What is an EMI gasket?

What the gasket does at a seam

A shielded enclosure works as long as its walls stay electrically continuous. Every joint breaks that: a cover, a door or a bolted panel leaves a gap, and the 2000 Parker Chomerics® handbook states that a gasket is needed only because such joints are imperfect, since a welded closure or perfectly flat, stiff and conductive flanges would need none [3]. A conductive gasket conforms to both flange faces and carries the current across the joint, so the seam no longer behaves like a slot that radiates [3]. Most conductive elastomer gaskets also seal the same joint against water, dust and pressure, so one part can do both jobs [4].

The EMI gasket design guide covers the groove, the deflection and the flange finish that make both jobs work, and the forms guide compares the gasket types.

When an enclosure needs one

The requirement comes from the equipment, not the gasket. Each market has a document that limits what equipment may emit, what it must withstand, or both, and the enclosure designer decides whether the seams need gaskets to meet it. None of the documents below specifies a gasket; they set limits the finished equipment is tested against.

At the seam level, the signs that a joint needs a conductive gasket are a cover, door or panel that must open and so cannot be welded shut, a flange whose mating face would otherwise be painted, a long seam between fasteners, and a top frequency high enough that the gaps approach half a wavelength [3]. The handbook's rule is that once fastener spacing approaches half a wavelength at the highest frequency of interest, the seam performs only as well as its point of least pressure [3].

Half a wavelength, by frequency

Because the half-wavelength rule sets how long a gap or a fastener spacing can be, it helps to see the numbers. The table is our own arithmetic from the speed of light, not a design limit; the design guide gives the handbook's fastener and deflection rules.

A gap that is a tiny fraction of a wavelength at 100 MHz is a large fraction of one at 10 GHz, so the same seam can pass at one frequency and leak at another.

EMI, RFI and EMC

The three terms describe one problem from three sides. The EU EMC Directive defines electromagnetic compatibility as “the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to other equipment in that environment”, and immunity as performing as intended without degradation in the presence of a disturbance [1]. Electromagnetic interference is the disturbance itself, in the Directive's words “any electromagnetic phenomenon which may degrade the performance of equipment” [1]. Radio-frequency interference is the part of it in the radio spectrum, which the FCC defines as 9 kHz to 3,000,000 MHz [2].

In the gasket trade the words are used interchangeably: an EMI gasket, an RFI gasket and an EMI/RFI shielding gasket are the same parts. EMC is the goal the equipment is tested against; the gasket is one of the ways the enclosure gets there.

Conductive gasket or grounding gasket

Both conduct, but they do different jobs. A shielding gasket runs the full length of a seam and keeps it continuous, so it is specified by shielding effectiveness in dB and by volume resistivity, and it usually seals the joint as well [3][4]. A grounding gasket or pad makes a low-resistance bond at a point or over a small area, joining two conductive parts to the same ground: the handbook describes grounding pads kiss-cut from hollow D extrusion for peel-and-place assembly, and 3M® lists grounding an EMI cage to a board's ground trace among the uses of a conductive tape [3][9]. A grounding part is judged by its contact resistance, not by a shielding figure.

A third group is easy to confuse with both. Some conductive materials are made for static dissipation, and a compounder notes that materials which conduct only slightly do not perform as shields at higher frequencies [10]. If a drawing says conductive, the resistivity or the shielding figure on it tells you which of the three it means; the testing guide explains those figures.

The forms, and where each is covered

The material behind every form is covered on conductive elastomers (fillers and binders), form-in-place beads, knitted mesh, fingerstock and conductive foam are compared on the forms guide, and the military specification on MIL-DTL-83528 explained. When a drawing calls out MIL-DTL-83528, the material comes from manufacturers on the QPL-83528 list.

Medical and avionics enclosures: what drives the choice

No gasket is best for every enclosure, and the drawing names the filler, the binder and the form; what changes between markets is what the drawing has to account for. For medical electrical equipment, IEC 60601-1-2 covers basic safety and essential performance in the presence of electromagnetic disturbances and sets no shielding figure for the enclosure [8]. Where the device is cleaned or disinfected in service, fluid resistance belongs to the binder rather than the filler, so the failure modes guide and the conductive elastomers page are where that is compared.

For avionics, DO-160 sections 20 and 21 set the equipment's RF susceptibility and emission, and military platforms add MIL-STD-461G [7][6]. Where an avionics drawing calls out MIL-DTL-83528, the MIL-DTL-83528 guide explains the type letters one by one. Aluminum housings, salt fog and fuel exposure make the flange finish and the binder part of the decision; the galvanic compatibility guide covers the filler and finish pairing, and the sheet page lists specific gravity by filler where weight matters.

What to send for a quote

• The form (cut sheet, O-ring or molded shape, extrusion, spliced ring) and its drawing, or the profile and its dimensions.
• The filler and binder, or the MIL-DTL-83528 type letter and sheet number your drawing names.
• The hardness, if the drawing names one.
• The flange metal and its finish, with the filler your drawing names; the galvanic guide explains why both matter.
• The environment: temperature range, fuels or cleaning agents, salt fog or weather.
• Adhesive backing yes or no, and conductive or not.
• The quantity, and the paperwork the order needs.

Send them through request a quote, and a person replies within one business day. A certificate of conformance ships with every order, naming the part number, the specification, the manufacturer, the lot and the quantity, and Conquest can also supply the manufacturer's certification on the raw material or from the maker of the finished part.

Chomerics is a trademark of Parker Intangibles LLC, a Parker Hannifin company; 3M is a trademark of 3M Company. No affiliation or endorsement is implied.

Sources

1. [1] Directive 2014/30/EU of the European Parliament and of the Council of 26 February 2014 (electromagnetic compatibility), Article 3 definitions; https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014L0030; read 2026-09-24.
2. [2] 47 CFR 15.3, Definitions (radio frequency energy, unintentional radiator), eCFR text via Cornell Legal Information Institute; https://www.law.cornell.edu/cfr/text/47/15.3; read 2026-09-24.
3. [3] Parker Chomerics, EMI Shielding Engineering Handbook, November 2000 edition (distributor-hosted copy): gasket design guide, fastener spacing, grounding pads; https://sealingdevices.com/wp-content/uploads/2022/02/CHO-HB.pdf; read 2026-09-24.
4. [4] Parker Chomerics, Finding the Right Elastomer EMI Gasket; https://discover.parker.com/finding-the-right-chomerics-elastomer-emi-gasket; read 2026-09-24.
5. [5] 47 CFR 15.109, Radiated emission limits, eCFR text via Cornell Legal Information Institute; https://www.law.cornell.edu/cfr/text/47/15.109; read 2026-09-24.
6. [6] MIL-STD-461G, Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, 11 December 2015 (EverySpec listing); https://everyspec.com/MIL-STD/MIL-STD-0300-0499/MIL-STD-461G_53571/; read 2026-09-24.
7. [7] Interference Technology, P. Albersman, Overview of the DO-160 Standard, 5 October 2018 (sections 20, 21 and 22); https://interferencetechnology.com/overview-of-the-do-160-standard/; read 2026-09-24.
8. [8] IEC 60601-1-2:2014+AMD1:2020 CSV, edition 4.1, 1 September 2020 (IEC webstore, title and scope); https://webstore.iec.ch/en/publication/67554; read 2026-09-24.
9. [9] 3M, XYZ-Axis Electrically Conductive Tape 9713, technical data, April 2009 (application ideas); https://multimedia.3m.com/mws/media/66127O/3m-xyz-axis-electrically-conductive-tape-9713.pdf; read 2026-09-24.
10. [10] Specialty Silicone Products (a compounder), Conductive Gaskets and EMI Shielding, 6 April 2025; https://sspinc.com/conductive-gaskets/; read 2026-09-24.