Conductive coatings provide a clear alternative to mesh windows
As large LCDs have become an increasingly popular choice of user interface - particularly on portable electronics equipment such as medical devices - designers have had to address the EMI shielding challenges that result from having a significant aperture in an otherwise shielded equipment housing. Shielded windows fitted over the aperture, in front of the display, can restore the EMI-blocking property of the conductive enclosure.Many shielded windows comprise an acrylic, polycarbonate, polyester or glass substrate material encasing a conductive mesh. Although this achieves high EMI attenuation, it also blocks illumination from the display backlight and can impair the readability of the display. This can be particularly problematic in outdoor or other high ambient light environments.
Technologies, such as Chomerics’ WIN-SHIELD material, addresses this challenge by using a one-piece, lightweight window that has a transparent conductive coating such as indium-tin oxide (ITO). Where moderate levels of shielding effectiveness are required, this approach provides an alternative solution that has minimal effect on light transmission.
The surface resistance of the ITO coating is a key parameter governing shielding effectiveness. A thin coating is desirable to maximise light transmittance; on the other hand, increasing the coating thickness increases conductivity and hence improves the window’s shielding properties. For most applications, windows having ITO surface resistance in the region of 12 Ohms/sq provide an optimal balance between shielding effectiveness and optical performance.
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Parker Hannifin Ltd's Seal Group, Chomerics Division Europe, High Wycombe, Bucks, UK. www.parker.com
