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Electromagnetic interference (EMI) can disrupt military electronics systems and endanger the lives of war-fighters who depend on those systems. The causes of EMI are many, occurring wherever electromagnetic (EM) fields are produced. Sources include electric motors, radio transmitters, computer circuits, and power lines.

In some cases, such as military jammers, EMI is intentionally generated to disrupt tactical radios and other military electronic systems. EMI can also cause problems for civilian electronic systems—e.g., different types of vehicular electronic systems—and can upset commercial communications and medical electronic devices. Fortunately, properly-applied EMI shielding can solve the problem.

Fig. 1Historically, EMI shielding has been fabricated from metal sheets and formed into shapes as required to fit an electronic housing (a radio enclosure, for instance) as a barrier against EMI. Sheets made from aluminum, copper, and steel provide rigidity and strength, but can deform under the mechanical pressure required for sealing. Once deformed, metal EMI shields tend to remain in that shape and may not effectively block EMI, allowing leakage to and from electronic circuitry. Some metals are also susceptible to rust, corrosion, and oxidation, and can lose integrity over time.

Modern EMI/RFI shielding materials include flexible metal screens, metal wires, and metal foams. Coatings made of metallic inks are applied to the interiors of electronic enclosures to add EMI shielding. Each EMI shielding method has its advantages, but different electronic devices have different requirements. Silicone shielding elastomers filled with metal particles or metal-coated particles represent a versatile shielding solution, since these compounds combine the material properties of silicone rubber with the electrical properties of metals.

Sealing and Shielding

Silicones are a family of synthetic rubbers that provide thermal stability over a wide temperature range (typically -55 °C to +300°C) and resist the passage of water, ozone, and ultraviolet (UV) light. Silicone rubber is also capable of forming tight environmental seals, remains flexible at low temperatures, and stiffens at high temperatures. In addition, silicone retains elastic properties even after long periods of compressive stress. Conductive silicones are available as sheet stock, as extrusions, and as ready-to-mold compounds.

Fig. 2When filled with conductive particles such as silver-plated aluminum or nickel-coated graphite, silicone compounds combine excellent environmental sealing capabilities with consistent electrical conductivity and proven EMI shielding. MIL-DTL-83528, a Defense Logistics Agency (DLA) specification for electrically conductive elastomer shielding gaskets, establishes minimum shielding effectiveness (SE) levels from 20 MHz to 10 GHz for 12 material types. MIL-DTL-83528 also specifies the hardness or durometer (Shore A) of the different material types, with some designated as low, medium, or high durometer.

Due to recent advances in silicone compounding, newer particle-filled silicones can meet MIL-DTL-83528 SE requirements; such SE performance also serves as a useful benchmark for other demanding applications in commercial and industrial areas. Historically, particle-filled silicones came with significant drawbacks and were not a first choice for solving EMI shielding issues in military electronic designs. Older silicone EMI shielding compounds have been harder, higher-durometer rubbers with poor compressibility. They were also limited to higher-costing conductive fill materials, such as silver-plated copper and silver-plated aluminum.

Newer particle-filled silicones provide much improved EMI shielding performance. Still, doubt remains whether these newer, softer silicones can meet the needs of the most demanding EMI shielding requirements, especially for SE performance. To better understand the capabilities of such flexible materials for rigorous aerospace and defense-based EMI shielding requirements, it may help to evaluate several particle-filled silicones from Specialty Silicone Products (SSP), and how they were used in an actual application.

Softer Silicones, Higher SE

Newer particle-filled silicones for EMI shielding include lower-durometer compounds that resist tearing—a problem that can occur during gasket fabrication due to “pulling” during cutting. For example, softer silicones are available from SSP with durometer values of 30 and 40 and tensile strengths of 90 and 120 psi, respectively. For applications requiring increased tear resistance, harder particle-filled silicones with greater tensile strength are also available.

In addition, for greater resistance to tearing, lower-durometer, particle-filled silicones can be reinforced by means of an inner layer of conductive fabric or mesh. In comparison to older particle-filled silicone compounds lacking tear resistance, newer silicone compounds enable rugged shielding products that are thinner, smaller, and lighter than those earlier compounds, while providing improved EMI shielding capabilities.

Material mechanical properties such as durometer, tensile strength, and tear resistance are important for an EMI shield, but only some of the factors to consider when selecting a material for an EMI shield. Conductive silicones containing silver or silver-coated particles can meet the minimum SE requirements detailed in MIL-DTL-83528. As an example, one such material from SSP, a 65-durometer, silver-plated aluminum silicone, has been independently tested and certified to MIL-DTL-83528, Type B (Fig. 1). MIL-DTL-83528, Type B materials are silver-plated, aluminum-filled silicones with an operating temperature range of  -55 to +160°C that are capable of 100-dB plane-wave SE at 10 GHz.

Table 1Table 1 offers a summary of the results from an independent testing laboratory for this silicone filled with silver-plated aluminum at different frequencies. As a comparison, Table 2 provides independent test results for a silicone filled with nickel-coated graphite particles.

While material properties and SE are important in selecting an EMI shielding material, so is manufacturability. For example, some EMI gaskets require an electrically conductive adhesive backing to keep the seal in place during gasket installation and product refurbishment. Conductive sheet stock provides excellent SE and is available in the right form factor for many applications, but may result in material waste with bezel-style gaskets.

In some cases, the availability of EMI shielding silicones as ready-to-mold compounds can help solve design challenges. As one example, a requirement for an EMI gasket for a military touchscreen display was handled by Stockwell Elastomerics using particle-filled silicones to meet all the application requirements. These include material performance, SE, manufacturability, and—perhaps most importantly—cost. While this example involves a military application, it applies to other market areas just as well, including for commercial and industrial applications.

Table 2In this particular application, an EMI gasket material with outstanding mechanical capabilities was needed. The military touchscreen would be deployed globally, often in rugged environments. A candidate material would need to maintain a mechanical seal under environmental extremes of desert heat and arctic freeze, keeping out dust, rain, and water during a wash down.

The customer also wanted the EMI gasket to provide some cushioning for protection from mechanical shock. In addition, the touchscreen gasket needed to be soft enough to avoid distorting or interfering with the display’s touch functionality. Any candidate EMI gasket required an electrically conductive adhesive and had to meet a specific price point.

To win this defense contract, Stockwell selected a cost-effective material for both environmental sealing and EMI attenuation. The material is a silicone filled with nickel-graphite particles supplied by SSP (Fig. 2). Use of the elastomer also supported the project’s two distinct timelines. The first involved completing an engineering build where EMI gaskets were needed quickly to prototype testing. The second challenge was to provide EMI gaskets for production parts delivered in high quantities.

To meet the tight engineering-build deadline, Stockwell Elastomerics waterjet-cut sheets laminated with an electrically conductive adhesive from 3M. The waterjet cutting process made it possible to deliver custom-cut parts within two days and without tooling costs. Once functional and EMI testing were completed, production tooling was assembled for the larger quantiles.

The same nickel-graphite-filled silicone compound used to make the sheets was now used to mold rough blanks for the touchscreen display gasket. These molded blanks greatly reduced material waste while still allowing for proper adhesive lamination of the narrow wall gasket. The adhesive-backed blanks were then cut to final gasket geometry and tolerances.

This two-step approach allowed Stockwell’s customer to meet its timeline and test parts without any tooling investment. In turn, this provided the client with a pricing advantage that helped win the Department of Defense (DoD) bid by delivering a water-sealed touchscreen display that met EMI attenuation requirements. This military touchscreen-display project also demonstrated the value of nickel-graphite-filled silicones for demanding EMI shielding applications.

Dominic J. Testo, Product Manager

Specialty Silicone Products, Inc., 3 McCrea Hill Rd., Ballston Spa, NY 12020; (518) 885-8826

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