Next-generation automotive applications might include Bluetooth technology as a connectivity solution for portable devices and in-vehicle electronic systems. A possible application would be a Bluetooth-based hands-free cellular phone.

Table I provides an overview of typical operating frequencies for Bluetooth and other wireless automotive devices. All of these devices and systems are susceptible to electromagnetic interference (EMI). If not properly contained, EMI not only causes a device's own circuitry to malfunction but also affects the performance of other devices that are in close proximity.

Government agencies, such as the Federal Communications Commission (FCC), set specific requirements to limit EMI emissions from electronic devices. These requirements ensure that, under most circumstances, electronic devices will not interfere with one another. Moreover, automobile manufacturers have their own standards and specifications that these devices must meet. To meet the standards, manufacturers often need to make certain that devices are properly shielded. To address this problem, manufacturers employ a variety of shielding techniques to diminish EMI. As board-level components become more tightly packed on a board, shielding needs increase and surface-mount shielding options become the preferred method of shielding.

Engineers need to address many board-level EMI shielding issues, including the material composition, design, production, installation, and cost of the shields. With manufacturers' greater emphasis on speed and profitability, the need for cost-effective and efficient EMI shielding is greater than ever.

Surface-mount shields are made from a variety of metals and can be plated with tin or other solderable materials as needed. Common shielding materials include copper, BMI's (Schaumburg, IL) Shield Lite, aluminum 6061, brass, beryllium copper, iron, magnesium AZ91D, cold-rolled steel, nickel silver, and Permalloy 80 (see Table II for the pertinent properties of these materials).

Technology Typical Operating Frequency Bluetooth 2.4­2.483 GHz Satellite radio 2.32­2.34 GHz Electronic toll collection 900­928 MHz (U.S. only) 2.45 GHz 5.8 GHz Adaptive cruise control 35, 60, and 86 GHz 76­77 GHz (globally) Collision warning systems 76­77 GHz Remote door openers 300­960 MHz

Table I. A selection of wireless technologies and their typical operating frequencies.

Shielding Effectiveness. The shielding performance of different materials can be analyzed using the transmission theory of shielding first derived by S. A. Schelkunoff.¹ The transmission theory examines an incident plane wave onto a flat shield. The shielding performance has two major components: absorption and reflection. The absorption losses (measured in dB) in the shield material can be written as

The reflection losses (measured in dB) occurring at the shield-air interface can be written as

A^_r = 168 + 10 log (K^_r f^­1 µ^_r­1). (2)

In actual service conditions, often other parameters are the limiting factor for shielding effectiveness. As shown in Figure 1, the theoretical shielding effectiveness for the materials examined is greater than 100 dB, whereas in practice, lower values are observed because of the nonideality of the system and the presence of openings and apertures.

Material Density (g/cm3) Relative Magnetic Permeability µr (rounded) Relative Conductance Kr Thermal Conductivity (W / [m·K]) Pure copper 8.92 1 1.00 391 Shield Lite (BMI) 3.32 1 0.40 190 Aluminum 6061 2.70 1 0.46 180 Brass 8.53 1 0.28 120 Beryllium copper 8.36 1 0.20 105 Pure iron 7.87 1000 0.17 76 Magnesium AZ91D 1.81 1 0.12 72 Cold-rolled steel 7.87 200 0.12 50 Nickel silver 8.70 1 0.06 29 Permalloy 80 8.74 75,000 0.04 20

Table II. Properties of some common shielding materials.

Weight. In the past, the densities of materials used in surface-mount shielding were greater than 8 g/cm³. The high density can increase the overall weight of the board to unacceptable levels. Recent breakthroughs have allowed a reduction in density (and weight) without sacrificing the effectiveness of the shield.

Heat Dissipation. Another issue manufacturers face when designing the composition of shields is how to address the level of heat emitted by electronic devices. The operating temperature of circuitry can reach high levels and, if not properly managed, thermal stress can cause system malfunction. In various attempts to reduce heat within devices, special materials have been created with enhanced thermal conductive properties that help to dissipate the heat generated by the electronics, resulting in increased reliability.

Surface-mount technology has quickly become one of the easiest and most cost-effective ways to install components onto circuit boards in an assembly line system. Existing standard pick-and-place equipment can place the board-level, surface-mount shields just prior to reflow, after all the other components have been placed. Surface-mount shields can even be used on double-sided boards and are compatible with the new lead-free reflow processes. This allows the boards to be manufactured quickly and inexpensively.

Shielding designers continue to work to overcome a variety of EMI obstacles. Through relationships with industry leaders in cellular phone, nonvoice wireless, computer, medical electronics, and automotive technology, many innovative products have been created that tackle key issues such as weight, heat, size, production, and cost.

Figure 1. Theoretical shielding effectiveness (SE) for cold-rolled steel (CRS), nickel silver (NS), and Shield Lite (SL).

To meet the weight-reduction demands of modern electronic devices, a product is needed that is half as dense as traditional materials. EMI shields using a specially engineered material provide a lightweight solution. Such shielding also offers enhanced thermal properties, effectively removing the heat generated from circuit boards, providing better long-term reliability.

Multicompartment shields customized for each device are another solution to modern shielding problems. One multicompartment shield can take the place of three or more individual shields, consolidating space in small devices.

In addition to solving issues of space constraint, this type of shielding has benefits in production and cost. For example, there is no longer a need to maintain and manage several shield inventories because each multicompartment shield is specifically designed to replace multiple shields. Furthermore, multicompartment shields have a single-process installation, which reduces cost and simplifies a manufacturer's production.

Overall, the returns on the use of EMI shielding remain positive with continuous improvements in technology. In modern devices, the importance of selecting proper shielding for each unique application has increased.

For example, as automotive electronics technology evolves and advances with the increased application of wireless systems, more-innovative shielding options will be required for safe and seamless functioning. Knowledgeable and experienced suppliers are key to meeting these shielding needs.