EMC screening plastic enclosures with large viewing windows
07 February 2016
Paul Hoath describes two non-ITO options for extending the conductive coating across the large viewing window of a plastic enclosure while maintaining its optical transparency.
Hand-held controllers for use in industrial applications are becoming increasingly powerful as the Bluetooth, Wi-Fi and touchscreen drivers are now so cost-effective that they can be migrated into lower volume commercial products. However, industrial products, which can be used in a wide range of environments, often require IP rated enclosures and robust physical protection for the electronics.
Standard plastic enclosures, produced in a variety of designs and sizes, are cost-effective housings for low to medium volume requirements where a bespoke design cannot be justified. However, EMC screening may be required, and plastic enclosures have no intrinsic RFI attenuation properties.
Vero Technologies can modify standard, unscreened enclosures with the addition of internal conductive coatings, and offers three alternative standard coating materials and thicknesses to provide varying levels of attenuation at different price points. The company says this covers more than 98 percent of possible applications, but for specialised applications, other coatings are available.
For basic commercial requirements, the lowest cost entry-level 50micron nickel coating provides attenuation of 50dB at 1GHz. For the majority of applications, the most appropriate coating is a 10 or 20micron silver-coated copper particle coating, which typically attenuates by 78dB at 10GHz. The highest attenuation is achieved using a 22.5-micron silver coating, which gives in excess of 80dB at 1.5GHz. It provides an effective shield against RFI and EMI and can act as a ground plane to protect against electrostatic discharge.
Transparent viewing windows are becoming increasingly common; they provide physical protection for a capacitive touchscreen and also simplify maintaining the integrity of an environmentally sealed enclosure. The various internal coatings used are extremely effective, but they suffer from one major weakness in that they are all opaque, so for small enclosures with viewing windows, they are unsuitable. However, Vero has extended its capability, offering two options for extending the conductive coating across a viewing window while maintaining optical transparency.
Indium Tin Oxide (ITO) is a widely used material to make transparent conductive coatings for various types of displays, for antistatic coatings and EMI shielding. ITO offers a combination of transparency and conductivity, but at a high cost. It is typically applied using vacuum deposition, a costly process requiring considerable capital investment.
For several recent projects, Vero has developed a technique by which a transparent conductive film is sprayed onto the window in the same way as the body of the device is coated. While there are some trade-offs against vacuum deposition in terms of coating uniformity, opacity and conductivity, the results are totally acceptable for a normal 'commercial' application. Across the visible light spectrum, wavelengths from roughly 390 to 700µm, the transmission is around 90 percent at thicknesses of up to 6µm, falling slightly to 85 percent at 12µm. The EMC attenuation is directly proportional to the thickness of the coating.
Although it is not yet at a point where Vero can offer it commercially, research into using other alternatives to ITO is showing great promise. Vero is currently researching a transparent ink loaded with silver nanowires, which maintains transparency while simultaneously being electrically conductive and flexible. When coated on a plastic (typically PET) substrate, the resultant film contains a network of highly conductive yet transparent silver nanowires, which is easy to apply and gives more than 90 percent visible light transmission. The results, so far, look promising.
To download a white paper, giving details of the options available, click here.
Paul Hoath is with Vero Technologies
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