EMI Material Guide to Electrically Conductive Elastomers

Many electronic designs need EMI materials that combine resistance to electromagnetic interference (EMI) with other application-specific requirements. For example, the EMI gaskets that are used in military touchscreens need to attenuate EMI emissions, provide electrical conductivity, and ensure environmental sealing. These shielding gaskets also must also cushion the unit from mechanical shock and be soft enough to avoid interfering with the display’s touch function.

The EMI shielding that’s used in automotive, aerospace, and medical electronics must also meet multiple requirements. For example, an EMI gasket that’s used with commercial aircraft may need to resist the splash of jet fuel or cleaning agents. EMI materials that are used in medical devices must combine required levels of shielding with corrosion resistance. Shielding that’s used with electric vehicle (EV) charging stations or robotics may require compliance with UL 94 standards for flammability. 

For electronic designers, EMI shielding decisions can be complex. Particle-filled silicones are used in many demanding applications, but can they meet all of your specific requirements? Are they cost-effective, and do they support design for manufacturability, too?  

Understanding Particle-Filled Silicones

Particle-filled silicones are elastomeric compounds that combine the advantages of silicone rubber with the electrical properties of metals. An inert, synthetic elastomer, silicone offers thermal stability over a wide temperature range along with resistance to ozone, water, and sunlight. When filled with tiny metal or metal-coated particles, silicone compounds combine EMI shielding and electrical conductivity with environmental sealing.

The table below shows the relationship between filler type, electrical conductivity, and typical volume resistivity (VR) as measured in ohms per centimeter. Direct methods for measuring shielding effectiveness can be expensive and complex, so VR is a commonly-used method for indicating EMI shielding effectiveness indirectly. Note that typically, the fill material in EMI gaskets is pure silver, or a silver-plated or nickel-coated material.    

Filler Type	Electrical Conductivity	Typical VR (ohms/cm)
Silver	Extremely Conductive	.0009
Silver-Aluminum	Super Conductive	.003
Silver-Copper	Super Conductive	.003
Silver-Glass	Very Conductive	.006
Nickel-Graphite	Conductive	.01
Carbon Black	Semi-Conductive	8.0

Electrical Conductivity, Material Properties, and Cost

Silicones have many desirable properties, but loading them with a high percentage of metal particles to increase electrical conductivity can have negative tradeoffs. That’s why historically, some designers have rejected particle-filled silicones as too hard or too brittle. Other designers have complained about part size limitations based on mold dimensions and long lead times for sheet materials. Some industry professionals also believe (incorrectly) that all particle-filled silicones are too thick to support thinner electronic designs.

The cost of older, particle-filled products also discouraged their use. For years, the filler of choice for shielding silicones was silver-aluminum. The U.S. military’s development of the MIL-DTL-83528 specification played an important role in this particle’s popularity. When silver began approaching $50 per Troy ounce in 2011, however, the fact that these elastomers were specified on thousands of gasket drawings and prints became problematic. Pure silver-filled silicones were even more expensive.

Today’s electronic designers can specify alternative particle fills. Choices such as nickel-graphite cost significantly less, as the table below shows. Note the difference between silver, silver-aluminum, and nickel-graphite.

Filler Type	Cost
Silver	$$$$$
Silver-Aluminum	$$$$
Silver-Copper	$$$$
Silver-Glass	$$$
Nickel-Graphite	$$
Carbon Black	$

Nickel-Graphite Silicones

Through research and material optimization, Specialty Silicone Products (SSP) of Ballston Spa, New York has developed cost-effective nickel-graphite silicones that perform at the shielding levels of silver-aluminum filled products. MIL-DTL-83528 provides a framework for comparison. This standard specifies a minimum shielding effectiveness of 100 dB. SSP’s nickel-graphite formula can achieve 125 dB.

The following table contains results from a third-party test report. It shows how SSP’s nickel-graphite silicones meet the shielding effectiveness requirements of MIL-DTL-83528 and are a suitable replacement for more expensive silver-aluminum products.

Frequency (MHz)	Reference Level (dB)	Dynamic Range (Analyzer Reading)	Test Sample (Analyzer Reading)	Dynamic Range (dB)	Nickel Graphite Gasket (Shielding Effectiveness) (dB)
20	95	-26.9	-25.1	121.9	120.1
30	100	-27.9	-24.5	129.9	124.5
40	100	-28	-24.3	128	124.3
60	100	-28.2	-25.1	128.2	125.1
80	100	-27.7	-25.5	127.7	125.5
100	100	-27.9	-25.2	127.9	125.2
200	100	-28.9	-27.7	128.9	127.2
400	100	-28.3	-26.3	128.3	126.3
601	100	-28.7	-26.1	128.7	126.1
800	100	-29.2	-27.1	129.2	127.1
1000	100	-17.8	-15.7	117.8	115.7
2000	100	-18.2	-15.5	118.2	115.5
4100	100	-17.9	-13.7	117.9	113.7
6000	100	-17.1	-13.1	117.1	113.1
8000	100	-17.2	-14.1	117.2	114.1
10000	100	-17.5	-15.7	117.5	115.7

SSP’s EMI materials include a range of products, but the table below describes nickel-graphite filled silicones in the 502 series. Note how softer materials with good tensile strength, elongation, and tear resistance combine desirable material properties with maximum VR levels for EMI shielding.

Product	Durometer (Shore A)	Tensile Strength (psi)	Elongation (%)	Tear B (ppi)	Maximum VR (ohm/cm)
SSP-502-30	30	100	400	N/A	0.300
SSP-502-40	45	150	200	25	0.030
SSP-502-55	55	150	200	25	0.040
SSP-502-65	65	200	200	35	0.040
SSP-502-75	75	270	250	35	0.040

SSP-502-Series silicones also resist salt spray and corrosion. These nickel-graphite elastomers have been independently tested according to ASTM B 117:2003, and test documentation is available.

Silver-Aluminum and Other Silver-Filled Conductive Elastomers

Electronic designers can still choose silver and silver-filled elastomers in various durometers for applications that require them. The table below describes silver and silver-filled elastomers from SSP that meet the requirements of MIL-DTL-83528. Included are two silver-aluminum products with a qualified product listing (QPL) from the Defense Logistics Agency (DLA), which is part of the U.S. Department of Defense.

Product	Fill Material	Base Elastomer	Durometer	Maximum VR (ohm/cm)	QPL
SSP-547-65	Silver-Copper	Silicone	65	0.004
SSP-2368-65	Silver-Aluminum	Silicone	65	0.008	Yes (Type B)
SSP-2486-70	Silver-Aluminum	Fluorosilicone	70	0.012	Yes (Type D)
SSP-550-45	Silver-Aluminum	Fluorosilicone	45	0.004
SSP-555-70 Passivated	Silver-Aluminum	Fluorosilicone	70	.012
SSP-555-65	Silver	Silicone	65	0.002
SSP-482-75	Silver-Nickel	Silicone	75	0.005
SSP-553-80	Silver-Copper	Silicone	80	0.005
SSP-416-65	Silver-Glass	Silicone	65	0.006

Fluorosilicones such as the silver-aluminum products in the table above have physical and mechanical properties that are very similar to standard silicones, but fluorosilicones provide improved resistance to fuels, oils, and solvents. On existing part drawings that reference the MIL-DTL-83528 specification, reinforced nickel-graphite fluorosilicones are a good choice for replacing older materials that contain expanded metal filters.

Overcoming Design and Manufacturing Challenges with Conductive Elastomers

Thanks to innovations in silicone compounding, particle-filled elastomers can meet demanding shielding requirements and many other project specifications. For example, because SSP-502 Series nickel-graphite silicones are available in 30, 40, and 45 durometer (Shore A), they’re soft enough for applications such as enclosure gaskets.

Unlike older shielding elastomers, these SpecShield™ products contain enough metal filler to ensure effective EMI shielding and electrical conductivity. SSP’s lower-durometer, nickel-graphite silicones can also be reinforced with an inner layer of conductive fabric for added conductivity and material strength, which helps to prevent brittleness and tearing during EMI gasket fabrication. 

As the only supplier of shielding elastomers that offers solid, heat-cured EMI silicones in continuous rolls, SSP supplies nickel-graphite silicones in higher durometers for applications that require harder materials. Compared to molded sheets, continuous rolls promote optimum yields for cost-effective conversion. Continuous rolls also support the use of automated equipment instead of time-consuming manual operations.

Various higher-durometer, nickel-graphite silicones are available, but SSP’s ArmourRFI™ is a special 65-durometer member of the SpecShield™ family that’s reinforced with an internal nickel-coated mesh. Other, higher-durometer materials in the SSP-502 series use fluorosilicone instead of silicone as the base material.  These flurosilicone shielding elastomers are available in 50, 60, and 80 durometers (Shore A).

In terms of manufacturability, SSP’s filled elastomers can meet requirements for larger sizes, thinner parts, shorter lead times, and multiple processing options. SSP’s particle-filled elastomers come in 15” wide continuous rolls, sheet sizes as thin as .010”, and as ready-to-mold compounds. EMI shielding materials are supplied in weeks rather than months, and support fabrication processes such as die cutting, water jet cutting, and press-cure molding.

During gasket cutting, SSP’s particle-filled elastomers won’t stretch or become deformed. Connector holes align properly, and the material’s structural properties support greater tear resistance – an important consideration for thinner wall gaskets. Product designers can also specify the use of an adhesive backing for ease-of-installation. For shielding applications where Z-axis conductivity is required, SpecShield™ and ArmouRFI™ materials support the use of electrically-conductive adhesives.

Get the EMI Materials You Need

Particle-filled silicones are good choice for meeting EMI shielding and many other application requirements. Electronic designers can choose from various types of filled elastomers, but it’s important to account for all of your project requirements – including cost and manufacturability. As silicone shielding elastomers are used in a growing number of military and commercial applications, designers can expect continued advancements in nickel-graphite and silver-aluminum materials.