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MAY 2021 I DESIGN007 MAGAZINE 33 5. Shielding Design e two issues with shielded enclosures are getting all pieces well-bonded to each other and to allow power or I/O cable to penetrate it without causing leakage of common mode currents. Bonding between sheet metal may require EMI gaskets or other bonding tech- niques. Figure 14 shows a handy chart for determining the 20 dB attenuation of a given slot length. For example, if a product design requires at least a 20 dB shielding effectiveness, then the longest slot length can be just one-half inch. Slots or apertures in shielded enclosures become issues when the longest dimension approaches a half wavelength. Figure 15 is a chart of wavelength vs. frequency. For exam- ple, a 6-inch (15 cm) slot has a half-wave res- onance at 1000 MHz. Generally, ventilation holes should be patterns of round holes no more than 1/4-inch diameter. Patterns of slots may be used, but they should be no longer than a half-inch in order to preserve an adequate shielding effectiveness. See References 11 and 12 for more detail on shielding. [11-12] Interference Technology also has a free downloadable 2016 EMI Shielding Guide with excellent information. [13] Summary Paying attention to these five product design faults will go a long way towards lowering the risk of EMI failure during formal compliance testing. Considering a proper EMC design early in project development will save tons of time and money in the end. DESIGN007 Figure 14: A chart of attenuation versus slot length. (Image source: Henry Ott)