capability. Volume resistivity is a better indicator of shielding effectiveness.





EMI shielding provides "immunity" for sensitive components from incoming electromagnetic interference (EMI) and/or prevents excessive emissions of EMI to other susceptible equipment. The primary methods for preventing emission or reception of EMI are proper circuit design/grounding and selection/placement of components (including specially designed filters).





When such preventive methods fail to meet agency standards, fail to achieve proper operation of the device, or are not economical, shielding with an EMI shielding compound is an excellent alternative.





Placing a Faraday Cage around an electrical device is the fundamental principle underlying the housing technique for


EMI shielding. Faraday Cages can be constructed using metal, metallized thermoplastic, or conductive thermoplastic-a compound material that is normally transparent to EMI but has been made opaque by the use of conductive fillers. Conductive thermoplastic compounds offer superior design and manufacturing freedom and are ready for use right out of the mold.





RTP. "EMI Shielding." 16 Oct. 2009 [http://www.rtpcompany.com/].

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