These considerations are changing markedly with the introduction of new and sophisticated sensing, control and communications modules in new vehicle designs. This article outlines the EMC challenges that automotive system designers face today, and the implications for their choice of EMI shielding devices.
A two-fold problem
The challenge for EMC design in automotive systems is two-fold.
First, the range of frequencies which need to be attenuated will be far greater in new cars than it was in the past. Until recently, the main frequencies of interest were the AM and FM bands used by radio broadcasts, and frequencies below 3GHz used by Bluetooth radios and mobile phone networks.
With the planned introduction of 5G mobile phone network coverage possibly starting as early as 2019 in some countries, the frequency coverage of EMI shielding materials will need to be extended. A higher frequency range is not the only issue, however. Cars are also going to support a much greater number of wireless communications systems to support demand for vehicle-to-vehicle (V2V), vehicle-to-cloud (V2C) as well as vehicle-to-person (V2P) communication. High-speed communications buses will also connect the growing number of electronics modules such as LIDAR (optical) detection and ranging systems, park-assist systems and safety and monitoring systems to central electronic control units (ECUs). And sophisticated infotainment devices, such as high-definition video displays in the front seat headrests, also require their own high-speed, high-frequency networks.
Broad frequency coverage, then, is one aspect of the EMC designer’s challenge: the second dimension is that the effectiveness of EMI shielding is likely to be more tightly specified in future as automotive manufacturers move towards a strict view of the functional safety of the electronics systems in cars, codified in the ISO 26262 functional safety standard. ISO 26262 requires car makers to identify the ‘failure modes’ of electronics systems, to quantify the risk of failure attributable to each mode, and to take steps to limit the probability of failure, known as the ‘failure in time’ or FIT rate, to a specified maximum value.
Since EMI is a known failure mode for almost any electronic circuit, measures to attenuate RF emissions to safe levels are likely to be more strictly implemented under the ISO 26262 regime than ever before. This is all the more likely as electronic systems take over more and more aspects of the car’s road-going operations. In an autonomous car, the communications link between a LIDAR object-detection camera and the ECU responsible for control of the speed and direction of movement is as safety-critical as the hydraulic link between the brake pedal and the brakes in a non- or semi-autonomous car.
So the developers of tomorrow’s automotive systems will need to provide for higher attenuation of a wider range of frequencies than before. And always in the automotive industry, the pressure to minimise space, weight and cost is intense. What does this mean for the specification of EMI shielding materials?
Development of new elastomer fillers
Various types of electrically conductive elastomers are commonly used in EMI gaskets for shielding. Special elastomers offer useful properties, including resistance to high temperatures and contaminants, and the ability to provide environmental sealing to protect circuits from the ingress of liquids.
For instance, CHO-SEAL conductive elastomer from Parker Chomerics consists of a silicone, fluorosilicone, EPDM or fluorocarbon-fluorosilicone binder with a filler of pure silver, silver-plated copper, silver-plated aluminium, silver-plated nickel, silver-plated glass, nickel-plated graphite, nickel-plated aluminium or non-plated graphite particles. These elastomer gaskets resist compression set, accommodate low closure force, and help control air flow. They are available as standard extruded products or in custom shapes (see Figure 2).