EMI/RFI shielding for the connected world
01 November 2019
EMC, or electromagnetic compatibility, means the ability of electronic & electrical equipment and systems to function satisfactorily in their electromagnetic environment, without introducing intolerable electromagnetic disturbances to other equipment in that environment.
This article was originally featured in the November 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
As David Wall, Chairman of EMI shielding & environmental sealing specialist, Kemtron explains, this is best achieved through a combination of good circuit board design, filtering and RFI/EMI shielding of the enclosure.
In the past, RFI shielding has mainly addressed frequencies up to 10 GHz, but requirements for RFI/EMI shielding continue to grow, as many more electronic devices are now entering the market to satisfy user demand for ubiquitous connectivity and intelligence. Requirements are also changing as frequencies used are getting higher and ultra-reliable low latency communications between devices is necessary for safety critical applications needing real-time access to rapidly changing data, such as advanced driver assistance systems (ADAS).
Electric vehicles (EVs) are also providing a new market for EMC products, with shielding required for battery management systems, DC-DC converters, LED lighting and sensors. Electric vehicle charging infrastructure is also important, as successful adoption of EVs will rely on the ease of charging your vehicle quickly. The fast chargers typically found at motorway service stations require shielding for their DC-DC converters. Lower power charging points, like those for home use, only need to meet the minimum EMC requirement to the same level as street lighting.
The vastly increased number of sensors, communication and control devices that will be required for autonomous vehicles means that EMC between these elements is of the utmost importance. The LiDAR used in vehicles for adaptive cruise control is changing from short range radar at 24GHz to long range at 77-84GHz, giving far greater accuracy. This higher frequency gives better separation between objects that can be confusing for 24GHz systems. Microwave absorbing pads are also a requirement in these LiDAR sensors, to narrow the beam of the radar, so that in a cruise control scenario, they only measure the distance from the vehicle in front, and not vehicles travelling alongside. EMC is safety critical for autonomous vehicles. Any interference to the vast number of sensors that will be utilised could be catastrophic.
Another area where frequencies are getting higher is the long awaited and possibly controversial 5G network. European regulators have identified the 3.4-3.8GHz band and plan to harmonise it to make it suitable for 5G. It will be the main frequency band for the launch of 5G. For higher data rates, higher frequencies are required in the millimetre wave bands of 24GHz up to 86 GHz. Even higher frequencies are on the table for the future, but controversy regarding these high frequencies is rife.
Comments that frequencies at 96GHz can be weaponised, with issues from skin heating effects to headaches or even fertility problems (which some have claimed are potential problems since the introduction of mobile communications technology), but no evidence of these effects have been found to be true. High frequency for high data rate 5G will need many more base stations than 4G, but the low power of these base stations should ensure health problems do not become an issue.
For industry, the introduction of 5G opens up huge advances in the IoT (internet of things) and IIoT (industrial internet of things), with connectivity of machines, vehicles and systems. Industrial 5G will provide a wireless network eliminating cables within industrial environments, giving greater flexibility, better production layouts and the very high speed of 5G to eliminate lag and improve productivity.
All these new systems based on 5G will need to meet EMC legislation, and RFI shielding of enclosures and components will be necessary. The challenge for RFI shielding component and material manufacturers is to meet the demands of these higher frequencies. As previously mentioned, demand up to 10GHz has been the norm, and there is a test standard for conductive elastomers in MIL-DTL-83528. This standard is good tool for comparison between shielding manufacturers and ensures good performance.
But the problem for these high frequency requirements is that the standard stops at 10GHz and the test method is not suitable above this. RFI shielding effectiveness testing at higher frequencies for shielding gaskets is available, but not to any recognised standard, so results may vary between test methods.
Electrically conductive elastomers are the preferred materials for shielding enclosure seams at higher frequencies. The manufacture of electrically conductive elastomer involves balancing electrically conductive particle loading and distribution throughout the elastomer base (mainly silicone or fluorosilicone). The distribution must be sufficient to ensure that the particles are in contact with each other, to provide a good conductive path through the elastomer, but the loading must not be so great as to cause the material to lose its elastomeric properties. In short, electrically conductive rubber. The electrical properties of the conductive elastomer are measured by volume resistivity in ohms-cm and shielding effectiveness stated in db – however, the two do not directly correlate with each other.
The volume resistivity of silver-plated aluminium in silicone will be max 0.008Ocm, giving a shielding effectiveness at 10Ghz of 102db, whereas nickel coated graphite will have a volume resistivity more than 10 times greater, but exhibits a similar shielding effectiveness.
Nickel coated graphite in silicone is the most cost-effective conductive elastomer, giving excellent shielding characteristics, even though the material is much more resistive than precious metal plated particle elastomers. This excellent performance can be accredited to the fact that the nickel graphite particles are very irregular in shape and have sharp edges. When the gasket is put under pressure, being compressed between two surfaces, the particles dig into the surface, giving very low contact resistance. Graphite is also a good microwave absorber, thereby enhancing the shielding performance.
Nickel graphite in silicone costs approximately 60% less than silver plated aluminium in silicone, and 70% less than silver plated copper in silicone. Kemtron produce a flame-retardant version of nickel graphite in silicone, tested and approved to the international standard UL94V-0 by Underwriters Laboratories for flame retardancy (file number E344902).
Electrically conductive elastomers can also be moulded, extruded and fabricated into complex shapes to suit customers’ requirements.
Based in Braintree, Essex in the UK, Kemtron, in addition to conductive elastomers, manufacture a full range of shielding gaskets and components to provide a comprehensive EMI shielding service to the electronics industry, particularly for enclosure applications. Kemtron also operates continuous research and development programmes for new materials to meet the growing demands of its customers. These include new elastomer fillers, elastomer compounds and additive manufacturing of elastomeric materials.
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