Dec. 23, 2024
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Nickel-graphite filled compounds have been developed with high shielding effectiveness values (over 100dB from 100MHz to 1GHz) at a cost far less than silver filled compounds. Further, nickel-graphite filled silicones withstand harsh environmental conditions including salt fog/spray, UV and ozone.
Nickel-graphite filled silicone gaskets are widely used for commercial applications where the higher cost of silver or copper filled materials is prohibitive. Industries such as lighting, signage, and instrumentation that historically haven't required EMI attenuation, now do as a result of integrated electronics for 'smart' features and tighter regulations. With more innovation and mobile devices, EMI emissions will need to be addressed by designers of OEM equipment ' and often at the end of the design cycle. Stockwell Elastomerics offers nickel-graphite filled silicones, readily available from inventory, in sheet form with thicknesses: .020', .032', .045', .062', .093', and .125' to support prototyping and fast-turn gasket production. Customers can send a CAD file, .stp or .dxf of the gasket design so Stockwell Elastomerics can determine the best choice of material and provide a waterjet cut gasket for testing.
Designers in defense and aerospace related applications are now specifying nickel-graphite filled silicone gaskets for custom molded gaskets, O-rings and fabricated gaskets, thereby reducing costs of applications where silver filled or silver plated particle filled compounds were previously specified. These compounds have been specified for applications ranging from seals for power supply gaskets to enclosure gaskets. The SNE-540, 40 durometer and SNE-556, 65 durometer nickel-graphite filled silicone compounds in particular have been subjected to extensive testing by several independent testing facilities. Along with shielding effectiveness values (see chart below), independent testing determined the thermal conductivity of SNE-556 to be 0.8 W/m-K and Low Temperature Flex, TR10 to pass at -40°C.
Stockwell Elastomerics inventories uncured nickel-graphite filled silicone compound as well as cured sheets from .020' to 0.125' thicknesses. If sheets are not immediately available, Stockwell Elastomerics' on-site Molding operations can quickly produce sheets to support requirements for die cut and fabricated gaskets. The standard sheet size is 15' x 20'.
If 15' x 20' standard sheets are not suitable due to large gasket dimensions, continuous roll material SNE-540-C is available in .010', .020' thick and .032' thick rolls up to 15' wide, custom thicknesses are available upon request. Continuous format offers designers the ability to design EMI shielding gaskets for larger enclosures with fewer splices. Stockwell Elastomerics has on-site water jet cutting for precise corner jointed gaskets and tightly fitting splices for environmental and shielding integrity.
Stockwell Elastomerics offers 3M conductive acrylic adhesive or 3M conductive silicone adhesives. Electrically conductive adhesives are laminated to EMI / RFI materials for ease of installation. Request a sample swatch of SNE-5XX series EMI gasket materials via the contact form.
SNE-540-R and SNE-556-R are 40 and 65 durometer (Shore 'A'), respectively, with low volume resistivity that challenges silver coated aluminum filled silicones. SNE-540-R and SNE-556-R have conductive fabric reinforcement that enhances the conductivity and tensile strength while maintaining Z-axis compression for EMI and environmental sealing.
It is typical for particle filled silicone rubber materials to have lower strength as a result of heavy loading of conductive filler. This lower strength is often problematic for gaskets with thin walls that are susceptible to breaking from over stretching. The fabric reinforcement in the SNE-540-R and SNE-556-R materials addresses these issues.
SNE-540-R and SNE-556-R fabric reinforced EMI silicones are manufactured in continuous rolls that are 12' and 15' wide. Continuous EMI gasket material enables large EMI gaskets to be made in one piece. Standard thicknesses are .020' and .032'; custom thicknesses are also available.
Stockwell Elastomerics offers 3M conductive acrylic adhesive or 3M electrically conductive silicone adhesives that can be laminated to these reinforced EMI gasket materials for ease of installation.
More complex designs, extending beyond flat cut shapes, are custom molded by Stockwell Elastomerics on-site in the United States. Compression molds are used to produce custom EMI gaskets and custom conductive parts. Three dimensional designs allow for tighter tolerances and the use of shape factors to reduce the closure force on the filled conductive polymers.
In the right quantities, molding custom EMI gaskets offers engineers significant unit cost savings over die cutting. EMI / RFI gasket materials tend to be higher in cost compared to unfilled silicone compounds; this is especially true for silver filled EMI compounds. The material savings associated with molding very often offsets the higher tooling cost of a custom mold. Generally, if the production quantities are expected to exceed pieces, the initial tooling investment in a compression mold will pay off with a reduced unit cost.
Stockwell Elastomerics inventories nickel-graphite EMI sheet material made from the same compounds used to mold custom EMI components. Stockwell Elastomerics can waterjet cut low volume EMI / RFI gaskets to prove out engineering designs, then these designs can be easily tooled up for molding when parts move into pre-production and production phases.
Stockwell Elastomerics is based in Philadelphia, PA and all gaskets and components are manufactured at this single location. Stockwell Elastomerics is an ITAR registered manufacturer, Cage Code . Additionally, all electrically conductive molding compounds and molds used by Stockwell Elastomerics are manufactured in the United States and certified with supporting test data. Stockwell Elastomerics is committed to supporting medical, defense and other sensitive industries that require the highest quality and material lot traceability.
Stockwell Elastomerics has partnered with Specialty Silicone Products (SSP) to develop product offerings in nickel-graphite filled silicone and fluorosilicone compounds, supplied with physical test reports to ensure properties are consistently met.
Nickel Graphite Filled Materials Available Molded and Fabricated for EMI Shielding Gaskets
MaterialDuro 'A'Tensile Strength (PSI)Elongation at Break (%)Tear Strength PPI of WidthVolume Res. (Ohm'cm)Comments, Additional Info SNE'.75' Soft EMI gasket material, available with conductive adhesive backing SNE'.05' Widely used for commercial EMI gasket applicationsNickel Graphite Filled Materials Test Results
MaterialTest FrequencyShielding Effectiveness EffectivenessTest MethodComments, Additional Info SNE-540 & SNE- MHz (E field)125.2MIL?DTL?DThird party tested ? DLS SNE-540 & SNE- MHz (E field)126.3MIL?DTL?DThird party tested ? DLS SNE-540 & SNE- MHz (E field)126.1MIL?DTL?DThird party tested ? DLS SNE-540 & SNE- GHz (Plane Wave)115.7MIL?DTL?DThird party tested ? DLS SNE-540 & SNE- GHz (Plane Wave)115.5MIL?DTL?DThird party tested ? DLS SNE-540 & SNE- GHz (Plane Wave)76.3MIL?DTL?DThird party tested ? DLSRogers Corporation's EC- nickel-graphite filled silicone rubber is a compressible 30 durometer electrically conductive gasketing material. EC- is the only nickel-graphite filled gasketing material available in 36' wide continuous rolls making it ideal for large EMI gaskets. Available in .062' and .125' thickness, EC- is UL 94HB flame rated. Application of 3M # conductive fabric supported acrylic adhesive provides both a conductive adhesive surface for EMI shielding integrity and dimensional stability to reduce tears during handling and assembly of this very conformable material.
Note that Rogers EC-, 40 durometer nickel-graphite filled silicone was recently discontinued. However, SNE-540, 40 durometer nickel-graphite filled silicone is now produced in continuous rolls .020' thick x 15' wide and .032' thick x 15' wide to replace EC- in Stockwell Elastomerics' product line.
Ohms Per Square
Conductive fabrics are made up of different fibers (e.g. nylon, cotton) and conductive metals (e.g. stainless steel, silver, copper). The resistance of a particular fabric depends on what conductor is used and how it is made. When purchasing conductive fabric the unit of resistance will be listed as Ohm/Sq or Ω /', meaning Ohms per Square.This unit of measurement calculates the sheet resistance of a material.
What does it mean?
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If a fabric is labeled as 2 Ω per ' it means that when the material is cut in a square, no matter how large or small that square is, it should be 2 Ohms. If cut in another dimension, such as a rectangle, the Ohms per inch are multiplied by the aspect ratio. For example:
If we define a 1" square an one unit and cut a rectangle that is 1" x 3", the aspect ratio if that rectangle is 3.
2 Ω (per ') x 3 (aspect ratio) = 6 Ohms
The thickness is also taken into account when coming up with this unit. If you wanted to calculate your own sheet resistance, two multimeters and 4 probes would be needed. We won't go over how to do this, the resistance you measure using your one multimeter will be more useful to you and your projects. If you would like to learn more, check out this explanation of Four Point Probe Resistivity Measurements.
Figuring Out Resistance
So, now we know what Ohm/Sq. means. This measurement is different than the resistance you will need when figuring out what voltage and current you will need and so on. It is helpful when buying material, look at this measurement as a guide to get a sense of it's conductivity.
The circuits you build will be made of specific sizes and shapes created by you. To get the resistance, cut you basic shapes and keep a multimeter by to test each trace or shape that you make. You may be able to come up with a unit of measurement yourself. If you are cnc cutting swirls and know that one swirl is 6 Ω, you then know that when you make your larger circuit, comprised of 10 swirls, the resistance will roughly be 60 Ω.
Voltage/Current Ratings
Most fabrics I have found and worked with do not state the current or voltage that the material can handle. If it is not available, be cautious when working and the manufacturer for advisement. Remember that it's uninsulated, so you if you are pumping a good amount of power through an exposed circuit, it can be dangerous. Be extremely careful not to create a short, you could get electrocuted! Jump to the step on insulation to learn how to protect and insulate yourself and the circuit from contact and weather conditions.
Conductive Vs. Resistive
Electrical conductivity measures a material's ability to conduct electrical current. If a material has high conductivity and low resistance, current moves freely through it.
Electrical resistivity is the measurement of how strongly a material opposes the flow of electrical current. If something has high resistance, it therefore has low conductivity.
Fabrics with electrical properties can be put into either category. I can't put a particular cut off point where one material becomes resistive and not conductive, because it will always be conductive and have resistivity. From my experience, when a fabric is called resistive, it usually means that it will measure to be 1K Ω/' or more.
When thinking of what makes a fabric conductive, I remind myself that wire typically used for traces and connections can be anywhere from .02 - 10Ω. This is dependent of length too. Always grab the multimeter to test for yourself!
How to Choose for a Specific Purpose
When used to replace traces in an electrical circuit, the fabric you want to use is the one with the lowest resistance.
For contactswitches, the same is true, choose a fabric that has low resistance. You can get away with high resistive fabrics sometimes, but it's easier to stick to one rule.
Capacitive touchswitches can be made using material that has a fairly high resistance, the change in voltage is all that is being detected.
Resistors can be replaced using resistive materials by cutting the right dimension of a resistive material to equal the value you are looking to replace.
When pressure or force is applied to piezoresistive materials the electrical resistance changes. This makes them ideal for creating sensors, especially force sensing resistors (FSRs), bend sensors and stretch sensors.
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