Holland Shielding provides a basic overview of RFI shielding and the important points on using Faraday cages to reduce interference.
❱ ❱ A wide variety of seals and other
shielding devices are available to protect against radio frequency interference (image courtesy of Holland Shielding)
Tricks of the Shielding Trade
will be required. The shielding can be used to block external interference as well as to prevent emissions.
FARADAY CAGES The principle of shielding is to create a conductive layer completely surrounding the object you want to protect. The layer is known as a Faraday Cage after the inventor, Michael Faraday. Ideally, the shielding layer should comprise conductive
sheets or layers of metal that are welded or soldered together to ensure a continuous conductive path. However, a complete absence of interruptions isn’t realistic since the Faraday cage will need apertures for displays, ventilation, cooling, power supplies etc.
Shielding works in both directions so the Faraday cage
protects items inside the enclosure from external RFI and also prevents unwanted emissions from within the enclosure. The quality of the cage is expressed as the ratio of the field
strength in Volts/meter (V/m) inside the cage and outside the cage, measured in a logarithmic scale in dB. According to the scale, 40dB represents a reduction of field
strength by 100 times with each increase of 20dB representing about a ten times increase in attenuation up to 1 million times at 120dB.
FREQUENCY CONSIDERATIONS The frequency of the interference will have an effect on the material used for the cage and its thickness. Thicker materials are needed for lower frequencies. At 10kHz, for example, a thick sheet of as much as 6mm of mild steel is needed to achieve 80dB reduction.
hilst the prevention of emissions through electronics design considerations might be considered the ideal, there is no way of completely avoiding unwanted radio frequencies and some kind of shielding
In contrast, at 30MHz 0.03mm thick copper foil is sufficient for the same level of attenuation. For higher frequencies in the GHz region, the thickness of the shield depends more on mechanical properties than attenuation levels.
SHIELDING LEVELS For electronic devices, there are three common levels of shielding that complement each other and provide the best level of protection. These levels are at the device or component level, at the PCB or board level and at the enclosure level. Shielding at source (device level) is usually the most cost-
effective method through the application of a “shielding can”, which can be mounted over the device using spring clips or soldered pins. For clip mounting, the spring clips are first surface mounted to the PCB. Once the board assembly is complete, the can is simply pushed onto the clips. This allows the can to be removed and replaced as required.
Pin mounting is more permanent and is usually employed
as part of the PCB assembly process using through-the-hole technology. Board level protection can be achieved by covering the entire PCB in shielding material. This can be done by means of a small housing, custom-made to exactly the right shape, or by simply wrapping or sticking material around the PCB. Foils, textiles, stretch material, and wrapshields, cut to the appropriate shape, are easy to apply. Insulation layers will usually be needed to prevent short circuits. Enclosure level protection involves shielding the box
the electronic devices are enclosed in as well as the cabling that provides external connectivity. Housings are equipped with displays, entries for power and signal lines and cooling air-vents, all of which disrupt the continuity of the Faraday cage and therefore need their own protection. This protection usually takes the form of conductive gaskets and sealants.
EMC Testing Vol 2 No. 1 /// 9
| Page 2
| Page 3
| Page 4
| Page 5
| Page 6
| Page 7
| Page 8
| Page 9
| Page 10
| Page 11
| Page 12
| Page 13
| Page 14
| Page 15
| Page 16
| Page 17
| Page 18
| Page 19
| Page 20
| Page 21
| Page 22
| Page 23
| Page 24
| Page 25
| Page 26
| Page 27
| Page 28
| Page 29
| Page 30
| Page 31
| Page 32