Shielding for Automotive EMC Design

Gary Fenical

EMI suppression techniques such as RF gasketing and PCB shields are essential to the automotive emissions environment.

Automobiles have a myriad of electrical and electronic devices that must work together in a relatively close environment. The vehicle and the electronics within it are also exposed to a vast external electromagnetic environment. Along with these conditions, these components must meet mandatory electromagnetic compatibility (EMC) requirements imposed by the auto manufacturers.

The concerns from an EMC point of view are not only the electromagnetic emissions from these devices, but also their susceptibility to electromagnetic emissions from other devices both within and outside the vehicle (see Figure 1). The equation for emissions is:

where E = field strength in µV/m, A = loop area in square centimeters, I = drive current in amps, F = frequency in megahertz, d = separation distance in meters, and S = shielding ratio between source and point of measurement.

Figure 1. Automotive electronics must be immune to interference from one another.

Analyzing Equation 1, it is clear that frequency is the biggest culprit because the emissions increase as the square of the frequency (F) increases. For current (I), emissions increase linearly, which is also true for loop area (A). The distance (d) is set by the specification, and 1.316 is a constant. The system designer has no control over these last two parameters, so they must not be considered. The equation for susceptibility is:

where Vi = volts induced into the loop, A = loop area in square meters, E = field strength in volts per meter, F = frequency in megahertz, B = bandwidth factor (in band: B = 1; out of band: B = circuit attenuation), and S = shielding (ratio) protecting circuit.

Equation 2 indicates that immunity is directly proportional to loop area (A), frequency (F), and the bandwidth factor (B). Frequency (F) cannot be considered in this case because it is dictated by the specification, as is the field strength (E). Of course, the engineer has no control over 2π, or 300, which is the speed of light divided by 1,000,000 for this equation.

From the equations, it is possible to determine some key information. Emission levels are:

• Directly related to loop area.

• Directly related to signal current.

• A function of frequency squared.

• Inversely related to shielding effectiveness.

Susceptibility levels are:

• Directly related to loop area.

• Directly related to bandwidth.

• Directly related to the transmitted frequency and field strength.

• Inversely related to shielding effectiveness.

From the point of view of an EMC engineer, loop area is defined as the length times the separation distance of the conductors on which a signal travels from the time it leaves its source until it returns. In an automotive electronic system, this can be very hard to define, especially if the signal return path is the frame or metal shell of the vehicle. There are two common ways to minimize loop area for circuits in which the signal travels along wires or along other conductors within the vehicle. One method is to place the signal conductor as close as possible to the metal ground of the vehicle. The second is to provide a signal-return conductor that runs along the signal conductor (see Figure 2).

Operating at the lowest possible frequency and minimizing loop area, circuit current, and bandwidth do not always reduce emissions sufficiently. If the design still cannot meet its requirements, the only parameter left in the equation is shielding.

Shielding Options

Shielding, which is noninvasive and does not affect high-speed operation, works for both emissions and susceptibility. It can be a stand-alone solution, but it is more cost-effective when combined with other suppression techniques such as filtering, grounding, and proper design to minimize loop area. It is also important to note that shielding usually can be installed after the design is complete. However, it is much more cost-effective and generally more efficient to design shielding into the device from the beginning as part of the design process. It is important to keep in mind that the other suppression techniques generally cannot be added easily once the device has gone beyond the prototype stage.

The use of shielding can take many forms, from radio-frequency (RF) gaskets to printed circuit board (PCB) shields. A device housed in a metal case is generally a good candidate for RF gasketing materials. PCB shields are better suited for devices in nonconductive cases.

Many new shielding materials are currently offered that were not on the market just a few years ago. RF gaskets are now available in materials made from beryllium copper, copper, bronze, phosphor bronze, stainless steel, aluminum, and cold- rolled steel.

Knitted wire-mesh gaskets are available in many different profiles, with several wire choices such as Monel, aluminum, tin-plated copper-clad steel, stainless steel, and beryllium copper. The beryllium-copper knitted wire-mesh gasket is the only material that is an excellent spring and does not require an elastomer core to function as a gasket. Also, the beryllium copper knitted wire-mesh gasket is generally 20 dB more conductive than any other material. It can also be plated for galvanic compatibility.

Conductive elastomers with many different material choices, and elastomer-conductive filler combinations, are available. Conductive fillers include, but are not limited to:

• Carbon (C).

• Passivated aluminum (IA).

• Silver-plated aluminum (Ag/Al).

• Silver-plated copper (Ag/Cu).

• Silver-plated glass (Ag/G).

• Silver-plated nickel (Ag/Ni).

• Nickel-coated carbon (Ni/C).

• Silver (Ag).

Elastomer options include:

• Silicone rubber.

• Fluorosilicone rubber.

• Ethylene propylene diene monomer (EPDM).

• Fluorocarbon rubber, Viton, or Fluorel.

Oriented wire is a conductive elastomer in which individual conductive wires of either Monel or aluminum are impregnated into solid or sponge silicone. Newer types of RF gaskets include conductive fabric-over-foam, conductive foam, form-in-place conductive elastomers, mold-in-place conductive elastomers, and printed conductive gaskets.

Selecting Proper Materials

Many factors affect the proper selection of RF gasket materials. The following list identifies some of the key issues that must be considered. The list has been developed over many years and includes essential considerations for choosing RF gasket materials to ensure that the materials achieve their advertised specifications.

• Operating frequency.

• Materials compatibility.

• Corrosive considerations.

• EMC compliance specification (SAE, FMC, GM, etc.).

• Operating environment.

• Load and forces.

• Cost.

• Attenuation performance.

• Fastening and mounting methods.

• Storage environment.

• Nuclear, biological, chemical (NBC).

• Cycle life.

• Shielding, grounding, etc.

• Electrical requirements.

• Materials thickness/alloy.

• Space and weight considerations.

• Product safety.

• Recyclability.

Longevity of the RF gasket material is affected by many factors. Compression, vibration, and number of cycles all affect longevity. In addition, whether the gasket is in shear or compression can play a role. However, galvanic compatibility is arguably the most important factor for the longevity of RF performance, especially in the automotive environment.

Corrosion is a natural phenomenon that converts material from an unstable to a stable state. When corrosion is caused by an electrochemical reaction in the presence of a dissimilar metal, it is called galvanic corrosion. Several conditions can increase galvanic corrosion: two dissimilar metals at a junction, electrical contact between two dissimilar metals, or the presence of an electrolyte. To reduce the effects of the dissimilar metals, either plate the metals or choose RF gasket materials that are galvanically compatible.

It is usually easier and less expensive to control EMI at its source. An RF solution that works closer to the source is a PCB shield. PCB shields can be a cost-effective way to control electromagnetic interference (EMI). PCB shields are an excellent way to achieve this control.

When electrical and electronic circuits are in nonconductive enclosures, or when it is difficult or impossible to use RF gasketing, PCB shields provide the best option for EMI suppression. A properly designed and installed PCB shield can actually eliminate the entire loop area because the offending or affected circuit will be contained within the shield.

If PCB shields are considered during the design stage, sections of the PCB can be used as part of the shield. PCB shields can be designed for maximum efficiency and minimum size. Installing PCB shields late in the design can render them less effective. Moreover, it can increase costs because changes in the PCB layout are usually required. When shields are installed later, the PCB design will be less efficient and will take up more board space than necessary.

Heat can be an issue when using PCB shields. Ventilation is usually an adequate way to address this problem. However, if ventilation does not provide enough heat dissipation, PCB shields are available with integral heat sinks (see Figure 3).

Figure 3. A PCB shield with integral heat sinks can provide additional heat dissipation.

Conclusion

In today's automotive market, electronic devices are a fundamental part of automobile design and function. These devices cannot interfere with other devices, and they certainly should not receive and react to interference, especially if they are critical to vehicle operation.

Generally, controlling factors such as frequency, current, bandwidth, and loop area can address automotive EMI. When these methods are insufficient, however, suppression techniques such as RF gasketing and PCB shields are often useful. Understanding the potential problems of the circuit during the design stage allows designing in these techniques rather than using them as a Band- Aid approach. Designing for these suppression techniques is more cost-effective than trying to install them at a late stage in the design.

Gary Fenical is the EMC technical support engineer for Laird Technologies (Delaware Water Gap, PA). He is a NARTE-certified EMC and ESD engineer. He can be reached at 570-424-8510, ext. 1177, or via e-mail at gfenical@lairdtech.com.