Particle-filled silicones for EMI shielding and environmental sealing: Page 2 of 3

June 01, 2015 // By Dominic Testo, Specialty Silicone Products
Particle-filled silicones are elastomeric compounds that combine the advantages of silicone with the electrical properties of metals. An inert, synthetic rubber, silicone offers thermal stability over a wide temperature range and resists ozone and ultraviolet (UV) light.
Material hardness

Conductive silicones don't have to be "too hard". For good reason, seal designers worry that a gasket made from a hard rubber will make an enclosure difficult to close, or allow the ingress of contaminants, chemicals, or water. Softer seal materials can improve compression, but it's still essential to avoid metal-on-metal contact. To address customer requests and industry complaints about metal-filled elastomers, SSP has developed ultra-soft silicones as low as 30 Shore A.

These specialty silicones are available with silver-coated or nickel-coated particles, and combine EMI shielding with environmental sealing. For example, SSP2426-30 is a 30-durometer silicone compound that's filed with silver-plated aluminum particles. SSP502-30 is a 30~ durometer nickel-coated, graphite-filled silicone elastomer. Both particle-filled silicones are supplied as fully-cured, conductive sheet stock; continuous extrusion profiles; and ready-to-mold compounds.

Tensile strength and tear strength

In addition to concerns about material hardness, some gasket fabricators believe that particle-filled silicones are too brittle and will tear easily. When a material lacks sufficient tensile strength, cutting a small part with a thin cross-section may result in tearing during packaging or application. With more expensive compounds, there are also concerns about the high cost of material waste. Even worse, a tear during product application can cause the loss of a conductive seal and allow the entry of RF waves.

To solve these problems, SSP has the capability to reinforce its EMI/RFI silicones with an inner layer of conductive fabric that enhances material strength while ensuring conductivity. Whereas other highly-filled compounds have an insufficient tear strength, SSP's nickel-coated graphite-filled silicones enable thinner, smaller, and lighter weight EMI/RFI solutions. For example, SSP502-65-032-Composite-NG is a nickel-coated, graphite-filled elastomer with a tensile strength of 950 psi and a Tear "B" ppi of 165.

Cost and performance

Gasket fabricators, die cutters, and molders are concerned about more than just the material properties of particle-filled conductive silicones. Cost is an important consideration, especially in commercial applications. Compounds that use silver-coated particles provide excellent shielding effectiveness, but silver is subject to volatility in the precious metals market. If the price of silver rises dramatically, an EMI/RFI shielding gasket that costs $15 today could cost several times that amount next year. For fabricators and their customers who need to manage costs, such uncertainty is undesirable.

To address these concerns about cost, SSP supplies superior nickel-coated graphite silicones that have the performance levels of silver-coated products. For example, the minimum shielding levels accepted by MIL-DTL-83528C is 100 dB from 20 MHz through 10 GHz. The high performance nickel-coated graphite formulations averages 115 dB from 20 MHz through 10 GHz. All third-party testing results are available upon request. This includes data on corrosion and salt spray.

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