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Stockwell Elastomerics, Inc.
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What Engineers Need to Consider When Selecting an EMI Shielding Material
Advances in particle-filled silicones for EMI shielding enable engineers to meet requirements for material performance, shielding effectiveness, and manufacturability, among others.
Dominic J. Testo
EMI/RFI Elastomers Product Manager
Specialty Silicone Products
Ballston Spa, N.Y
Electromagnetic interference (EMI) can disrupt military electronics and endanger the lives of the war-fighters who depend on them. The causes of EMI are numerous, and include everything from electric motors and radio transmitters to computer circuits, power lines, switches, and nearby electronic devices. Military radios, cameras, sensors, touchscreens, and unmanned systems also need protection against electronic jamming or intentional EMI, a threat that’s capturing the attention of U.S. military planners.
EMI shielding is also used with automotive electronics that direct drivers and manage vehicle safety and performance. Adaptive cruise control, emergency braking, and pedestrian detection systems are all susceptible to the unintentional generation, propagation, and reception of electromagnetic energy. Gasoline and diesel-powered cars aren’t the only vehicles that face potential problems, however. Electric vehicles and their charging systems are also susceptible to EMI, as are medical electronics, wireless devices, and satellite communications.
To prevent electromagnetic induction or electromagnetic radiation from interfering with electrical circuits, EMI shielding is used. Historically, EMI shields have been made of metal sheets and formed or fabricated into custom shapes. Sheets of copper, aluminum, and steel provide rigidity and strength, but can deform under the mechanical pressure that’s required for sealing. Once deformed, metal EMI shields tend to maintain their deformed shape and may admit electronic interference. Some metals are also susceptible to rust, corrosion, and oxidation.
Today, materials for EMI shielding include flexible metal screens, metal wires, and metal foams. Coatings made of metallic inks are applied to the interiors of electronic enclosures. Each EMI shielding method has its advantages, but electronic devices have different requirements. Silicone shielding elastomers that are filled with metal or metal-coated particles are often a good choice because 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 water, ozone, and ultraviolet light. Silicone rubber also forms tight seals, remains flexible at low temperatures, and stiffens up at high temperatures. In addition, silicone retains its elastic properties even after long periods of compressive stress. Conductive silicones are available as sheet stock, and as extrusions and ready-to-mold compounds, too.
When filled with particles such as silver-plated aluminum or nickel-coated graphite, silicone compounds combine excellent environmental sealing with reliable 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 levels for 12 material types over a frequency range of 20 MHz to 10 GHz. MIL-DTL-83528 also specifies hardness or durometer (Shore A) material types. In addition, some types are designated as low, medium, or high durometer.
Thanks to recent advances in silicone compounding, newer particle-filled silicones can meet the shielding effectiveness requirements of MIL-DTL-83528, which is also a useful benchmark for other demanding applications. Historically, however, particle-filled silicones came with significant drawbacks. These older compounds were harder, higher-durometer rubbers with poor compressibility. They were also limited to higher-cost conductive fills, such as silver-plated copper and silver-plated aluminum.
Today, some engineers remain skeptical that particle-filled silicones can meet all of their application requirements, especially for shielding effectiveness. Their concerns are important to address because they provide a framework for evaluating EMI materials.
Softer Silicones, Higher Shielding Effectiveness
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. Softer silicones are available in 30 and 40 durometers and have tensile strengths of 90 psi and 120 psi, respectively. Harder particle-filled silicones with greater tensile strengths for increased tear resistance are also available.
For additional resistance to tearing, lower-durometer, particle-filled silicones can be reinforced with an inner layer of conductive fabric or mesh. Whereas older, highly-filled silicone compounds had insufficient tear strength, these newer silicone compounds can support ruggedized products and thinner, smaller, lighter EMI shielding. Depending on a project’s specific requirements, these EMI materials can be used in military touchscreens and other applications.
Material properties such as durometer, tensile strength, and tear resistance are important, but they’re only part of what engineers need to consider during material selection. Importantly, conductive silicones that contain silver or silver-coated particles can meet the dB shielding effectiveness levels in the MIL-DTL-83528 specification. For example, Specialty Silicone Products (Ballston, Spa, N.Y.) supplies a 65-durometer, silver-plated aluminum silicone that’s been independently tested and certified to the requirements of MIL-DTL-83528, Type B.
As the MIL-DTL-83528 specification explains, Type B materials are silver-plated, aluminum-filled silicones capable of 100 dB of plane wave shielding effectiveness at 10 GHz, with a continuous use temperature range of -55°C to +160°C. The table below shows the results that an independent testing laboratory obtained for SSP’s material.
This silver-aluminum silicone is also part of the DLA’s Qualified Product List (QPL), which designates products where manufacturers have met military requirements and consistently followed internal quality standards.
Cost, Performance, and Manufacturability
Silver-filled silicones provide high levels of shielding effectiveness, but silver is subject to price volatility in the precious metals market. Consequently, an EMI gasket that’s cost-effective today can result in cost overruns next year. Shielding silicones filled with nickel-coated graphite particles provide a cost-effective alternative, and are comparable to nickel-graphite products in terms of shielding effectiveness.
Minimum plane-wave shielding effectiveness varies by MIL-DTL-83528 material type, but the lowest value is 80 dB (Type J) and the highest is 110 dB (Types C, F, G, K L, and M). Specialty Silicone Products’ 502-series 40-durometer, nickel-graphite filled silicone was tested according to this spec using a spectrum analyzer and signal generator in conjunction with appropriate power amplifiers and antennas.
The 26-inch x 26-inch test sample was compressed 10% under test, and test levels were recorded at each frequency. Attenuation values were determined by calculating the difference between the reference level and the test level. The table below contains the test results for this soft, 40-durometer silicone.
Shielding effectiveness and material properties are important, but so is manufacturability. For example, some EMI gaskets require an electrically-conductive adhesive backing that can keep the seal in place during installation and product refurbishment. Conductive sheet stock is the right form factor for many applications, but may result in material waste with bezel-style gaskets. That’s where the availability of shielding silicones as ready-to-mold compounds can help solve design challenges.
As the following case study from Stockwell Elastomerics shows, engineers can source particle-filled silicones that meet all of their application requirements, including material performance, shielding effectiveness, manufacturability, and, importantly, cost. The EMI gasket that Stockwell fabricated was for a military touchscreen, but the company’s example is applicable to other industries with demanding requirements.
Particle-filled Silicone Meets Demanding EMI Gasket Requirements
Stockwell Elastomerics, a custom component manufacturer that serves the defense and aerospace industry, needed an EMI gasket material that could withstand a wide range of physical demands. The military touchscreen would be deployed globally in rugged environments. The gasket needed to seal under the extremes of desert heat or arctic freeze, keeping out dust, rain, and water during wash-down. In addition to sealing, the customer wanted the EMI gasket to offer some cushioning to help protect the unit from mechanical shock.
Stockwell’s touchscreen gasket also needed to be soft enough to avoid distorting or interfering with the display’s touch function. Other requirements were that the EMI gasket needed an electrically-conductive adhesive backing and had to meet a specific price point. As Stockwell headed into a bidding process for the defense contract, the company selected a cost-effective material for both environmental sealing and EMI attenuation.
The particle-filled silicone that Stockwell chose, a nickel-graphite compound from SSP, met all of its requirements. Using this elastomer also supported the project’s two distinct timelines. The first was an engineering build, where EMI gaskets were needed very quickly to meet a deadline for testing. The second challenge was for production parts delivered in high quantities.
In order to meet the tight engineering-build deadline, Stockwell Elastomerics waterjet cut sheets that were laminated with an electrically-conductive adhesive from 3M. The waterjet cutting process allowed Stockwell to deliver custom-cut parts within two days and without tooling costs. Once the functional and EMI testing was completed, production tooling was made.
The same nickel-graphite filled silicone compound that was used to make the sheets was now used to mold rough blanks for the touch screen 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 the 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 DOD bid by delivering a water-sealed touchscreen that met the EMI attenuation requirements. The military touchscreen project also demonstrated the value of nickel-graphite filled silicones for demanding applications.
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