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Effective EMI shielding for viewing windows and displays

09 October 2014

Electromagnetic interference, or EMI, has long been the bane of manufacturers and users of electronic devices. The effects of EMI are well documented, but what is less well known is the reason why it’s becoming an ever-increasing problem.

There are two key issues driving the growing need for optical products that are shielded from EMI – the growing demand for equipment that is not just portable but can be used in uncontrolled environments and tougher regulations on electromagnetic compatibility (EMC). 

In 2016 The European Union will enforce the directive on EMC compliance(2014/30/60) and failure to comply will carry strong penalties. At the very least, a non-compliant product will have to be recalled and replaced and maximum penalties will be a £5,000 fine and three months in prison. 

There are also several acts of product-specific legislation governing EMI, including the Medical Device Directive (MDD). As for user demand, there is now a general expectation that equipment will operate anywhere with zero interference or disruption. Reliability should be the norm.

These changing circumstances have meant that shielded windows are no longer only of interest to those in military, aerospace or test and measurement applications - areas where it has always been vital that performance is protected from EMI. It’s fair to say in the current environment, all industries and areas using integrated displays in optical equipment must be as free from EMI leakage as possible.

The problem stems from the fact that in many items of equipment using display screens there is an inevitable gap between the equipment’s case and its screen – a gap that EMI will automatically exploit.

Although equipment is usually manufactured with built-in shielding against EMI (metallic casings, metallised plastic mouldings, conductive gaskets and even an additional environmental seal can all help to block radiation), any enclosure that has an aperture to accommodate a screen has an inherent weakness in terms of EMI and the larger the screen, the greater the problem.

The accepted solution in such cases is to fit a shielded window over the aperture, in front of the display to restore EMI-blocking capabilities of the conductive enclosure. With this the quality of the window and of the materials used can make a sizeable difference.

Essentially, a shielded window is a laminated clear panel made up of acrylic, polycarbonate, and polyester or a glass substrate material incorporating a conductive mesh. By maintaining electrical continuity, the mesh weakens any EMI signals that would otherwise pass through the gap between the screen and the casing.

With this mesh there is a compromise – or, rather, a balance – to be achieved between attenuating EMI and allowing sufficient light to reach the user's eye so the display can be read in a variety of environments. The size of the mesh aperture is crucial to how effective the shielding is. A more dense mesh would certainly do the job but would significantly reduce any illumination coming from the display backlight. 

Other factors governing how effective a shield is against EMI are the electrical connectivity between intersecting wires and the techniques and materials used to terminate the wire at the frame edge. Glass is the best material for use in a shielded window because it combines optical clarity with good scratch resistance.

On the other hand, windows that use polycarbonate or acrylic materials are lightweight and impact and shatter resistant. They can also operate at temperatures up to +100°C. Depending on the application, coatings can be added that are hydrophobic, oleophobic and anti-fogging.

Chomerics Europe, a division of Parker Hannifin, took all these factors into consideration when developing its shielded cast windows. For a product that has been manufactured at the company’s Grantham facility for optical windows for nearly a decade, the best optically performing, optically matched adhesives and optical-grade polycarbonate substrates are used.

The company also constructs mesh from copper or stainless steel, blackened through anodising to minimise reflections and to boost the optical performance of the window.

Its latest product in the battle against EMI is the WIN-SHIELD Mesh 100P, designed to be used in glass or plastic windows and shield equipment vents and displays without compromising airflow or the user's viewing experience. The mesh is easy to form into shapes and, if needed, can be stretched across small vent openings in an enclosure.

A noticeable difference in Mesh 100P is that it combines a shielding effectiveness of 55dB to 60dB (100 MHz to 1GHz) with a surface conductivity of <0.100 ohms/sq. The 61 percent open area copper mesh coated with an inert blackened polymer features 100 openings per inch (OPI) and wire diameter of 0.056mm. Not only does the mesh reduce glare for better visibility but the coating is designed not to fade, degrade, or react with moisture in the air.

With an operating temperature range of -40°C to +70°C, the shield is available in sheet form (maximum size 30,000 x 2,000mm) or can be supplied in custom die-cut shapes.

Chomerics has also developed WIN-SHIELD C, a cast monomer material that combines electromagnetic shielding with optical clarity. Applications include display filtering in test equipment, medical instrumentation, public information displays and military radios and computers.

Unlike most shielded window materials that consist of an electrically conductive medium such as a mesh laminated to a substrate, the WIN-SHIELD C is a single component material which boasts 90 percent light transmission and can be cast in thicknesses of between 1.5mm and 6.0mm. Capable of operating at temperatures from -60°C to +100°C, the material can be used in a range of challenging environments.

Also available are touchscreens that can be simply 'dropped in' where EMI shielding is needed. Ideal for hand-held mobile devices, rugged LCDs, devices used in critical patient care environments, shielded cabinets and racks and digital signage, the screens have EMI shielding built into them with no air gaps or voids thanks to optical grade pressure sensitive adhesives. They also have EMI shielding as thin as 0.177mm with 4 and 8 ohms/sq conductive films and plated stainless steel mesh with 50, 80 or 100 OPI.

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