EMI/RFI Shielding Materials

Shielding effectiveness (SE) is the ratio of incident-to-transmitted RF field amplitude across an enclosure wall or gasket joint, expressed in decibels per the 20·log₁₀ convention used in MIL-DTL-83528 and IEEE Std 299. So 20 dB equals 10× field reduction; 60 dB equals 1,000×; 100 dB equals 100,000×. Targets: commercial FCC Part 15 / EN 55032 Class B typically calls for 40–60 dB; defense MIL-STD-461 typically 60–100 dB; high-margin avionics and EW (electronic warfare) housings often 100–120+ dB. SE is strongly frequency-dependent; a single-number SE claim without a frequency band is uninformative [2].

MIL-DTL-83528 defines conductive elastomer types by filler chemistry: Type A is silver-plated copper in silicone (highest SE, highest cost, galvanic risk on aluminum). Type B is silver-plated aluminum in silicone (balanced SE and corrosion compatibility — the workhorse for aluminum chassis). Type C is silver-plated copper in fluorosilicone (Type A conductivity plus fuel and solvent resistance). Type D is silver-plated aluminum in fluorosilicone (Type B chemistry plus fuel and solvent resistance).

Type M is nickel-coated graphite in silicone (cost-effective, > 100 dB SE, the standard non-silver alternative). Specify the Type letter (or chemistry name) within the same paragraph as any MIL-DTL-83528 reference [1].

Conductive elastomer gaskets require approximately 7–10% deflection of uncompressed thickness to achieve specified shielding effectiveness, per MIL-DTL-83528 Section 4.5.12 reference compression condition. Below this range, intermittent metal-to-metal contact between filler particles and flange surfaces produces unreliable SE. Above ~25% deflection, compression set accumulates and the gasket permanently deforms. Design the flange clamp force and fastener spacing to land in the 7–10% window across manufacturing tolerance and the full thermal expansion range of your operating envelope.

Yes — significantly. Standard sulfuric-acid anodize on aluminum creates a 15–25 µm dielectric oxide layer that is electrically insulating (volume resistivity > 10¹⁴ Ω·cm). Under a gasket footprint it breaks the conductive path from cover to chassis: the gasket itself may perform at spec, but the flange becomes an open circuit at DC and degrades to near-zero SE at RF.

The correct flange finish under a gasket is a conductive conversion coating — Alodine (hexavalent chromate), SurTec® 650 or similar Cr-free alternatives, or electroless nickel plating. Paint, powder coat, and hard anodize are all insulating and must be physically masked off the gasket footprint during surface treatment.

Total shielding effectiveness has three components per Schelkunoff’s equation: SE = R + A + B, where R is reflection loss, A is absorption loss, and B is the multiple-reflection correction term. Reflection shielding dominates at low frequencies (< 10 MHz) and is maximized by high-conductivity materials — copper, silver, aluminum foil. Absorption shielding dominates at high frequencies (> 100 MHz) and is maximized by high-permeability materials — nickel-graphite, ferrite-loaded compounds, mu-metal.

Most EMI gaskets provide both, but the balance shifts by frequency. Choose silver-based compounds for low-frequency reflection-dominated designs; choose nickel-graphite or ferromagnetic compounds for high-frequency absorption-dominated designs.

Field measurement of installed enclosure SE follows IEEE Std 299-2006 [2]: a transmit antenna outside the enclosure at a known distance, a receive antenna inside the shielded volume, and a spectrum analyzer comparing received signal with and without the enclosure present. Testing is frequency-sweep across the specified band — typically 20 MHz to 18 GHz. Field measurements rarely match MIL-DTL-83528 laboratory values because installed geometry includes apertures, seams, cable pass-throughs, and imperfect flange conditions.

A delta of 20–40 dB between lab material data and field enclosure SE is normal and indicates that the enclosure (not the gasket material) is the dominant leak path.

A solid conductive elastomer (e.g., SSP502 series) compresses by deformation of the cured silicone polymer; it delivers maximum SE, durability, and environmental sealing but requires meaningful closure force. A soft conductive material like BISCO® EC-2130 (despite its sponge-like positioning, it is technically a soft solid silicone, not a foam) and true conductive foam variants compress at much lower force, conform better to uneven mating surfaces, and are preferred for handheld electronics, low-fastener-count consumer designs, and seam designs where the closure force is fixed.

Solid elastomers dominate defense, aerospace, and industrial applications where compression set and continuous conductivity matter over years of service.

Conductive pressure-sensitive adhesive (PSA) is appropriate for non-load-bearing applications: board-level shields, I/O bezels, seam coverage, and retrofits on existing hardware where drilling is prohibited. PSA simplifies assembly but has limited temperature range — typically −40 to +105°C for 3M™ XYZ-axis conductive tape. Mechanical fastening (bolts plus a frame gasket) is required for structural loads, high-vibration environments, extended temperature service (−60 to +220°C with silicone-based SSP502), and any application requiring repeated disassembly. When in doubt, specify mechanical fastening — conductive PSA cannot be specified above its temperature rating without SE degradation.

Ground bond degradation is almost always corrosion at the metal-to-metal interface. Aluminum chassis faying surfaces oxidize naturally (aluminum oxide Al₂O₃ is insulating at > 10¹⁴ Ω·cm) and the oxide layer grows under humid or saline conditions. Stainless fastener paired with an aluminum chassis accelerates galvanic corrosion due to the ~0.6 V potential difference between the two metals.

Mitigations: apply conductive grease at installation; specify conductive finishes on both faying surfaces; match materials for fastener and chassis where possible; and specify corrosion-resistant EMI gaskets (SSP2529 nickel-aluminum or SSP550 silver-aluminum fluorosilicone) instead of standard nickel-graphite in marine or humid service.

Generally no. Conductive elastomer gaskets undergo permanent compression set when deflected — typical values are 10% compression set per ASTM D395 Method B after 22 hours at 100°C. After one compression cycle, the gasket’s recovery is partial, and second-installation contact pressure is reduced. This can drop SE by 10–30 dB. Foil tape gaskets with conductive PSA cannot be reused at all — the adhesive is single-service.

If a design requires repeated access (service panels, test ports), specify a higher-recovery elastomer with lower filler loading, or a spring-finger gasket (beryllium-copper or stainless) rather than a compression elastomer.

Unopened conductive silicone and fluorosilicone elastomers have indefinite shelf life when stored at 25°C / 50% RH or below, away from UV exposure and ozone sources (electrical equipment, photocopiers, corona-generating devices). Silver-filled compounds may darken cosmetically over years but retain full SE and volume resistivity performance. Conductive PSA backings have a typical shelf life of 12–24 months from the date of manufacture — after that, adhesion degrades. Nickel-graphite compounds have no meaningful shelf life limit; silver-aluminum and silver-copper compounds should be used within 10 years of manufacture per MIL-DTL-83528 Group A life testing provisions.

W. L. Gore discontinued the GS2100 and GS5200 conductive silicone gasket lines. The closest functional replacements are SSP502-40-V0 (matching the GS2100 durometer and SE band) and SSP502-60-V0 (matching the GS5200). Both are nickel-graphite silicone with UL 94 V-0 flame rating and equivalent published SE performance per third-party MIL-DTL-83528 testing. Program-specific qualification against the buyer’s drawing remains the program’s responsibility — the cross-reference matches base chemistry, durometer, and SE envelope, not full MIL-DTL-83528 QPL listing.

Both Type A (silver-copper, e.g., SSP2569 — industry cross-reference: Parker® CHO-SEAL® 1215) and Type B (silver-aluminum, e.g., SSP2368 — comparable to Parker® CHO-SEAL® 1285) deliver > 110 dB SE across 20 MHz–10 GHz per MIL-DTL-83528 methodology. The decision is corrosion. Silver-copper has a ~0.4 V galvanic potential difference vs. aluminum — in salt-fog, marine, or humid outdoor service, the copper filler galvanically attacks the aluminum chassis surface, eventually destroying the conductive path.

Silver-aluminum filler is nearly galvanically inert against aluminum chassis. For aluminum-chassis defense / aerospace applications, Type B is the safer default unless the program explicitly requires Type A peak SE.

H-O is a converter; the Qualified Products List (QPL) listings on MIL-DTL-83528 belong to the source-material manufacturers (SSP Inc., Parker Chomerics, etc.), not to converters. H-O converts MIL-DTL-83528 QPL-listed materials — including the SSP2569 (Type A), SSP2368 (Type B), SSP2486 (Type D), and SSP2571 (Type K) families — into die-cut gaskets, sheet stock, and custom-geometry parts. Material traceability and lot-code TDS records accompany every order, performed under our ISO 9001:2015 certified quality management system.

Yes, extruded EMI gasket profiles (D-strip, P-strip, hollow-O, custom cross-sections) are available through our partner network on typical 4–6 week tooling lead times for new profiles. Stock standard profiles are available faster. H-O’s in-house capability is die-cutting, kiss-cutting, slitting, lamination, and PSA application performed in Winsted, Connecticut; extrusion is routed through partners who specialize in continuous-profile conductive elastomer extrusion.

Key test methods cited on every conductive elastomer TDS: [9] ASTM D991 for volume resistivity of conductive rubber; [6] ASTM D2240 for Shore A durometer; [7] ASTM D575 for compression-deflection; ASTM D395 Method B for compression set; [3] ASTM B117 for salt-fog corrosion; [4] UL 94 for flammability; [5] ASTM E595 for outgassing (TML, CVCM); [2] IEEE Std 299-2006 for installed enclosure SE measurement; [1] MIL-DTL-83528 for the QPL conductive elastomer envelope.

Yes — flexible graphite (NeoGraf SpreaderShield™ SS300 through SS1500) has high in-plane thermal conductivity (300–1,500 W/m·K) and intrinsic electrical conductivity that provides secondary EMI attenuation. Peer-reviewed literature reports flexible graphite SE up to ~130 dB in the microwave band [18]. The material excels in applications where one component must spread heat and dampen RF simultaneously — LED driver enclosures, EV battery module shields, mmWave radio housings, server rack heat spreaders.

The trade-off versus a dedicated conductive elastomer is closure-force flexibility and sealing: flexible graphite is rigid by EMI gasket standards and does not seal moisture.

MIL-DTL-83528 SE testing is a material-level test on a flat gasket coupon in a defined test fixture; it characterizes the conductive elastomer compound’s intrinsic SE envelope across 20 MHz–10 GHz. IEEE Std 299-2006 is an installed-enclosure SE measurement: transmit and receive antennas paired across an installed enclosure wall, sweeping across the band. MIL-DTL-83528 tells you what the material can do under ideal conditions; IEEE 299 tells you what your enclosure actually delivers in service.

Expect a 20–40 dB delta between the two — the difference is dominated by seam quality, aperture geometry, and cable pass-through bonding, not by the gasket material.

Samples typically ship in 3–5 business days for common configurations on materials we keep on hand. Standard production runs are 2 weeks; special orders run custom lead times depending on material, geometry, and volume. Minimum order quantities (MOQs) vary by material and part — prototype quantities through full production runs are equally accepted. For extruded profiles (partner-network), allow 4–6 weeks for tooling on new geometries.

Standard shipping documentation includes packing slip, material lot code, and vendor technical data sheet (TDS) for the specific lot. On request: First Article Inspection (FAI) reports, Certificate of Analysis (CoA), Certificate of Conformance (CoC), and material traceability documentation back to the supplier’s QPL lot. H-O performs all converting work under our ISO 9001:2015 certified quality management system; ITAR-controlled programs are handled under our ITAR-registered processes.