EMI Shielding Theory & Gasket Design Guide

SECTION CONTENTS PAGE

Theory of shielding and gasketing 192

Conductive elastomer gasket design 196

Gasket junction design 196

Corrosion 198

Selection of seal cross section 202

General tolerances 204

Gasket mounting choices 205

Fastener requirements 206

Designing a solid-O conductive elastomer gasket-in-a-groove 209

Mesh EMI gasketing selection guide 214

Glossary of terms 218

Part number cross reference index 220

Theory of Shielding wave impedance is greatly different transmitted across the boundary and Gasketing from the intrinsic impedance of the and supports a current in the metal Fundamental Concepts discontinuity, most of the energy will as illustrated in Figure 2. The be reflected, and very little will be amount of current flow at any depth A knowledge of the fundamental transmitted across the boundary. in the shield, and the rate of decay concepts of EMI shielding will aid Most metals have an intrinsic is governed by the conductivity of the designer in selecting the gasket impedance of only milliohms. For the metal and its permeability. The inherently best suited to a specific low impedance fields (H dominant), residual current appearing on the design. less energy is reflected, and more opposite face is the one responsible All electromagnetic waves consist is absorbed, because the metal for generating the field which exists of two essential components, a is more closely matched to the on the other side. magnetic field, and an electric field. impedance of the field. This is why These two fields are perpendicular it is so difficult to shield against Ei to each other, and the direction of magnetic fields. On the other hand, wave propagation is at right angles the wave impedance of electric Et to the plane containing these two fields is high, so most of the energy Jo components. The relative magnitude is reflected for this case. between the magnetic (H) field and Jt Consider the theoretical case the electric (E) field depends upon of an incident wave normal to how far away the wave is from its the surface of a metallic structure Figure 2 Variation of Current Density source, and on the nature of the as illustrated in Figure 1. If the with Thickness for Electrically Thick generating source itself. The ratio conductivity of the metal wall is Walls of E to H is called the wave infinite, an electric field equal and Our conclusion from Figures 2 impedance, Z . w opposite to that of the incident and 3 is that thickness plays an If the source contains a large electric field components of the important role in shielding. When current flow compared to its potential, wave is generated in the shield. skin depth is considered, however, such as may be generated by a This satisfies the boundary condition it turns out that thickness is only loop, a transformer, or power lines, that the total tangential electric field critical at low frequencies. At high it is called a current, magnetic, or must vanish at the boundary. Under frequencies, even metal foils are low impedance source. The latter these ideal conditions, shielding effective shields. definition is derived from the fact should be perfect because the two The current density for thin shields that the ratio of E to H has a small fields exactly cancel one another. is shown in Figure 3. The current value. Conversely, if the source The fact that the magnetic fields are density in thick shields is the same operates at high voltage, and only in phase means that the current flow as for thin shields. A secondary a small amount of current flows, the in the shield is doubled. reflection occurs at the far side of source impedance is said to be the shield for all thicknesses. The high, and the wave is commonly x only difference with thin shields is referred to as an electric field. At Perfectly Ei that a large part of the re-reflected very large distances from the Conductive Plane z=0 wave may appear on the front source, the ratio of E to H is equal surface. This wave can add to or for either wave regardless of its H E i subtract from the primary reflected origination. When this occurs, the Hr wave depending upon the phase wave is said to be a plane wave, Er relationship between them. For this and the wave impedance is equal H reason, a correction factor appears to 377 ohms, which is the intrinsic z y in the shielding calculations to impedance of free space. Beyond account for reflections from the this point all waves essentially lose Figure 1 Standard Wave Pattern of a far surface of a thin shield. their curvature, and the surface Perfect Conductor Illuminated by a A gap or slot in a shield will allow containing the two components Normally Incident, + X Polarized Plane electromagnetic fields to radiate becomes a plane instead of a Wave through the shield, unless the section of a sphere in the case current continuity can be preserved of a point source of radiation. Shielding effectiveness of metallic across the gaps. The function of an The importance of wave enclosures is not infinite, because EMI gasket is to preserve continuity impedance can be illustrated by the conductivity of all metals is finite. of current flow in the shield. If the considering what happens when an They can, however, approach very gasket is made of a material electromagnetic wave encounters a large values. Because metallic identical to the walls of the shielded discontinuity. If the magnitude of the shields have less than infinite conductivity, part of the field is

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σg < σm shielded enclosure. (5) In the previous section, it was A = 3.338 x 10–3 x t √µfG shown that electromagnetic waves Metal Shield where incident upon a discontinuity will be A is the absorption or penetration loss Ei partially reflected, and partly trans- expressed in dB, and t is the thickness Et mitted across the boundary and into of the shield in mils. the material. The effectiveness of the shield is the sum total of these two The factor B can be mathematically effects, plus a correction factor to positive or negative (in practice it is Gasket account for reflections from the back always negative), and becomes surfaces of the shield. The overall insignificant when A>6 dB. It is expression for shielding effectiveness usually only important when metals is written as: are thin, and at low frequencies (i.e., below approximately 20 kHz). Figure 4 Lines of Constant Current S.E. = R + A + B (1) B (in dB) = 20 log (6) Flow Through a Gasketed Seam where 10 (K – 1)2 S.E. is the shielding effectiveness2 expressed in dB, 1 – 10 –A/10 e –j.227A appear on the far side of the shield. ( (K + 1)2 ))( ( ) This increased flow causes a larger R is the reflection factor expressed in dB, where leakage field to appear on the far A is the absorption term expressed in dB, and A = absorption losses (dB) side of the shield. Second, leakage B is the correction factor due to reflections from     µ 2 1/ 2 can occur at the interface between K = Z S /Z H = 1.3( /fr G) the far boundary expressed in dB. the gasket and the shield. If an air Z S = shield impedance

Z H = impedance of the incident References magnetic field 1. Much of the analysis discussed in this section was performed by Robert B. Cowdell, as published in “Nomograms Simplify Calculations of Magnetic Shielding Effectiveness” EDN, page 44, September 1, 1972. 2. Shielding Effectiveness is used in lieu of absorption because part of the shielding effect is caused by reflection from the shield, and as such is not an absorption type loss. 3. Vasaka, G.J., Theory, Design and Engineering Evaluation of Radio-Frequency Shielded Rooms, U.S. Naval Development Center, Johnsville, Pa., Report NADC-EL-54129, dated 13 August, 1956.

The preceding equation was current to flow in the 300 See text details and correction for thin sheets Copper solved in two parts. A digital computer shield in a vertical Shielding effectiveness = absorption + reflection loss Iron Copper was programmed to solve for B with direction. A gasket 1 2 Absorption loss per mil σ = 1   250 thickness µ = 1 a preselected value of A, while K placed transverse to 3 4 Reflection loss Ð Electric fields Iron –4 3 3 σ = .17 varied between 10 and 10 . The the flow of current is 5 6 Reflection loss Ð Plane waves µ = 200

results are plotted in Figure 9. less effective than 200 7 8 Reflection loss Ð Magnetic fields The nomograph shown in Figure one placed parallel 4 8 was designed to solve for K in to the flow of current. 150 equation (6). Note that when ZH A circularly 5 LOSS (dB) becomes much smaller than ZS polarized wave (K>1), large positive values of B may contains equal 100 6 result. These produce very large and vertical and unrealistic computed values of S.E., horizontal compo- 50 7 and imply a low frequency limitation nents, so gaskets 2 8 on the B equation. In practical cases, must be equally 1 absorption losses (A) must be cal- effective in both 0 culated before B can be obtained.1 directions. Where 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 1GHz 10GHz FREQUENCY A plot of reflection and absorption polarization is loss for copper and steel is shown in unknown, gasketed Figure 5 Shielding Effectiveness of Metal Barriers Figure 5. This illustration gives a junctions must be designed Some care must be exercised good physical representation of the and tested for the worse condition; in using these charts for behavior of the component parts of that is, where the flow of current is ferrous materials because µ an electromagnetic wave. It also parallel to the gasket seam. varies with magnetizing force. illustrates why it is so much more Nomographs difficult to shield magnetic fields Magnetic Field Reflection – than electric fields or plane waves. The nomographs presented in Figure 7: To determine magnetic Note: In Figure 5, copper offers more Figures 6 through 9 will aid the field reflection loss RH: shielding effectiveness than steel in designer in determining absorption a. Locate a point on the G/µ all cases except for absorption loss. and magnetic field reflection losses scale for one of the metals This is due to the high permeability directly1. These nomographs are listed. If the metal is not listed, of iron. These shielding numbers are based on the equations described compute G/µ and locate a theoretical, hence they are very high in the previous section. point on the numerical scale. (and unrealistic) practical values. Absorption Loss – Figure 6: b. Locate the distance between If magnetic shielding is required, Given a desired amount of absorption the energy source and the particularly at frequencies below loss at a known frequency, determine shield on the r scale. 14 kHz, it is customary to neglect all the required thickness for a known c. Place a straight-edge between terms in equation (1) except the metal: r and G/µ and locate a point absorption term A. Measurements of a. Locate the frequency on on the unmarked X scale numerous shielded enclosures bears the f scale and the desired (Example: r =10 inches for this out. Conversely, if only electric absorption loss on the A scale. hot rolled steel). field, or plane wave protection is required, reflection is the important Place a straight-edge across d. Place a straight-edge between factor to consider in the design. these points and locate a point the point on the X scale and The effects of junction geometry, on the unmarked X scale the desired frequency on the contact resistance, applied force (Example: A = 10 dB, f scale. and other factors which affect f =100 kHz). e. Read the reflection loss from

gasket performance are discussed b. Pivot the straight-edge about the RH scale. (For f = 10 kHz, in the design section which follows. the point on the unmarked X RH = 13 dB). scale to various metals noted Polarization Effects f. By sweeping the f scale while on the G x µ scale. A line holding the point on the X Currents induced in a shield flow connecting the G x µ scale essentially in the same direction as scale, RH versus frequency and the point on the unmarked can be obtained. (For the electric field component of the scale will give the required f = 1 kHz, RH = 3.5 dB). inducing wave. For example, if the thickness on the t scale. (Note that thickness is not a factor electric component of a wave is (Example: for copper t = 9.5 mils, in calculating reflection losses.) vertical, it is known as a vertically cold rolled steel t = 2.1 mils). polarized wave, and it will cause a

References 1. Robert B. Cowdell, “Nomograms Simplify Calculations of Magnetic Shielding Effectiveness” EDN, page 44, September 1, 1972.

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A = 5.0 dB 0 A = 6.0 dB

A = 3.0 dB A = 4.0 dB

-5 A = 2.0 dB

A = 1.5 dB 1 kHz -10 A = 1.0 dB A = .8 dB B in dB

A = .6 dB -15

A = .4 dB |K| = 1.3[µ/fr2G]1/2 -20

A = .2 dB |K| = 2.2 x 10 -2 -25 10 -4 10 -3 10 -2 10 -1 1 |K| Figure 9 Solving for Secondary Reflection Loss (B)1

Figure 8 Magnetic Field Secondary Reflection Loss Factor Nomograph1

Magnetic Field Secondary Reflec- Find B at 1 kHz. c. At its intersection with the K tion Losses K Figures 8 and 9: a. Draw a line between copper scale, read K = 2.2 x 10–2. To determine the magnetic field on the G/µ scale and r = 2 d. Proceed to Figure 9. secondary reflection loss factor K inches on the “source to shield e. On Figure 9, locate K = 2.2 x to solve for B: distance scale.” Locate a point 10–2 on the horizontal scale. on the X scale. Given: r = 2 inches for 0.0162 in. f. Move vertically to intersect the thick copper and A = 1.3 dB. b. Draw a line from the point on A = 1.3 curve (interpolate), the X scale to 1 kHz on the and then horizontally to find f scale. B = –8.5 dB.

Gasket Junction Design At this stage of the design every become widely accepted. Zinc is The ideal gasketing surface is effort should be given to choosing a primarily used with steel. Consult the rigid and recessed to completely flange that will be as stiff as possible applicable specifications before house the gasket. Moreover, it consistent with the construction used selecting a finish. A good guide to should be as conductive as and within the other design finishing EMI shielded flanges for possible. Metal surfaces mating constraints. aerospace applications has been published by SAE Committee AE-4 with the gasket ideally should be 1. Flange Materials non-corrosive. Where reaction with (Electromagnetic Compatibility) Flanges are generally made of the the environment is inevitable, under the designation ARP 1481. A same material as the basic enclosure the reaction products should be discussion of corrosion control for reasons of economy, weldability, electrically conductive or easily follows later in this guide. strength and resistance to corrosion. penetrable by mechanical abrasion. Wherever possible, the flanges 2. Advantages of Grooved Designs It is here that many gasket designs should be made of materials with the All rubber materials are subject to fail. The designer could not, or did highest possible conductivity. It is “Compression Set,” especially if not treat the mating surface with the advisable to add caution notes on over compressed. Because flange same care as that given to the drawings not to paint the flange surfaces cannot be held uniformly selection of the gasketing material. mating surfaces. If paint is to be flat when the bolts are tightened By definition, a gasket is necessary applied to outside surfaces, be sure (unless the flanges are infinitely only where an imperfect surface that the contact surfaces are well stiff), gaskets tend to overcompress exists. If the junction were perfect, masked before paint is applied, and in the areas of the bolts. Proper which includes either a solidly then cleaned after the masking tape groove design is required to avoid welded closure, or one with mating is removed. If the assembled units this problem of over compression. A surfaces infinitely stiff, perfectly flat, are subject to painting or repainting groove also provides metal-to-metal or with infinite conductivity across in the field, add a cautionary note in contact between the flange members, the junction, no gasket would be a conspicuous location adjacent to thereby reducing contact resistance necessary. The more imperfect the the seal that the seal areas are to be across the junction. mating surfaces, the more critical is masked before painting. A single groove will suffice for most the function of the gasket. Perfect Ordinarily, the higher the conduc- designs. Adding a second groove surfaces are expensive. The final tivity of a material, the more readily it parallel to the first adds approximately solution is generally a compromise oxidizes – except for noble metals 6 dB to the overall performance of between economics and performance, such as gold and silver. Gold is a single-groove design. Adding but it should not be at the expense impervious to oxidation, and silver, more grooves beyond the second of neglecting the design of the although it oxidizes, forms oxides does not increase the gasketing flange surfaces. that are soft and relatively conductive. effectiveness significantly. The important property that Most oxides, however, are hard. makes a conductive elastomer 3. Flange Design Considerations Some of the oxide layers remain thin gasket a good EMI/EMP seal is its while others build up to substantial Most designers fight a space ability to provide good electrical thickness in relatively short time. limitation, particularly in the vicinity conductivity across the gasket- These oxides form insulating, or of the gasketed seam. Complex flange interface. Generally, the better semi-conducting films at the fasteners are often required to make the conformability and conductivity, boundary between the gasket and the junctions more compact. the higher the shielding effectiveness the flanges. This effect can be The ideal flange includes a of the gasket. In practice, it has overcome to a degree by using groove for limiting the deflection of a been found that surface conductivity materials that do not oxidize readily, gasket. The screw or bolt fasteners of both the gasket and the mating or by coating the surface with a are mounted outboard of the gasket surfaces is the single most important conductive material that is less to eliminate the need for providing property that makes the gasketed subject to oxidation. Nickel plating is gaskets under the fasteners. A seam effective; i.e., the resistance generally recommended for machined flange and its recom- between the flange and gasket aluminum parts, although tin has mended groove dimensions are should be as low as possible. shown in Figure 10. The gasket may

* Complete solid-O gasket design information starts on page 209.

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“roll over” or twist in its groove. bolts, require a thicker flange to

The minimum distance from the prevent excessive deflections. For G G G edge of the groove to the nearest calculations of elastic deformation, terminal edge, whether this terminal refer to pages 206 and 207. be the edge of a casting, a change O-shaped and D-shaped gaskets in cross section, or a fastening may also be used in sheet metal D device, should be equal to the flanges. The gaskets can be retained E = 2 E groove width, G. in a U-channel or Z-retainer, and are D = Diameter of Washer Bolts should be located a minimum deflection-limited by adjusting the W = Uncompressed Diameter of O-Ring distance, E (equal to one-half the channel or retainer dimensions with H = Groove Depth = 0.75-0.90 W diameter of the washer used under respect to gasket height. Suggested G = 1.1 W the head of the bolt) from the edge of retainer configurations are shown in Figure 10 Machined Flange with the flange. Figures 11a and 11b. Gasket Groove Square or rectangular cross A basic difference between flanges section gaskets can be used in the constructed from sheet metal and be an “O” or “D”-shaped gasket, same groove provided sufficient those which are machined from either solid or hollow. void is allowed for displacement of castings is that the bolts cannot be Solid conductive O-rings are the rubber. A good design practice located as close to the edge of the normally limited to a deflection of is not to allow the height of the part when the flange is made of 25 percent. Therefore, the minimum gasket to exceed the base width. A sheet metal. Note, in Figure 11a, F is compressed height of the O-ring better, or a more optimum ratio is a recommended to be 1.5 D, where D (also the groove depth) is related height-to-width ratio of one-half. Tall is the diameter of the washer. to the uncompressed height (or gaskets tend to roll over when loaded. Flat gaskets are ordinarily used diameter) by the expression H = 0.75 with sheet metal or machined W, where W is the uncompressed Cover D flanges as typically illustrated in diameter. The width of the groove, G, Figure 12. Bolt holes in the flanges should be equal to 1.1 W. Allow should be located at least 1.5 times

sufficient void in the groove area to W the bolt diameter from the edge of H provide for a maximum gasket fill of the flange to prevent tearing when 95 percent. Conductive elastomer the metal is punched. If the holes gaskets may be thought of as G Overlapping are drilled, the position of the “incompressible fluids.” For this Spot Welds holes should be not less than the reason, sufficient groove cross F = 1.5 D F thickness of the gasket material from sectional area must be allowed for the edge of the flange. If holes must the largest cross-sectional area of Figure 11a Shaped Sheet Metal be placed closer to the edge than the gasket when tolerances are taken Container the recommended values, ears or into account. Never allow groove Gasket Cavity and gasket tolerance accumulations t to cause an “over-filled” groove (see gasket tolerances in section which follows). When a seal is used to isolate pressure environments in addition to Sheet Metal Housing F = t EMI, the bottom of the gasket groove "Z" Retainer should have a surface finish of 32- F 64 µin. (RMS) to minimize leakage along the grooves. Avoid machining Figure 11b Z-Retainer Forms Gasket Figure 12 Flat Gasket on Sheet Cavity Metal Flange

sealed with molded circular D-Section exposure or exposure in gaskets, and contact flanges with uncontrolled warehouses. molded rectangular D-gaskets in a Class C. Marine Environment suitable groove (both in CHO-SEAL Shipboard exposure or land 1212 material). exposure within two miles of salt The peak power handling capa- water where conditions of Class A bilities of waveguide flanges are are not met. limited primarily by misalignment and Class D. Space Environment sharp edges of flanges and/or gaskets. Exposure to high vacuum and high Average power handling is limited by radiation. the heating effects caused by contact resistance of the flange-gasket ■ Finishes interface (“junction resistance”). Table I shows the minimum finish Figure 13 Ears or Slots in Sheet necessary to arrest chemical Metal Flanges or Flat Gaskets Corrosion corrosion and provide an electrically All metals are subject to corrosion. conductive surface for the common slots should be considered as metals of construction. Only the shown in Figure 13. Holes in flat That is, metal has an innate tendency to react either chemically or electro- Class A, B, and C environments are gaskets should be treated in a shown in the table because the similar manner. chemically with its environment to form a compound which is stable in space environment is not a cor- 4. Dimensional Tolerances the environment. rosive one (i.e., metals are not Grooves should be held to a Most electronic packages must generally affected by the space machined tolerance of ±0.002 in. be designed for one of four general environment). Holes drilled into machined parts environments: Some metals require finishing should be held to within ±0.005 in. Class A. Controlled because they chemically corrode. with respect to hole location. Location Environment Temperature and These are listed in Table I, and of punched holes should be within humidity are controlled. General should be finished in accordance ±0.010 in. Sheet metal bends indoor, habitable exposure. with the table. To select a proper finish for metals not given in Table I, should be held to +0.030 and Class B. Uncontrolled refer to the material groupings of –0.000 in. Gasket tolerances Environment Temperature and Table II. Adjacent groups in Table II are given in the “Selection of Seal humidity are not controlled. Exposed are compatible. Another excellent Cross Section,” later in this guide. to humidities of 100 percent with source of information on corrosion- occasional wetting. Outdoor 5. Waveguide Flanges compatible finishes for EMI shielded The three concerns for waveguide flanges are to ensure maximum Table I transfer of electromagnetic energy MINIMUM FINISH REQUIREMENTS FOR STRUCTURAL METALS across the flange interface to prevent RF leakage from the interface, and to ENVIRONMENT maintain pressurization of the wave- Metal Class A Class B Class C Carbon and 0.0003 in. cadmium plate 0.0005 in. cadmium 0.003 in. nickel guide. Conductive elastomeric Alloy Steel 0.0005 in. zinc plate 0.001 in. zinc 0.001 in. tin gaskets provide both an electrical and 0.0003 in. tin 0.0005 in. tin a seal function. For flat cover flanges, Corrosion- No finish required No finish required; No finish required; a die-cut sheet gasket (CHO-SEAL Resistant Steels 0.0005 in. nickel to 0.001 in. nickel to 1239 material), incorporating expanded prevent tarnish prevent tarnish metal reinforcement to control gasket Aluminum 2000 Chromate conversion Chromate conversion coat 0.001 in. tin & 7000 series coat (MIL-C-5541, Class 3) (MIL-C-5541) plus conductive creep into the waveguide opening, epoxy or urethane provides an excellent seal. Raised Aluminum 3000, No finish required, unless Chromate conversion coat Chromate conversion lips around the gasket cut-out 5000, 6000 series shielding requirements coat plus conductive improve the power handling and and clad are high (see above) epoxy or urethane pressure sealing capability of the Copper and 0.0003 in. tin 0.0005 in. tin 0.003 in. nickel gasket. Choke flanges are best Copper Alloys 0.001 in. tin Magnesium 0.0003 in. tin 0.0005 in. tin 0.001 in. tin Zinc Base Castings No finish required 0.0003 in. tin 0.0005 in. tin

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The easily Silver Filled Elastomers – Silver Filled Coatings mixed three-component system allows Titanium – Nickel – Monel – Cobalt – Nickel and Cobalt Alloys – Nickel Copper Alloys – Copper – Bronze – Brass – Copper Alloys – Silver Solder – Commercial Yellow Brass and Bronze – minimum waste with no weighing 3 Leaded Brass and Bronze – Naval Brass – Steels AISI 300 Series, 451, 440, AM 355 and of components, thus eliminating PH hardened – Chromium Plate – Tungsten – Molybdenum – Certain Silver Filled Elastomers weighing errors. Because of the filler Leaded Brass and Bronze – Naval Brass – Steels AISI 431, 440, 410, 416, 420, AM 355 and loading of the 2000 series coatings, PH hardened – Chromium Plate – Tungsten – Molybdenum – Tin-Indium – Tin Lead Solder – 4 it is recommended that an air agitator Lead – Lead Tin Solder – Aluminum 2000 and 7000 Series – Alloy and Carbon Steel – cup be incorporated into the spray Certain Silver Filled Elastomers – CHO-SHIELD 2000 Series Coatings system to keep the conductive Chromium Plate – Tungsten – Molybdenum – Steel AISI 410, 416, 420, Alloy and Carbon – particles in suspension during the 5 Tin – Indium – Tin Lead Solder – Lead – Lead Tin Solder – Aluminum – All Aluminum Alloys – Cadmium – Zinc – Galvanized Steel – Beryllium – Zinc Base Castings spraying sequence. It is recom- 6 Magnesium – Tin mended that approximately 7 mils of wet coating be applied. This * Each of these groups overlaps, making it possible to safely use materials from adjacent groups. thickness can be achieved by spraying multiple passes, with a flanges is ARP 1481, developed and Organic Finishes ten minute wait between passes. published by SAE’s Committee AE-4 Organic finishes have been used A 7-mil wet film coating will (Electromagnetic Compatibility). with a great deal of success to yield a dry film thickness of 4 mils, When a finish is required to make prevent corrosion. Many organic which is the minimum thickness two mating metals compatible, finish finishes can be used, but none will required to attain the necessary the metal which is found in the lower be effective unless properly applied. corrosion and electrical values numbered grouping of Table II. Metals The following procedure has been referenced in Chomerics’ Technical given in Table II will, because of used with no traces of corrosion Bulletin 30. The coating thickness their inherent corrodibility, already after 240 hours of MIL-STD-810B plays an important role in the electrical be finished and the finish metal salt fog testing. and corrosion properties. Thinner will be subject to the same rule. Aluminum panels are cleaned coatings of 1-3 mils do not exhibit For example, to couple metals with a 20% solution of sodium the corrosion resistance of 4-5 mil separated by two or more groups hydroxide and then chromate coatings. (e.g., 4 to 2), find a finish which conversion coated per MIL-C-5541 The coating will be smooth to the appears in Group 3 and 4. The Class 3 (immersion process). The touch when cured. It is recommended Group 3 metal should be plated conversion coated panels are then that the coating be cured at room onto the Group 2 metal to make coated with MIL-C-46168 Type 2 temperature for 2 hours followed metals 2 and 4 compatible. The urethane coating, except in the by 250°F +/-10°F for one-half hour reason for this is, if the finish metal areas where contact is required. For whenever possible. Alternate cure breaks down, or is porous, its area maximum protection of aluminum cycles are available, but with will be large in comparison to the flanges, a CHO-SHIELD 2000 series significant differences in corrosion exposed area of the Group 2 metal, conductive coating and CHO-SEAL and electrical properties. Two and the galvanic corrosion will 1298 conductive elastomer gasket alternate cure schedules are two be less. material are recommended. For hours at room temperature followed On aluminum, chromate conversion additional information, refer to by 150°F for two hours, or 7 days at coatings (such as Iridite) can be Design Guides for Corrosion room temperature. considered as conductive finishes. Control on page 201. Full electrical properties are MIL-C-5541 Class 3 conversion The finish coat can be any achieved at room temperature after coatings are required to have less suitable urethane coating that 7 days. It should be noted that the than 200 milliohms resistance when is compatible with the MIL-C-46168 250°F cure cycle reflects the measured at 200 psi contact pressure coating. It is important to note that ultimate in corrosion resistance after 168 hours of exposure to a test specimens without the MIL-C- properties. The 150°F/2 hour and 5 percent salt spray. Recommended 46168 coating will show some signs room temperature/7 day cures will MIL-C-5541 Class 3 coatings are of corrosion, while coated test speci- provide less corrosion resistance Alodine 600, or Alodine 1200 and mens will show no traces of corrosion. 1200S dipped.

than the 250°F cure, but are well aluminum or steel, and most EMI Table III within the specification noted in gaskets contain Monel, silver, tin, CORROSION POTENTIALS OF VARIOUS Technical Bulletin 30. etc.). The second condition is METALS AND EMI GASKET MATERIALS satisfied by the inherent conductivity 1091 Primer (in 5% NaCI at 21°C of the EMI gasket. The last condition after 15 minutes of immersion) Because of the sensitivity of could be realized when the electronic Ecorr vs. SCE* surface preparation on certain package is placed in service, where Material (Millivolts) substrates and the availability of salt spray or atmospheric humidity, if Pure Silver –25 equipment to perform the etching of allowed to collect at the flange/gasket Silver-filled elastomer –50 aluminum prior to the conversion interface, can provide the electrolyte coating, Chomerics has introduced for the solution of ions. Monel mesh –125 1091 primer, which is an adhesion Many users of EMI gaskets select Silver-plated-copper –190 filled elastomer promoter for CHO-SHIELD 2000 Monel mesh or Monel wire-filled series coatings. When used in materials because they are often Silver-plated-aluminum –200 filled elastomer conjunction with an alkaline etch or described as “corrosion-resistant.” chemical conversion coating per Actually, they are only corrosion- Copper –244 MIL-C-5541 Class 3, the 1091 resistant in the sense that they do Nickel –250 primer will provide maximum not readily oxidize over time, even Tin-plated Beryllium-copper –440 adhesion when correctly applied. in the presence of moisture. Tin-plated copper-clad –440 (See Technical Bulletin 31.) This However, in terms of electrochemical steel mesh primer is recommended only for the compatibility with aluminum flanges, Aluminum* (1100) –730 2000 series coatings on properly Monel is extremely active and its Silver-plated-aluminum filled –740 treated aluminum and is not use requires extensive edge sealing elastomer (die-cut edge) recommended for composites. and flange finish treatment to For further assistance on the *Standard Calamel Electrode. Aluminum Alloys prevent galvanic corrosion. Most approximately –700 to –840 mV vs. SCE in 3% NaCl. application of CHO-SHIELD 2000 galvanic tables do not include Mansfield, F. and Kenkel, J.V., “Laboratory Studies of series coatings on other metallic Galvanic Corrosion of Aluminum Alloys,” Galvanic and Monel, because it is not a commonly Pitting Corrosion – Field and Lab Studies, ASTM STP and non-metallic substrates, contact used structural metal. The galvanic 576, 1976, pp. 20-47. Chomerics’ Applications Engineering table given in MIL-STD-1250 does Department. include Monel, and shows it to have and 2) the retention of conductivity ■ Galvanic Corrosion a 0.6 volt potential difference with by the elastomer after exposure to respect to aluminum – or almost the The most common corrosion a corrosive environment. same as silver. concern related to EMI gaskets is Instead of using a table of A common misconception is galvanic corrosion. For galvanic galvanic potentials, the corrosion that all silver-bearing conductive corrosion to occur, a unique set of caused by different conductive elastomers behave galvanically as conditions must exist: two metals elastomers was determined directly silver. Experiments designed to capable of generating a voltage by measuring the weight loss of an show the galvanic effects of silver- between them (any two unlike aluminum coupon in contact with the filled elastomer gaskets in aluminum metals will do), electrically joined by conductive elastomer (after exposure flanges have shown less corrosion a current path, and immersed in a to a salt fog environment). The than predicted. Silver-plated- fluid capable of dissolving the less electrical stability of the elastomer aluminum filled elastomers exhibit noble of the two (an electrolyte). In was determined by measuring its the least traces of galvanic corrosion short, the conditions of a battery resistance before and after exposure. and silver-plated-copper filled must exist. When these conditions Figure 14a describes the test fixture elastomers exhibit more. (See do exist, current will flow and the that was used. Figure 14b shows the Table III). extent of corrosion which will occur aluminum weight loss results for Tables of galvanic potential do will be directly related to the total several different silver-filled not accurately predict the corrosivity amount of current the galvanic cell conductive elastomers. The of metal-filled conductive elastomers produces. aluminum weight loss shows a two because of the composite nature of When an EMI gasket is placed order of magnitude difference be- these materials. Also, these tables between two metal flanges, the first tween the least corrosive (1298 do not measure directly two important condition is generally satisfied silver-plated-aluminum) and most aspects of conductive elastomer because the flanges will probably corrosive (1215 silver-plated- “corrosion resistance”: 1) the not be made of the same metal as copper) filled elastomers. For silver- corrosion of the mating metal flange the gasket (most flanges are containing elastomers, the filler

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 200 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 Figure 14a Test Fixture substrate that the silver is plated on bowing between fasteners and 1/4-20 CRES Fastener is the single most important factor in tolerance buildups. If the gasket is Non-Conductive 1.25" Dia. Sealing Gasket determining the corrosion caused by designed and applied correctly, it the conductive elastomer. will exclude moisture and inhibit Upper Delrin Block Figure 14c shows the weight loss corrosion on the flange faces and 1.00" Conductive Gasket results for nickel and carbon-filled inside the package. Aluminum Alloy elastomers compared to 1298. The Bare aluminum and magnesium, Coupon nickel-filled materials are actually as well as iridited aluminum and

Non-Conductive Sealing Gasket more corrosive than the silver- magnesium, can be protected by

Lower 1.00" plated-aluminum filled elastomers. properly designed conductive Delrin Block The carbon-filled materials are elastomer gaskets. It is important to extremely corrosive. note that magnesium is the least noble 1.50" Dia. Figure 14d compares the structural metal commonly used, and 1.75" Dia. electrical stability of several a silver-filled elastomer in contact conductive elastomers before and with magnesium would theoretically 300 Figure 14b Average Weight 281 Loss of CHO-SEAL 237 after salt fog exposure. In general, produce an unacceptable couple. Elastomers silver-containing elastomers are Some specific design suggestions more electrically stable in a salt fog for proper corrosion control at EMI 167 environment than nickel-containing flanges are: 50 elastomers. 1. Select silver-plated-aluminum 37

Weight Loss (mg) filled elastomers for best overall

22 Design Guides for 19 sealing and corrosion protection. Corrosion Control CHO-SEAL 1298 material offers 2.2 The foregoing discussion is not more corrosion resistance than any 0 1298 1287 1285S6304 1350 1224 1215 intended to suggest that corrosion other silver-filled elastomer (see CHO-SEAL Material should be of no concern when Figure 15, next page). (for composition, see Specifications Table, pgs. 32-34.) flanges are sealed with silver-bearing 2. For aircraft applications, conductive elastomers. Rather, consider “seal-to-seal” designs, with 40 Figure 14c Weight Loss of 6061-T6 corrosion control by and large Aluminum Coupons in Contact 35.2 same gasket material applied to with Conductive Elastomers presents the same problem whether During 168 hr. Salt Fog both flange surfaces (see Figure 16). 30 the gasket is silver-filled, Monel wire- 25.5 filled, or tin-plated. Furthermore, the designer must understand the factors 20 which promote galvanic activity and

Weight Loss (mg) strive to keep them at safe levels. By 10.3 10 “safe”, it should be recognized that Non-conductive Non-conductive sealant CHO-SEAL sealant some corrosion is likely to occur 1287 2.2 (and may be generally tolerable)

0 CHO-SEAL Nickel Nickel Carbon at the outer (unsealed) edges of a 1298 Powder Filled Fiber Filled Filled flange after long-term exposure

to salt-fog environments. This is Figure 14d Volume Resistivity (mohm-cm) of Conductive especially true if proper attention Elastomers Before and After 168 hr. Salt Fog Exposure Figure 16 “Seal-to-seal” design 1052.2 has not been given to flange ® 200 incorporating CHO-SEAL 1287 Before materials and finishes. The objective 445.1 conductive silver-aluminum fluorosilicone After should be control of corrosion within gaskets on both mating flange surfaces. acceptable limits. 150 Gaskets are bonded and edge sealed to 132.4 The key to corrosion control in 126.7 prevent moisture from entering the gasket/ flanges sealed with EMI gaskets is 102.3 flange area. 100 proper design of the flange and 84.8 gasket (and, of course, proper 3. To prevent corrosion on outside selection of the gasket material). A edges exposed to severe corrosive Volume Resistivity (mohm-cm) 50 properly designed interface requires environments, use dual conductive/ a moisture-sealing gasket whose non-conductive gaskets (see page 7.3 7.3 thickness, shape and compression- 0 55) or allow the non-conductive CHO-SEAL Nickel Nickel Carbon deflection characteristics allow it to 1298 Powder Filled Fiber Filled Filled protective paint (normally applied to fill all gaps caused by uneven or outside surfaces) to intrude slightly unflat flanges, surface irregularities, under the gasket (see Figure 17).

of materials which will not c. Stability throughout the corrode in the use temperature range; environment. In these d. Freedom from outgassing; cases, the outside edges of e. Compatibility with other EMI-gasketed flanges might materials in the assembly; also require peripheral sealing as defined in MIL- f. Resistance to flame and arc; STD-1250, MIL-STD-889 or g. For outdoor applications, MIL-STD-454. MIL-STD- ability to withstand weathering. 1250 deserves special mention. Although it was Selection of developed many years Seal Cross Section prior to the availability Selection of the proper con- of CHO-SEAL 1298 ductive elastomer gasket cross conductive elastomer section is largely one of application, Figure 15 Comparison of corrosion results obtained from compromise, and experience with ® and CHO-SHIELD 2000 CHO-SEAL 1298 conductive elastomer (left) and pure series conductive coatings, similar designs used in the past. silver-filled elastomer (right) mated with aluminum after it offers the following useful Some general rules, however, can 168 hours of salt fog exposure. corrosion control methods be established as initial design applicable to electronic guidelines in selecting the class Paint Paint of gasket to be used. enclosures: EMI Gasket 1. Bonds made by conductive 1. Flat Gaskets gaskets or adhesives, and involving When using flat gaskets, care Figure 17 Non-Conductive Paint dissimilar contact, shall be sealed must be taken not to locate holes Intrudes Slightly Under Gasket to with organic sealant. closer to the edge than the thickness Provide Edge Protection 2. When conductive gaskets are of the gasket, or to cut a web used, provision shall be made in narrower than the gasket thickness. 4. If moisture is expected to reach design for environmental and This is not to be confused with the the flange interfaces in Class C electromagnetic seal. Where criteria for punching holes in sheet (marine) environments, flange practical, a combination gasket with metal parts discussed earlier. surfaces should be coated or plated conductive metal encased in resin Keep in mind also that flat to make them more compatible with or elastomer shall be preferred. gaskets should not be deflected the EMI gasket material. Chomerics’ more than about 10 percent, CHO-SHIELD 2000 series coatings 3. Attention is drawn to compared with 15 to 30 percent for are recommended for silver-filled possible moisture retention when molded and solid extruded gaskets elastomer or Monel wire gaskets, sponge elastomers are used. and 50% for hollow gaskets. Standard and tin plating for tin-plated gaskets. 4. Because of the serious loss thicknesses for flat gaskets are in conductivity caused by corrosion, 5. Avoid designs which create 0.020, 0.032, 0.062, 0.093 and special precautions such as environ- sump areas. 0.125 in. (see General Tolerances on mental seals or external sealant 6. Provide drainage and/or drain page 204.) bead shall be taken when wire mesh holes for all parts which would Where possible, the flange gaskets of Monel or silver are used become natural sumps. should be bent outward so that the in conjunction with aluminum or screws or bolts do not penetrate the 7. Provide dessicants for parts magnesium. which will include sumps but cannot shielded compartment (see Figure 5. Cut or machined edges of be provided with drain holes. Dessi- 18a). If the flange must be bent laminated, molded, or filled plastics cant filters can also be provided for inward to save space, the holes in shall be sealed with impervious air intake. the gasket must fit snugly around materials. the threads of the bolts to prevent 8. Avoid sharp edges or 6. Materials that “wick” or are leakage along the threads and protrusions. hygroscopic (like sponge core mesh directly into the compartment. This 9. Select proper protective gaskets) shall not be used. calls for closely toleranced holes finishes. 7. In addition to suitability for the and accurate registration between The definition of a “safe“ level of intended application, nonmetallic the holes in the flange and the holes galvanic activity must clearly be materials shall be selected which in the gasket, and would require expanded to include the requirements have the following characteristics: machined dies (rather than rule of the design. If all traces of corrosion dies) to produce the gasket. An a. Low moisture absorption; must be prevented (e.g., airframe alternate solution can be achieved applications) the structure must be b. Resistance to fungi and by adding an EMI seal under the properly finished or must be made microbial attack; heads of bolts penetrating the

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 202 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 enclosure, or by using an insert T similar to an acorn nut that has been H W A inserted in the flange and flared to

W make the joint RF-tight. “Blind nuts” can also be welded or attached with Deflection W Deflection Deflection Deflection a conductive epoxy adhesive Range Dia. Range H Range T Range A (see Figure 18b). 0.007-0.018 0.070 0.006-0.012 0.068 0.001-0.002 0.020 0.025-0.080 0.200 (0.178-0.457) (1.778) (0.152-0.305) (1.727) (0.025-0.051) (0.508) (0.635-2.032) (5.08)

0.010-0.026 0.103 0.008-0.016 0.089 0.001-0.003 0.032 0.030-0.125 0.250

(0.254-0.660) (2.616) (0.203-0.406) (2.261) (0.025-0.076) (0.813) (0.762-3.175) (6.35)

0.013-0.031 0.125 0.012-0.024 0.131 0.003-0.006 0.062 0.075-0.250 0.360 (0.330-0.787) (3.175) (0.305-0.610) (3.327) 0.076-0.152) (1.575) (1.905-6.35) (9.144)

0.014-0.035 0.139 0.014-0.029 0.156 0.003-0.009 0.093 (0.356-0.889) (3.531) (0.356-0.737) (3.962) (0.076-0.229) (2.362) Figure 18a External Bolting Prevents 0.016-0.032 0.175 EMI Leakage (0.406-0.813) (4.445)

(mm dimensions in parentheses)

Figure 20 Gasket Deflection Ranges

3. Hollow Gaskets be molded in one piece and placed

Hollow gasket configurations are into the desired groove, or a strip

very useful when large gaps are gasket can be spliced to length and Figure 18b Insert Pressed-In and encountered, or where low closure fitted to the groove. To properly seat Flared Makes EMI Tight Joint forces are required. Hollow gaskets a spliced solid “O” cross section (Alternate: Weld or Cement with gasket, the inner radius of the Conductive Epoxy) are often less expensive, and they can be obtained with or without groove at the corners must be equal attachment tabs. Hollow gaskets to or greater than the gasket cross 2. Shaped or Molded Gaskets with tabs are referred to in the text section width. Other cross sections Groove designs for O- or D- and in the tables as “P-gaskets”. The need greater inner radius and may shaped configurations are effective minimum wall thickness of hollow not be practical due to twisting because gasket deflection can be gaskets is 0.020 in. depending when bent around corners. Splices controlled and larger deflections on material. Contact Chomerics’ can be simply butted (with no can be accommodated. O-ring Applications Department for details. adhesive) or bonded with a cross sections are preferred Hollow gaskets will compensate for conductive or non-conductive because they can be deflected a large lack of uniformity between compound. If it has been decided more easily under a given load. mating surfaces because they can that a spliced gasket will provide a D-shapes or rectangular cross be compressed to the point of satisfactory seal, the decision sections are excellent for retrofit eliminating the hollow area. between splicing and molding applications because they can be should be based on cost. When a made to accommodate almost any 4. Compression Limits standard extrusion is available, groove cross section. Groove When compression cannot be splicing is generally recommended. designs also provide metal-to-metal controlled, compression stops For custom extrusions, splicing is flange contact, and require fewer should be provided to prevent generally more cost effective in fasteners, thereby minimizing the gasket rupture caused by over- quantities over 500 feet. number of paths where direct compression. Grooves provide leakage can occur. built-in compression stops. Figure 7. Gasket Limitations Imposed Fasteners should be located such 20 gives nominal recommended by Manufacturing Methods that pressure distribution is uniform compression ranges for CHO-SEAL Current manufacturing tech- at the corners (see Figure 19). and CHO-SIL materials, assuming nology limits conductive elastomer standard tolerances. gasket configurations to the X X/2 following dimensions and shapes : 5. Elongation ■ Die-cut Parts The tensile strength of conductive Maximum Overall Size: 32 in. long X/2 elastomer gaskets is not high. It is x 32 in. wide x 0.125 in. thick good practice to limit elongation to (81 cm x 81 cm x 3.18 mm) less than 10 percent. Minimum Cross Section: Width-to- X 6. Splicing thickness ratio 1:1 (width is not When grooves are provided for to be less than the thickness of gasket containment, two approaches the gasket). are possible. A custom gasket can Figure 19 Fastener Location Near Corners

■ Molded Parts given to the barrier as a pressure MOLDED GASKETS Currently available in any solid inch (mm) TOLERANCE seal if gas pressures of significant cross section, but not less than magnitude are expected. The gasket Overall Dimensions 0.040 in. in diameter. The outer 0.100 to 1.500 (2.54 to 38.10) ±0.010 (0.25) will blow out if the pressure is too dimensions of the gasket are 1.501 to 2.500 (38.13 to 63.50) ±0.015 (0.38) high for the gap. limited to 34 inches in any 2.501 to 4.500 (63.53 to 114.30) ±0.020 (0.51) The minimum gap allowed in direction. Larger parts can be 4.501 to 7.000 (114.33 to 177.80) ±0.025 (0.64) the junction is determined by the made by splicing. Molded parts >7.000 (>177.80) ±0.35% allowable squeeze that can be Nom. Dim. will include a small amount of tolerated by the gasket material. Cross Section flash (0.008 in. width and 0.005 in. 0.040 to 0.069 (1.02 to 1.75) ±0.003 (0.08) Deflection of conductive elastomer thickness, maximum). 0.070 to 0.100 (1.78 to 2.54) ±0.004 (0.11) gaskets was given in Figure 20. Flat ■ Extruded Parts 0.101 to 0.200 (2.57 to 5.08) ±0.005 (0.13) gaskets may be deflected as much 0.201 to 0.350 (5.11 to 8.89) ±0.008 (0.20) No limitation on length. Minimum as 6-10 percent (nominal), depending solid cross-section is limited to Flash Tolerance 0.005 (0.13) on initial thickness and applied force. 0.028 in. extrusions. Wall thickness Max.Thickness O-shaped and D-shaped gaskets 0.008 (0.20) are normally deflected 10 to 25 of hollow extrusions varies with Max. Extension material but 0.020 in. can be percent; however, greater deflections achieved with most materials. can be achieved by manipulating EXTRUDED STRIP cross section configuration. GASKETS inch (mm) TOLERANCE 8. General Tolerances Determination of the exact gasket The following tables provide general Cut Length thickness is a complex problem <1.000 (25.40) ±0.010 (0.25) involving electrical performance, tolerances for conductive elastomer 1.0 to 30.000 (25.40 to 762) ±0.062 (1.58) gaskets. It is important to note that all > 30.000 (762) ±0.2% Nom. Dim. flange characteristics, fastener flat die-cut, molded, and extruded spacing and the properties of the Cross Section gaskets are subject to free-state gasket material. However, an initial < 0.200 (5.08) ±0.005 (0.13) estimate of the necessary thickness variation in the unrestrained condition. 0.200-0.349 (5.08-8.86) ±0.008 (0.20) The use of inspection fixtures to verify 0.350-0.500 (8.89-12.70) ±0.010 (0.25) of a noncontained gasket can be conformance of finished parts is > 0.500 (12.70) ±3% Nom. Dim. determined by multiplying the common and recommended where difference in the expected minimum appropriate. 9. Gasket Cross Section and maximum flange gaps by a Based on Junction Gaps factor of 4, as illustrated in Figure Also note that “Over-all Dimensions” Gasket geometry is largely deter- 21. A more detailed discussion, and for flat die-cut gaskets and molded mined by the largest gap allowed a more accurate determination of gaskets includes any feature-to-feature to exist in the junction. Sheet metal gasket performance under loaded dimensions (e.g., edge-to-edge, edge- enclosures will have larger variations flange conditions, can be found in to-hole, hole-to-hole). than machined or die castings. The the Fastener Requirements section, ultimate choice in allowable gap page 206. FLAT DIE-CUT GASKETS tolerance is a compromise between inch (mm) TOLERANCE cost, performance and the reliability Overall Dimensions required during the life of the device. ≤10 (254) ±0.010 (0.25) When a value analysis is conducted, >10 to ≤15 (254 to 381) ±0.020 (0.51) >15 (>381) ±0.20% Nom. Dim. it should be made of the entire junction, including the machining G max G min t o Thickness 0.020 (0.51) ±0.004 (0.10) required, special handling, treatment 0.032 (0.81) ±0.005 (0.13) of the surfaces and other factors 0.045 (1.14) ±0.006 (0.15) required to make the junction tO = 4 (Gmax Ð Gmin) (tO = Original thickness of gasket) 0.062 (1.57) ±0.007 (0.18) functional. Often, the gasket is the 0.093 (2.36) ±0.010 (0.25) Figure 21 Gasket Deflection Along a 0.125 (3.18) ±0.010 (0.25) least expensive item, and contributes >0.125 (>3.18) Contact a Chomerics to cost-effectiveness by allowing Flange Applications or loosely-toleranced flanges to be Sales Engineer made EMI-tight. Hole Diameters The maximum gap allowed to exist >0.060 (1.52) dia. if sheet thickness is... in a junction is generally determined ≤0.062 (1.57) ±0.005 (0.13) by the minimum electrical perfor- >0.062 (1.57) ±0.008 (0.20) mance expected of the seal. A secondary consideration must be

Our various EMI gasket mounting techniques offer designers cost-effective choices in both materials and assembly. These options offer aesthetic choices and accommodate packaging requirements such as tight spaces, weight limits, housing materials and assembly costs. Most Chomerics gaskets attach using easily repairable systems. Our Applications Engineering Department or your local Chomerics representative can provide full details on EMI gasket mounting. The most common systems are shown here with the available shielding products.

Pressure-Sensitive Adhesive Friction Fit in a Groove Adhesive Compounds Quick, efficient attachment strip Prevents over-deflection of gasket Conductive or non-conductive ■ Conductive Elastomers Retaining groove required spot bonding ■ SOFT-SHIELD ■ POLASHEET ■ Conductive ■ MESH STRIP ■ Conductive Elastomers ■ SPRING-LINE ■ POLASTRIP Elastomers ■ POLASTRIP ■ MESH STRIP ■ SOFT-SHIELD ■ SPRINGMESH

Robotically Dispensed Form-in- Friction Fit on Tangs Spacer Gaskets Place Conductive Elastomer Accommodates thin walls, Fully customized, integral conductive Chomerics’ Cho-Form® automated intricate shapes elastomer and plastic spacer provide technology applies high quality ■ Conductive Elastomers economical EMI shielding and grounding conductive elastomer gaskets to metal in small enclosures. Locator pins or plastic housings. Manufacturing ensure accurate and easy installation, options include Chomerics facilities, manually or robotically. authorized Application Partners, and turnkey systems.

Metal Clips Rivets/Screws Frames Teeth bite through painted panels Require integral compression stops Extruded aluminum frames and strips Require knife edge mounting flange Require mounting holes on flange add rigidity. Built-in compression stops ■ Conductive Elastomers ■ Conductive ■ SHIELDMESH for rivets and screws. ■ METALKLIP Elastomers ■ COMBO STRIP ■ Conductive Elastomers ■ SPRING-LINE ■ SPRING-LINE ■ MESH STRIP

Fastener Requirements 3. Flange Deflection where 1. Applied Force The flange deflection between h is the thickness of the flange. Most applications do not require fasteners is a complex problem b is the width of the flange. more than 100 psi (0.69 MPa) to involving the geometry of the flange d is the spacing between fasteners. achieve an effective EMI seal. and the asymmetrical application of Waveguide flanges often provide forces in two directions. The one- b ten times this amount. Hollow strips dimensional solution, which treats require less than 10 pounds per in. the flange as a simple beam on an h Compression deflection data for elastic foundation, is much easier to 1 many shapes, sizes and materials is analyze and gives a good first included in the Performance Data order approximation of the spacings d section of this handbook. required between fasteners, because most EMI gaskets are sandwiched The force required at the point of Figure 22 Bolt Spacings for Flanges least pressure, generally midway between compliant flanges. between fasteners, can be obtained Variation in applied forces by using a large number of small between fasteners can be limited Assume the flange is to be made fasteners spaced closely together. to ±10 percent by adjusting the of aluminum. Alternatively, fasteners can be constants of the flange such that To maintain a pressure distribution spaced further apart by using stiffer βd= 2, between bolts of less than ±10 flanges and larger diameter bolts. where percent, ßd must be equal to 2 Sheet metal parts require more 4 (see Figure 23 and discussion). fasteners per unit length than β = √ k Assume an average foundation 4 E I castings because they lack stiffness. f f modulus (k) of 12,500 psi for the To calculate average applied where seal. If the actual modulus is known force required, refer to load-deflection k = foundation modulus of the seal (stress divided by strain), substitute curves for specific gasket materials Ef = the modulus of elasticity of the flange that value instead. and cross sections (see Performance lf = the moment of inertia of the flange and seal The bolt spacings for aluminum Data, page 80). d = spacing between fasteners flanges for various thicknesses and widths have been calculated for the 2. Fastener Sizes and Spacing The modulus of elasticity (Ef) for previous example and are shown in 7 Fastener spacing should be steel is typically 3 x 10 . The modulus Figure 24. 7 determined first. As a general rule, for aluminum is typically 1 x 10 , and The previous example does not 7 fasteners should not be spaced for brass it is about 1.4 x 10 . take into account the additional more than 2.0 inches (50 mm) apart The foundation modulus (k) of stiffness contributed by the box to for stiff flanges, and 0.75 inch (19 mm) seals is typically 10,000 to 15,000 psi. which the flange is attached, so the ( ) apart for sheet metal if high levels of The moment of inertia lf of results are somewhat conservative.

shielding are required. An exception rectangular sections, for example, 2.4 to the rule is the spacing between may be obtained from the following 2.2 §d = 1 fasteners found in large cabinet expression2: 2.0 doors, which may vary from 3 inches 1.8 3 1.6 lf = bh (76.02 mm) between centers to 1.4 n§d Ð §x single fasteners (i.e., door latches). 12 A 1.2 ∑ §d = 2 N Ð 1 N = 0 1.0 where §d = 3 The larger spacings are compen- 0.8 sated for by stiffer flange sections, b is the width of the flange in contact 0.6

very large gaskets, and/or some with the gasket (inches) and Array Factor 0.4 0.2 reduction in electrical performance h is the thickness of the flange (inches). §d = 4 0 §d = ∞ requirements. Ð0.2 Ð0.4 The force per bolt is determined Example 87654321012345678 by dividing the total closure force by Calculate the bolt spacings for (Ð) §x (+) the number of bolts. Select a fastener flanges with a rectangular cross- Figure 23 Array Factor vs. Spacing with a stress value safely below the section, such as shown in Figure 22, allowable stress of the fastener.

References 1. Galagan, Steven, Designing Flanges and Seals for Low EMI, MICROWAVES, December 1966. 2. Roark, R.J., Formulas for Stress and Strain, McGraw-Hill, 4th Ed., p. 74.

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 206 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 Actual deflection vs. distance Table IV lists a few recommenda- are used for any reason, refer to the between fasteners may be computed tions for bolts and bolt spacings in literature3 for the proper coefficient from the following expression: various thin cross section aluminum values to be applied. flanges. In soft materials, such as N –1 Bolt spacings for waveguide aluminum, magnesium and βp flanges are fixed by Military and EIA insulating materials, inserts should y = Anβd –βx 2k Σ Standards. Waveguide flanges be provided if the threads are normally have bolts located in the “working threads.” A thread is n = 0 middle of the long dimension of the considered a “working thread” if it where p is the force applied by the fastener, flange because the flow of current is will be assembled and disassembled and β and k are the constants of the flange as most intense at this point. ten or more times. determined previously. N represents the number Torque loss caused by elongation Table IV of bolts in the array. of stainless steel fasteners should MAX. TORQUE TO The array factor denoted by the PREVENT STRIPPING also be considered. High tensile summation sign adds the contri- SCREW CL-TO-CL THICKNESS FOR UNC-2A THREAD strength hardware is advised when SIZE (in.) (in.) (in.-lbs.) bution of each fastener in the array. this becomes a problem, but care 3 The array factor for various bolt #2 ⁄8 0.062 4.5 must be taken of the finish specified 3 spacings (βd) is shown in Figure 23. #4 ⁄4 0.125 10.0 to minimize galvanic corrosion. Although any value can be selected #6 1 0.125 21.0 Thermal conductivity of high 1 for βd, a practical compromise #8 1 ⁄4 0.156 37.5 tensile strength hardware is lower 3 between deflection, bolt spacing and #10 1 ⁄8 0.156 42.5 than most materials used in electro- mechanical packaging today, so electrical performance is to select a 4. Common Fasteners bolt spacing which yields a value βd Many different types of fasteners Table V equal to 2. are available, but bolts are the most RECOMMENDED TORQUE VALUES 2.8 widely used fastening devices. The Calculated for aluminum flanges with a FOR MILD STEEL BOLTS 2.6 rectangular cross section where: approximate torque required to apply adequate force for mild steel bolts is Max. Recommended 2.4 Ef = 10,000,000 k = 12,000 (average modulus for * Basic Pitch shown in Table V. Size Threads Torque Tension 2.2 conductive silicones) per in. (in.-lbs.) (lbs.) Dia.(inches) h = thickness These values are approximate 3 2.0 and will be affected by the type of #4 40 4 /4 0.0958 48 6 0.0985 1.8 h = 1/4" lubricants used (if any), plating, the type of washers used, the class and #5 40 7 0.1088 1.6 44 81/ 0.1102 finish of the threads, and numerous 2 h = 3/32" 3 1.4 #6 32 8 /4 0.1177 other factors. 40 11 0.1218 1.2 The final torque applied to the #8 32 18 0.1437 h = 1/16" fasteners during assembly should 1.0 36 20 0.1460 be 133 percent of the design value #10 24 23 0.1629 Fastener Spacing, d (inches) 0.8 to overcome the effect of stress- h = 1/32" 32 32 0.1697 0.6 relaxation. When torqued to this 1 /4" 20 80 1840 0.2175 0.4 value, the gasket will relax over a 28 100 2200 0.2268 period of time and then settle to the 5 0.2 /16" 18 140 2530 0.2764 design value. 24 150 2630 0.2854 0.0 3 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Torque may be converted to /8" 16 250 3740 0.3344 Width of Flange, b (inches) tension in the bolts by applying 24 275 3950 0.3479 the formula 7/ " 14 400 5110 0.3911 Figure 24 Fastener Spacing 16 20 425 5250 0.4050 Torque 1 Tension = /2" 13 550 6110 0.4500 For βd = 2, the flange deflection 0.2 x Bolt Dia. 20 575 6150 0.4675 fluctuates by ±10 percent. Minimum 9 /16" 12 725 7130 0.5084 deflection occurs midway between Frequently the rule of thumb value 18 800 7600 0.5264 of 0.2 for the coefficient of friction can 5 fasteners and is 20 percent less /8" 11 1250 11,040 0.5660 than the deflection directly under the result in torque and bolt estimates 18 1400 11,880 0.5889 which may be seriously in error. fasteners. The variation in deflection Torque Excessive bolt preload may lead to * Tension = is approximately sinusoidal. 0.2 x Diameter of Bolt † RF leakage. Therefore, if lubricants † Basic Pitch Diameter

3. Roehrich, R.L., Torquing Stresses in Lubricated Bolts, Machine Design, June 8, 1967, pp. 171-175.

that the enclosure expands faster 150 The average shielding effectiveness than the hardware and usually helps Electric Fields 400 MHz of the gasketed seam is a function 10 kHz to 10 MHz to tighten the seal. Should the of the mean applied pressure, pm. equipment be subjected to low 18 MHz For spacings which approach or temperatures for long periods 1 GHz are equal to one-half wavelength,

of time, the bolts may require 10 GHz the shielding effectiveness is a tightening in the field, or can be 100 function of the minimum pressure,

pretightened in the factory under Plane Waves 200 kHz p1. Therefore, the applied pressure similar conditions. must be 20 percent higher to Under shock and vibration, a 100 kHz achieve the required performance. stack up of a flat washer, split helical For this condition, the space lockwasher and nut are the least 14 kHz between the fasteners can be Magnetic Fields reliable, partly because of elongation 50 considered to be a slot antenna of the stainless steel fasteners, Shielding Effectiveness (dB) loaded with a lossy dielectric. If which causes the initial loosening. the slot is completely filled, then The process is continued under the applied pressure must be 20 shock and vibration conditions. percent higher as cited. Conversely, Elastic stop nuts and locking inserts if the slot is not completely filled

installed in tapped holes have 0 (as shown in Figure 27), the open proven to be more reliable under 0 200 400 area will be free to radiate energy shock and vibration conditions, Applied Pressure (psi) through the slot. but they cost more and are more Figure 25 Shielding Effectiveness vs. expensive to assemble. Applied Pressure w h 5. Electrical Performance as a Shielding effectiveness values for Function of Fastener Spacing typical silver-plated-copper filled, The electrical performance die-cut gaskets as a function of (shielding effectiveness) provided applied pressure are shown in Figure 25. The curves show that Figure 27 Unfilled Slot is Free to by a gasket sandwiched between 1 two flanges and fastened by bolts the shielding effectiveness varies Radiate When Spacing is Equal to /2 Wavelength spaced d distance apart is equivalent appreciably with applied pressure, and changes as a function of the to the shielding effectiveness The cut-off frequency for type of field considered. Plane wave obtained by applying a pressure polarizations parallel to the long attenuation, for example, is more which is the arithmetic mean of the dimension of the slot will be sensitive to applied pressure than maximum and minimum pressure determined by the gap height, h. electric or magnetic fields. applied to the gasket under the The cut-off frequency for the Thus, in determining the perfor- condition that the spacing between polarization vector perpendicular mance to be expected from a fasteners is considerably less than to the slot will be determined by the junction, find the value for an a half wavelength. For bolt spacings width of the slot, w. The attenuation applied pressure which is 10 equal to or approaching one-half through the slot is determined by percent less (for βd = 2) than the wavelength at the highest operating the approximate formula frequency being considered, the value exerted by the bolts directly λ shielding effectiveness at the adjacent to the gasket. For example, A(dB) = 54.5 d/ c examine a portion of a typical gasket point of least pressure is the where governing value. performance curve as shown in For example, assume that a Figure 26. d = the depth of the slot, gasket is sandwiched between and λ is equal to 2w or 2h, depending upon the two flanges which, when fastened Max. c together with bolts, have a value of Mean polarization being considered. p = Minimum Pressure βd equal to 2. Figure 23 shows that Min. 1 pm = Mean Pressure This example also illustrates why a value of βd = 2 represents a p2 = Maximum Pressure leakage is apt to be more for polari- deflection change of ±10 percent zations which are perpendicular to about the mean deflection point. the seam. Because applied pressure is directly β

Shielding Effectiveness For large values of d, the proportional to deflection, the applied percentage adjustments must be pressure also varies by ±10 percent. p1 pm p2 Pressure even greater. For example, the Figure 26 Typical Gasket Performance Curve

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This section presents the method gasket under-deflection and Designing a Solid-O for calculating groove and gasket loss of seal (see Figure 28f) Conductive Elastomer dimensions which will permit the groove over-fill, which can Gasket-in-a-Groove shielding system to function under destroy the gasket (see Figure worst-case tolerance conditions. The solid-O profile is the most 28e). Adherence to these general often specified conductive elastomer Designing to avoid these problems guidelines will result in optimum EMI gasket for several key reasons. is made more complicated by the shielding and sealing for typical Compared to other solid cross effects of: electronics “boxes”. It should be sections, it offers the widest deflection understood that they may not be worst-case tolerance conditions range to compensate for poorly suitable for designing shielding for toleranced mating surfaces and to deformation of the cover sheet metal cabinets, doors, rooms provide reliable EMI shielding and (cover bowing) or other large, unconventional pressure sealing. It can be installed enclosures. poor fit of mating surfaces. in a relatively small space, and is Important Notes: The guidelines the most easily installed and manu- The key to success involves presented here are intended to factured. It also tends to be less selection of the appropriate gasket consider only “solid O” gasket cross prone to damage, due to the absence size and material, and careful design sections. The calculations for hollow of angles, corners or other cross of the corresponding groove. O, solid and hollow D, and custom section appendages. Deflection Limits gasket cross sections differ from The “gasket-in-a-groove” design In nearly every solid-O appli- these guidelines in several key areas. offers five significant advantages cation, Chomerics recommends a Chomerics generally does not over surface-mounted EMI gaskets: minimum deflection of 10% of gasket recommend bonding solid O gaskets diameter. This includes adjustments 1. Superior shielding, due to in grooves. If for some reason your for all worst-case tolerances of both substantial metal-to-metal contact design requires gasket retention, the gasket and groove, cover achieved when the mating surfaces contact Chomerics’ Applications bowing, and lack of conformity are bolted together and “bottom Engineering Department for specific between mating surfaces. We out”. (Flat die-cut gaskets prevent recommendations, since the use of recommend a maximum gasket metal-to-metal contact between adhesives, dove-tailed grooves or deflection of 25% of gasket diameter, mating flange members, which “friction-fit” techniques require considering all gasket and groove reduces EMI shielding performance special design considerations not tolerances. – especially in low frequency covered here. Although sometimes modified magnetic fields.) Extreme design requirements or to accommodate application pecu- 2. Positive control over sealing unusually demanding specifications liarities, these limits have been performance. Controlling the size of are also beyond the scope of the established to allow for stress the gasket and groove can ensure guidelines presented here. Examples relaxation, aging, compression set, that required shielding and sealing would include critical specifications elastic limits, thermal expansion, etc. are achieved with less careful for pressure sealing, exceptionally assembly than is required for flat high levels of EMI shielding, excep- Maximum Groove Fill gaskets. In other words, the gasket- tional resistance to corrosion, harsh Solid elastomer gaskets (as in-a-groove is more foolproof. chemicals, high temperatures, opposed to foam rubber gaskets) seal by changing shape to conform 3. Built-in compression stop heavy vibration, or unusual mounting to mating surfaces. They cannot provided by the groove eliminates and assembly considerations. change volume. The recommended the risk of gasket damage due to limit is 100% groove fill under worst- excessive compression. Mechanical Considerations case tolerances of both gasket and Causes of Seal Failure 4. A gasket retention mechanism groove. The largest gasket cross can be provided by the groove, In order to produce a gasket-in-a- sectional area must fit into the smallest eliminating the need for adhesives groove system which will not fail, the cross sectional groove area. or mounting frames.

Analyzing Worst-Case Tolerances Figure 28a Figures 28a-c illustrate the issues Exploded View of Electronic Enclosure View for of concern, and identify the para- section AÐA meters which should be considered in developing an effective design. Cover Solid Figures 28d and e illustrate two O-profile different cases which can result in EMI gasket gasket damage in the area of torqued bolts. In Figure 28d, the

relationship between groove depth Groove and gasket diameter is critical in for gasket avoiding over-deflection. In Figure 28e, sufficient groove volume must Flange be provided for a given gasket Enclosure volume to permit the gasket to deflect without over-filling the groove. As shown in Figure 28f, cover deformation and groove sizing must be controlled to make sure the gasket is sufficiently deflected to seal the system. Since a single gasket and groove Figure 28b Bolt Spacing (L) are employed for the entire perimeter, View for Lack of Cut-away View section AÐA conformity the design must be optimized for (LOC) each of the worst-case examples of Assembly illustrated in Figures 28d-f.

Cover

Flange

“O” Strip conductive elastomer Flange width (FW) in rectangular groove

Figure 28c Bolt spacing (L) Cover thickness (T) Bolt outboard Cover bowing (CB) Section A-A of Assembled of groove C Enclosure Flange and Gasket B

(Sectioned midway through gasket

and groove)

B C Conductive elastomer gasket in groove

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Minimum groove Nominal depth groove depth

Solution: Over-deflection avoided with smaller maximum gasket diameter and/or

deeper minimum groove depth. Gasket deflected 25% or less

Minimum groove Nominal depth groove depth

Maximum Gasket damage diameter gasket Figure 28e Section B-B from Figure 28c – Worst Case Maximum Groove Fill (Maximum gasket diameter in minimum groove depth and width) Minimum groove width Minimum groove dimension cannot Problem: Nominal accommodate maximum gasket diameter, groove width Minimum resulting in gasket damage. groove Nominal depth groove depth

Solution: Groove over-fill avoided with smaller maximum gasket diameter and/or greater minimum groove depth and/or width. Nominal groove width

Nominal groove depth

Figure 28f Section C-C from Figure 28c – Worst Case Minimum Deflection (Minimum gasket Minimum diameter gasket diameter in maximum depth groove, aggravated Gasket by cover bowing and lack of conformity between deflected less than 10% mating surfaces) Nominal groove depth Problem: Gasket will not be deflected the Maximum recommended 10% minimum. Combined groove depth effects of tolerances, cover bowing, and lack of conformity can result in complete loss of cover-to-gasket contact over time, and consequent seal failure. Gasket Solution: Under-deflection avoided with deflected 10% or more larger minimum gasket diameter and/or Nominal groove depth shallower maximum groove depth. Maximum groove depth

Calculating the Dimensions and Tolerances for the Groove and EMI Gasket Figure 29 diagrams the calcula- SELECT A REASONABLE GASKET DIAMETER tion and decision sequence required to determine the dimensions for a properly designed solid-O gasket/ groove system. Because the relationship between groove depth CALCULATE NOMINAL GROOVE DEPTH (Formula 1) and gasket diameter is central to seal performance, groove depth is selected as the key variable to determine first. Start by making an educated guess as to reasonable values for ESTABLISH TOLERANCES groove and gasket sizes and tolerances, based on desired nominal gasket deflection of 18%. For example, if 0.025 in. of gasket deflection is desired, start with a CALCULATE MAXIMUM GASKET DEFLECTION (Formula 2) nominal gasket diameter of 0.139 in. Adjust parameters if this value is more than 25% This is calculated by dividing the desired total gasket deflection by 0.18 to estimate the required gasket size. (Total Gasket Deflection ÷ 0.18 = Approx. Nominal Gasket Size.) This relationship is an alternate form CALCULATE MINIMUM GASKET DEFLECTION (Formula 3) of Formula 1. Final groove Adjust parameters if this value is less than 10% dimensions can only be determined after completing all of the calculations called for in Figure 29, and arriving at values which remain within the recommended limits for gasket deflection and groove fill. CALCULATE NOMINAL GROOVE WIDTH (Formula 4)

VERIFY THAT FINAL GROOVE DIMENSIONS SATISFY BOTH MIN. AND MAX. GASKET DEFLECTION AND GROOVE FILL LIMITS UNDER WORST-CASE TOLERANCE CONDITIONS

Figure 29 Procedure for Calculating Gasket and Groove Dimensions

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GrDnom – Nominal groove depth 3. Minimum Gasket Deflection GrDT – Groove depth tolerance (difference (Worst Case, expressed as a % of gasket diameter) between max. and nom. or min. and nom.)

GaDfmax – Maximum gasket deflection (%) (GaDnom – GaT) – (GrDnom + GrDT) – CB – LOC a. GaDfmin = 100 GaDf – Minimum gasket deflection (%) [ (GaDnom – GaT) ] min where Lmax – Maximum bolt spacing FWmin – Minimum flange width GDF x L4 b. CB = max Tmin – Minimum cover thickness 3 FWmin x T min x E x 32 GDF – Gasket deflection force (ppi or Newtons (Note: Formula must be adjusted when using metric units) per meter). Note: For the purpose of this guide, the GDF value and should represent the worst-case minimum gasket deflection arising from cover bowing. For example, c. LOC = 0.001 in. for machined surfaces with surface roughness of the GDF is taken at 10% deflection for the 32-64 µin. RMS. calculation in Formula 3b. 7 (For discussion, see Terms.) E – Young’s modulus. (For aluminum, use 1 x 10 psi, or 7 x 105 kg/cm2.) 4. Nominal Groove Width CB – Cover bowing, generally calculated by modeling the elastic deformation of the cover as a 2 a. GaA max = 0.7854* (GaDnom + GaT) uniformly loaded beam with two fixed supports. (The moment of inertia of the cover is modeled as a GaAmax rectangular beam, the “height” of which is taken to b. GrWmin = GrDmin be equal to the cover thickness, while “width” is considered equal to flange width. The moment of c. GrW = GrW + GrWT inertia can be adjusted for cover configurations nom min other than flat. Refer to an engineering handbook π for the necessary revisions to Formula 3b.) An *Note: 0.7854 = 4 assumption is made that one side of a cover/flange interface is infinitely stiff, typically the flange. If this is not essentially true, elastic deformation of each is computed as though the other were infinitely stiff, and the two values combined. LOC – Lack of conformity, the measure of the mismatch between two mating surfaces when bolted together, expressed in inches. Experience has shown that machined surfaces with a surface roughness of 32-64 µin. RMS exhibit an LOC of 0.001 in. It is left to the engineer’s judgment to determine LOC for other surfaces. LOC can be determined empirically from measurements made of actual hardware. In this guide, LOC applies only to the surfaces which form the EMI shielding interface.

EMI Shielding the non-conductive portion. Spot Plus Environmental/ applications to the conductive Pressure Sealing area are permissible. Some gasket applications require 3. MESH STRIP – The all metal and only restoration of the shielding elastomer core versions of these integrity of an enclosure, and can with attachment fins can be held be satisfied with Chomerics’ simple in position with non-conductive MESH STRIP gasketing. In these adhesive or epoxy if it is restricted cases, the use of MESH STRIP with Poor Design Good Design to the mounting fins (see Figure Elastomer Core provides additional Figure 30 Allowing for Solid Elastomer 31c). resiliency. Elastomer cored strips Flow in Groove Capture Attachment 4. Frame Gasketing can be attached offer limited environmental sealing Method with a non-conductive adhesive or by positive blocking of dust and rain. epoxy restricted to the aluminum Additional environmental sealing or without adhesive backing. In many extrusion (see Figure 31d). exclusion of ventilating air or vapor cases customers purchase COMBO However, most Frame Gaskets requires a gasket such as COMBO STRIP or COMBO Gasket materials are attached mechanically with STRIP, which incorporates a smooth, for applications which don’t require fasteners. easily compressed, elastomer environmental sealing, but utilize the sealing strip in parallel with the EMI adhesive-backed rubber portion as 5. Dry Back Adhesive for Neoprene shielding strip. When an appreciable an inexpensive, temporary attach- Sponge COMBO Gaskets – pressure differential must be ment method (“third hand”) during Factory-applied solvent-activated maintained between the interior and installation. adhesive is recommended for several reasons: a) controlled exterior of an enclosure, in addition C. Bond Non-EMI Portion application guarantees restriction to EMI protection, materials such as of Gasket Non-conductive of the adhesive to the non- CHO-SEAL conductive elastomers or adhesives may be employed to conductive portion; b) controlled POLA gaskets should be used. bond an EMI gasket in position by adhesive thickness assures applying adhesive to the portion that reliable bonding without reducing Gasket Attachment is not the EMI gasket (and which compressibility; and c) the and Positioning can be insulated from the mating adhesive provides a permanent Substantial cost savings can surfaces by a non-conductive bond. result from the careful choice of material). gasket attachment or positioning Note: When specifying non- method, which often determines conductive adhesive attachment, the final choice of material. applicable drawings and standard (a)* (c)* procedures for production personnel A. Groove Capture This method is should emphasize that the adhesive strongly recommended if a groove

is to be applied only to the portion can be provided at relatively low cost, (b)* of the gasket which is not involved (d)* such as die-casting. (Caution: POLA- with the EMI shielding function. The STRIP gaskets are essentially *Areas where non-conductive assumption that the gasket “will adhesives can be used incompressible, although they seem hold better if all of it is bonded rather to compress because the material (c)* than half of it” will result in serious flows while maintaining the same degradation of EMI shielding volume. Extra space must be allowed Figure 31 a-d Application of Non- effectiveness. to permit the solid elastomer material Conductive Adhesive to flow (see Figure 30). 1. Figure 31a illustrates this method D. Bolt-Through Holes This is a used for COMBO STRIP and B. Pressure-Sensitive Adhesive common, inexpensive means to hold COMBO Gaskets, in which only This is often the least expensive gaskets in position (see Figure 32). the elastomer portion is bonded attachment method for mesh EMI For most Chomerics metal shielding to one of the mating surfaces. gasket materials. Installation costs products, providing bolt holes are dramatically reduced with only a 2. “Combo“ forms of POLASTRIP involves only a small tooling charge, slight increase in cost over gasketing may be bonded if, as in Figure with no additional cost for the holes 31b, the adhesive is restricted to

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 214 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 in the unit price of the gasket. Bolt- Friction, Abrasion and resiliency and compression set of holes can be provided in the fin Impact Considerations the gasket material. portion of MESH STRIP, or in EMI gaskets should be positioned Monel rectangular cross section MESH so that little or no sliding or shear STRIP if these are wide enough, occurs when compressed. In Figure This good all-purpose nickel- 3 (minimum width /8 in. (9.52 mm). 34a, the EMI gasket is subject to copper alloy resists oxidation sliding as the door is closed, which (thereby maintaining its conductivity), may lead to tearing, wearing out, or has good EMI qualities, and very

detachment. Figure 34b illustrates good mechanical strength and the preferred position, in which the resiliency. In controlled or protected EMI gasket is subjected almost atmospheres, it may be used in

contact with aluminum; but where

entirely to compression forces.

salt spray environments are

(a) Poor design, encountered, galvanic corrosion

door slides on is a problem. EMI gasket

Cabinet Note: In salt spray environments, Figure 32 Bolt-Through Gasket monel is corrosion-resistant, but Door Mounting when in contact with aluminum E. Special Attachment Means flanges, electrolytic currents will Knitted mesh fins provided on some cause corrosion of the aluminum versions of MESH STRIP, and (b) Good design, flange.

door compresses extruded aluminum strips on Frame on EMI gasket Ferrex® Gasketing are designed for attach- Cabinet Chomerics’ Ferrex tin-plated ment (see Figure 33). Attachment Door fins can be clamped under a metal copper-clad steel wire offers the strip held down by riveting or spot best EMI/EMP performance of the welding, or can be bonded with a standard mesh materials, especially Figure 34 a-b Sliding Motion vs. for H-field shielding. Its mechanical structural adhesive or epoxy. The Straight Compression aluminum extrusions in Frame properties are very close to monel, Gaskets can also be fastened by Mesh Gasketing Materials and it is more compatible with aluminum, but it has poorer intrinsic riveting or bolting. A. Knitted Wire Mesh corrosion resistance than monel. Knitted wire mesh can be produced With this understanding of Cover from any metal which can be drawn material characteristics, gasket EMI Mesh Strip Gasketing into wire form. However, the great metal is usually chosen using the Rivet or spot Cabinet majority of shielding requirements

weld strip over following guidelines:

mounting fin. Box are readily satisfied with a choice of For low frequency magnetic

Door two materials – monel or Ferrex – field shielding: recommended both of which are standard produc- gaskets are Ferrex versions of Cover tion materials for Chomerics’ mesh knitted mesh gasketing (provided

Frame Gasketing gaskets.

corrosion resistance requirements

Rivet or spot Cabinet weld aluminum Two design considerations should are not severe).

extrusion to influence the choice of EMI gaskets: cover or cabinet.Box For high frequency electric field

■ required shielding performance shielding: recommended gaskets Door are monel or Ferrex. in E-, H- and Plane Wave fields, For best corrosion resistance ■ required corrosion resistance Figure 33 Rivet or Spot Welding (except in contact with aluminum in of the gasket. salt spray environments where Additional considerations include corrosion will occur): monel is the mechanical strength, durability, recommended, preferably embedded

in elastomer (e.g., POLA). Aluminum or custom die-cut configurations. It mesh is sometimes selected when should only be used where joint sponge sponge equipment specifications permit unevenness is less than 0.002 in. no other metal to be used against (0.05 mm). aluminum mating surfaces, for

solid solid D. Expanded Metal Mesh galvanic corrosion compatibility. PORCUPINE METALASTIC However, it must be understood Figure 37 Normal and High Pressure gasketing is a material composed of that aluminum mesh oxidizes readily, COMBO STRIP Gaskets expanded Monel metal mesh, and is and shielding effectiveness there- MESH STRIP with Elastomer Core available with optional silicone filling. fore degrades. is available in round or rectangular It is produced in sheets of con- Chomerics Knitted Wire profiles, with solid or hollow tinuous length, 12 in. (30.4 cm) by Mesh Products elastomer, with an optional mesh 0.020 in. or 0.030 in. (0.51 mm or mounting fin (see Figure 38). 0.76 mm) thick. PORCUPINE MESH STRIP is available as METALASTIC gasketing is easily cut

resilient, single and dual all-metal into intricate shapes with inexpensive

strips or compressed shapes,

rule dies, has high uniformity in

with optional mounting fins. Both thickness, ±0.004 in. (0.010 mm), rectangular and round profiles are Figure 38 MESH STRIP with and withstands high compression offered in a large range of standard Elastomer Core Profiles forces without damage. Available dimensions for use as EMI gaskets as sheets and standard connector where no environmental sealing is Compressed Mesh Gaskets gaskets, it can also be supplied in required (see Figure 35). Note: See are jointless units made by die- custom die-cut configurations. It also SPRINGMESH highly resilient compressing knitted metal mesh, should only be used where joint wire mesh gaskets made from tin- usually in round or rectangular unevenness is less than 0.003 in. plated steel wire, page 111. forms, with a constant rectangular (0.08 mm) (see Figure 40). cross section. Standard waveguide

types are available, and Chomerics maintains a large selection of B existing tooling for other annular A Figure 35 MESH STRIP Gasketing types. Profiles B. Oriented Wire in Silicone A Wire Mesh Frame Gaskets offer POLASTRIP/POLASHEET are D combinations of one or two round- composite mesh and elastomer B A A profile mesh strips, or one mesh/ materials in which wire is one pressure-seal strip (round or embedded in part or all of the C rectangular) with a metal mounting silicone elastomer. The mesh is frame (see Figure 36). METALKLIP C E in the form of individual wires A clip-on strips consist of wire mesh oriented perpendicular to the joint over elastomer core gaskets mating surfaces, for maximum EMI Figure 40 PORCUPINE attached to metal mounting clips. shielding (see Figure 39). METALASTIC Die-Cut Gaskets (fully dimensioned drawings required)

Figure 39 POLA Materials Profiles

C. Woven Metal Mesh METALASTIC Gasketing is formed of Figure 36 Frame Gasketing Profiles woven aluminum mesh, filled with silicone or neoprene for pressure COMBO and COMBO STRIP sealing. It is produced in 8 in. (20.3 Gaskets combine a low-profile, solid cm) wide sheets in random lengths, or sponge elastomer strip in parallel in thicknesses of 0.016 in. (0.40 mm) with one or two rectangular mesh and 0.020 in. (0.51 mm). The 0.016 strips (see Figure 37). With solid in. (0.40 mm) material is the thinnest elastomers, the mesh strip has a available for EMI plus pressure seal higher profile than the elastomer, to gasketing. It can be obtained in allow for compression of the mesh. sheets, standard connector gaskets,

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 216 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 Wire Mesh EMI Gasket Selection Guide C F ® ° ° N/A F to 400 C to 204 ° ° POLASHEET C –62 F –80 ® ° ° See Note 7. H 0.125 H) (3.18) 2 2 / / 1 1 Only F to 500 C to 260 Special Special ° Use Silicone Base Adhesive ° POLASTRIP Combo Version ® F C C –57 F –70 ° ° ° ° F to 225 F to 500 C to 107 C to 260 ° ° ° ° METALASTIC –40 C –53 F –65 ® ° ° (5) N/A F to 400 C to 204 Monel, Aluminum Monel, Monel, Special Special ° ° Excellent Excellent Excellent Excellent Aluminum Only Aluminum Aluminum C –62 F F –80 (1) C –40 ° ° ° ° (3) (3) C to 66 F to 150 F to 400 C to 204 FRAME METALASTIC ° ° ° ° Formed Knitted ® GASKETING F –30 C –62 F –80 C –34 ° ° ° comparisons between products in this table since all tests were conducted under similar conditions. They cannot be used to compare other EMI gasket data unless those were obtained by the same methods. conductive adhesive. ° H 0.437 0.140 0.125 0.093/ H) (11.0) (3.56) (3.18) (2.36/ 2 Pressure sensitive adhesive is available for certain mesh over core gaskets. Contact Chomerics details. 2 / , Aluminum Monel, Ferrex / 1 * (6) These EMI ratings are based on MIL-STD-285 test methods and useful for making meaningful qualitative If more adhesive surface is needed, use yields excellent results, but use sparingly. (7) Non-conductive RTV 1 (1) C to 66 F to 150 F to 400 C to 204 ° ° ° ° Excellent Excellent . AND PORCUPINE STRIP COMBO Good-Excellent Poor ® ® Formed Knitted Wire Wire Strips Oriented Wire in Matrix of COMBO F –30 C –62 F –80 C –34 TM ° ° ° (2) ° , Monel, Ferrex (2) H 0.125/1 H) (3.18/1 2 2 (1) GASKETING GASKETING GASKETS GASKETING GASKETING GASKETING GASKETING / / 1 1 ® C to 66 F to 150 F to 400 C to 204 ° ° ° ° Fins Only MESH STRIP Possible with Versions with Versions Fin Versions –34 –30 –80 –62 TM H 0.62/ , Ferrex H) (1.57/ 2 2 / (1) / 1 1 * N/A COMPRESSED (ELASTOMER CORE) AND COMBO SHIELDMESH TM (2) H 0.062/ , Ferrex (2) Knitted Wire Mesh Elastomer Strips or Die-Cut Gaskets Extrusions in Elastomer in Elastomer sensitive adhesive) H) (1.57/ 2 2 / (1) / 1 Formed or Compressed Mesh Over Elastomer Strips; Aluminum Metal Wire Woven (available with pressure 1 Strips Gaskets Clip-On Strips Strips Strips EMI Strips Gaskets Gaskets Joining Strips Gaskets Strips, Strips, Gaskets Strips, Die-Cut Strips, Fab. Monel Monel Monel by Joining or Rectangular Joining Strips, by Joining Joined EMI with Joined Die-Cut Die-Cut Made by Die-Cut 30-40%25-30%20-25% 30% 25% 20% 30-50% 25-40% 20-30% 30% 30% 25% 30% 25% 25% 15% 10% 10%`10% 7% 7% 20% 17% 17% 20% 17% 17% Ferrex MESH STRIP Aluminum Aluminum Aluminum Gaskets Made Jointless Rings Made by Gaskets Made Elastomer with Lengths, Frames Sheets, Sheets, Strips, Gaskets Sheets, Fins Only Poor to Good Poor to Good Poor to Good N/A N/A No No Versions with Versions (mm) (1.57/12.70) (1.02/9.53) (3.18/19.05) (1.57/9.53) (2.36/6.35) (0.51/0.76) (0.41/0.51) (1.57/7.92) (0.76/6.35) Inches 0.062/0.500 0.040/0.375 0.125/0.750 0.062/0.375 0.093/0.250 0.020/0.030 0.016/0.020 0.062/0.312 0.030/0.250 Open-Close in Same Position Completely Interchangeable Permanently Closed (4) Bolt thru Possible with 14 kHz (H)18 MHz (E)1.0 GHz (P) >20- >30 dB >102 dB >83- >93 dB >25- >30 dB >102 dB >93 dB >25- >35 dB >102 dB >93 dBIn Slot >20- >30 dBPressure SensitiveAdhesive Bond Non- >20- >30 dBEMI Gasket >102 dB >83- >93 dBPortion Excellent >35 dBAdhesive N/A ExcellentBolt Holes >35 dB >93 dB >102 dBNeoprene Version N/A >46 dB Excellent >102 dB >85 dB Fin Versions N/A >102 dB >40 dB >35 dB N/A >102 dB Excellent >93 dB N/A >102 dB >93 dB Excellent No No N/A No N/A Good N/A Possible Special Excellent Conductive Silicone Version N/A N/A Class B – Class C – Class A – is Chomerics’ tradename for tin-plated, copper-clad steel EMIis Chomerics’ tradename for tin-plated, copper-clad gasketing. ® (6) or Elastomer Temperature Range PRODUCT TRADE NAME (ALL-METAL) MESH GASKETS METALKLIP Schematic Cross Section Dim. or Portion of Height) (mm)Pressure (1.57/ (kg/cm) (0.35-7.03) (0.35-7.03) (0.35-7.03) (1.41-7.03) (0.35-7.03) (1.41-7.03) (1.41-7.03) (1.41-7.03) (1.41-7.03) Recommended Compression psi 5-100 5-100 5-100 20-100 5-100 20-100 20-100 20-100 20-100 Min. Width (Greater of Actual Inches 0.062/ Minimum/Maximum Height Available Forms Available Construction Knitted Wire Strips in Parallel with Clamped in Expanded Silicone Elastomer Portion (others also available) Standard Metals Available in EMI Standard Metals Available Positioning EMI Rating Maximum Joint Unevenness, % of Gasket Height Attachment (1) Ferrex (2) Two versions,(2) Two (3) The aluminum extrusion is intended as a convenient means of attachment. (4) Most products for which this method is suitable are available with “dry back” (solvent-activated) adhesives already applied and have fins especially designed for easy attachment. (5) Available without elastomer in metal form only. (5) Available

Electrical Depth of Penetration: Distance Interference: Any electromagnetic which a plane wave must travel phenomenon, signal or emission, Absorption Loss: Attenuation of an through a shield to be attenuated 1/e, man-made or natural, which causes electromagnetic wave or energy or approximately 37 percent of its or can cause an undesired response, encountered in penetrating a shield original value. (Also called “skin malfunctioning or degradation of caused by the induction of current depth”). It is a function of the shield’s performance of electrical or electronic flow in the barrier and the resulting conductivity and permeability and the equipment. I2R loss. Usually stated in dB (decibels). wave’s frequency. Internal Loss: Attenuation of electro- Ambient Electromagnetic Electrical or E-Field: A field induced magnetic energy by the reflection Environment: That electromagnetic by a high impedance source, such as and re-reflection of electromagnetic field level existing in an area and a short dipole. waves within a shield or a barrier. emanating from sources other than Usually stated in dB. the system under test. Electromagnetic Compatibility (EMC): A measure of an equipment’s Magnetic or H-Field: An induction Attenuation: A reduction in energy. ability to neither radiate nor conduct field caused predominantly by a Attenuation occurs naturally during electromagnetic energy, or to be current source. Also called a low wave travel through transmission susceptible to such energy from impedance source, such as may be lines, waveguides, space or a other equipment or an external generated by a loop antenna. medium such as water, or may be electromagnetic environment. produced intentionally by inserting an Malfunction: A change in the attenuator in a circuit or a shielding Electromagnetic Interference (EMI): equipment’s normal characteristics absorbing device in the path of Undesired conducted or radiated which effectively destroys proper radiation. The degree of attenuation electrical disturbances, including operation. is expressed in decibels or decibels transients, which can interfere with Permeability: The capability of a per unit length. the operation of electrical or electronic material to be magnetized at a given equipment. These disturbances can Attenuator: An arrangement of fixed rate. It is a non-linear property of both occur anywhere in the electromagnetic and/or variable resistive elements the magnetic flux density and the spectrum. used to attenuate a signal by a frequency of wave propagation. desired amount. Emanation: Undesired electromag- Plane Wave: An electromagnetic netic energy radiated or conducted Cross Coupling: Coupling of the wave which exists at a distance from a system. signal from one channel to another greater than a wavelength from where it becomes an undesired signal. Gasket-EMI: A material that is inserted the source, where the impedance between mating surfaces of an of the wave is nearly equal to the Conductivity: Capability of a material electronic enclosure to provide low impedance of free space – 377 ohms. to conduct electrical currents. resistance across the seam and Radio Frequency (RF): Any frequency Decibel (dB): A convenient method thereby preserve current continuity at which coherent electromagnetic for expressing voltage or power ratios of the enclosure. radiation of energy is possible. in logarithmic terms. The number of Ground: A reference plane common Generally considered to be any such units of attenuation, N is to all electronic, electrical,electro- frequency above 10 kHz. P mechanical systems and connected N (dB) = 10 log 1 Radio Frequency Interference (RFI): to earth by means of a ground rod, P2 Used interchangeably with EMI. EMI where ground grid, or other similar means. is a later definition which includes the P1/P2 is a unitless power ratio. N can also be Hertz: An international designation entire electromagnetic spectrum, expressed in terms of a voltage ratio E1/E2 as for cycles per second (cps). whereas RFI is more restricted to follows: Insertion Loss: Measure of the radio frequency band, generally considered to be between the limits E improvement in a seam, joint or N (dB) = 20 log 1 10 kHz to 10 GHz. E2 shield by the addition of a conductive gasket. Usually stated in dB. Reflection Loss: Attenuation of the electromagnetic wave or energy Degradation: An undesired change caused by impedance mismatch in the operational performance of a between the wave in air and the wave test specimen. Degradation of the in metal. operation of a test specimen does not necessarily mean malfunction.

US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 218 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 Relative Conductivity: Conductivity Mechanical known as durometers and plasto- of the shield material relative to the Abrasion Resistance: The resistance meters. Durometers are used most conductivity of copper. of a material to wearing away by commonly. The higher the durometer Relative Permeability: Magnetic contact with a moving abrasive number, the harder the rubber, and permeability of the shield material surface. Usefulness of standard tests vice versa. relative to the permeability of free very limited. Abrasion resistance is a Hardness Shore A: Durometer space. complex of properties: resilience, reading in degrees of hardness using Shield: A metallic configuration stiffness, thermal stability, resistance a Type A Shore durometer. (Shore A inserted between a source and the to cutting and tearing. hardness of 35 is soft; 90 is hard). desired area of protection which has Cold Flow: Continued deformation Permeability: A measure of the ease the capability to reduce the energy under stress. with which a liquid or gas can pass level of a radiating electromagnetic Compression Set: The decrease in through a material. field by reflecting and absorbing the height of a specimen which has been Permanent Set, Stress and Strain energy contained in the field. deformed under specific conditions Relaxation: Permanent Set is defined Shielding Effectiveness: A measure of load, time, and temperature. as the amount of residual displacement of the reduction or attenuation in Normally expressed as a percentage in a rubber part after the distorting electromagnetic field strength at a of the initial deflection (rather than as load has been removed. Stress point in space caused by the insertion a percentage of the initial height). Relaxation, or Creep, is a gradual of a shield between the source and Durometer: An instrument for increase in deformation of an elastomer that point. Usually stated in dB. measuring the hardness of rubber. under constant load with the passage Shielding Increase: The difference Measures the resistance to the of time, accompanied by a of an electromagnetic field amplitude penetration of an indentor point corresponding reduction in stress emanating through a seam (measured into the surface of the rubber. level. under fixed test conditions) with and Elasticity: The property of an article Resilience: The ratio of energy given without the gasket in the seam, with which tends to return to its original up on recovery from deformation to the force joining the seam remaining shape after deformation. the energy required to produce the constant. The difference is expressed deformation – usually expressed in dB based on voltage measurements. Elastic Limit: The greatest stress in percent. which a material is capable of Skin Effect: Increase in shield developing without a permanent Tear Strength: The force per unit of resistance with frequency because of deformation remaining after complete thickness required to initiate tearing crowding of current near the shield release of the stress. Usually this in a direction normal to the direction surface because of rapid attenuation term is replaced by various load of the stress. of current as a function of depth from limits for specific cases in which the Tensile Strength and Elongation: the shield surface. resulting permanent deformations Tensile Strength is the force per unit Surface Treatment: Coating or plating are not zero but are negligible. of the original cross sectional area of mating surfaces of a junction. Elastomer: A general term for elastic, which is applied at the time of the Susceptibility: Measure of the rubber-like substances. rupture of the specimen during degradation of performance of a tensile stress. Elongation is defined Elongation: Increase in length system when exposed to an as the extension between benchmarks expressed numerically as a fraction electromagnetic environment. produced by a tensile force applied or percentage of initial length. to a specimen, and is expressed Total Shielding Effectiveness: The Hardness: Relative resistance of as a percentage of the original difference of an electromagnetic rubber surface to indentation by an distance between the marks. Ultimate amplitude emanating from a source indentor of specific dimensions under elongation is the elongation at the within an enclosure, and that from a a specified load. (See Durometer). moment of rupture. Tensile Stress, source in free space. The difference Numerical hardness values represent more commonly called “modulus,” is is expressed in dB based on voltage either depth of penetration or the stress required to produce a measurements. convenient arbitrary units derived certain elongation. Wave Impedance: The ratio of from depth of penetration. Devices electric field intensity to magnetic for measuring rubber hardness are field intensity at a given frequency expressed in ohms.

Use this table to identify product groups by Part Number and locate them in this Handbook.

CHOMERICS REFER DESCRIPTION CHOMERICS DESCRIPTION REFER PART NO. TO PAGE PART NO. TO PAGE

01-0101-XXXX MESH STRIP TM All Metal Gaskets 81-82 19-11-XXXX-XXXX Conductive Elastomer Die-Cut Parts 58 01-0104-XXXX 19-12-XXXX-XXXX Conductive Elastomer Molded Parts 61 ® 01-0199-XXXX SPRINGMESH Highly Resilient Gaskets 111 19-13-XXXX-XXXX Molded-In-Place Cover Seals 77-79 ® 01-02XX-XXXX COMBO STRIP Mesh/Rubber Gaskets 112-114 19-18-XXXX-XXXX Conductive Elastomer Extrusions 35-58 01-03XX-XXXX 19-24-XXXX-XXXX (see detailed index on pages 87-90) 01-06XX-XXXX 19-41-XXXX-XXXX Conductive Elastomers, Fabric Reinforced 76 01-07XX-XXXX 20-01-XXXX-XXXX Conductive Elastomer Waveguide Gaskets 67-71 01-04XX-XXXX MESH STRIP TM With Elastomer Core 109-110 20-02-XXXX-XXXX 01-05XX-XXXX 20-03-XXXX-XXXX SOFT-SHIELD® 1000 20-11-XXXX-XXXX 01-09XX-XXXX Low Closure Force Foam Gaskets 103-104 30-01-XXXX-XXXX Conductive Elastomer Connector Gaskets 72-75 01-1292-XXXX SOFT-SHIELD® 2000 01-1392-XXXX Low Closure Force Foam Gaskets 101-102 30-02-XXXX-XXXX SHIELDMESHTM Compressed 30-03-XXXX-XXXX 02-XXXX-XXXX Mesh Gaskets 115 30-XX-XXXX-XXXX 04-XXXX-XXXX METALASTIC ® EMI Gasketing 121 40-XX-XXXX-XXXX Conductive Elastomer Sheet Stock 58-60 05-XXXX-XXXX-XX SHIELD WRAPTM Knitted Wire Mesh Tape 190 41-XX-XXXX-XXXX 06-01XX-XXXX-XX Mesh Frame Gaskets and Strips 116-117 43-XX-XXXX-XXXX 06-02XX-XXXX-XX 50-XX-XXXX-XXXX Conductive Compounds 133-142 06-21XX-XXXX-XX 51-XX-XXXX-XXXX 06-03XX-XXXX-XX SHIELD CELL® Shielded Vent Panels 162 52-XX-XXXX-XXXX 06-05XX-XXXX-XX 53-XX-XXXX-XXXX 06-09XX-XXXX-XX 54-XX-XXXX-XXXX 06-X7XX-XXXX SLIMVENTTM Shielded Air Vent Panels 165 55-XX-XXXX-XXXX 06-XX15-XXXX-XX Steel Honeycomb Shielded Vents 164 70-2X-XXXX-XXXX CHO-SHRINK® Heat Shrinkable Tubing 186-188 06-07XX-XXXX-XX SHIELDSCREEN® Shielded Air Filters 166 ® Heat Shrinkable 71-XX-XXXX-XXXX CHO-SHRINK 186-188 06-13XX-XXXX-XX Molded Parts 06-14XX-XXXX-XX 80-10-XXXX-XXXX CHO-DROP® EMI Absorbers 183 ® Beryllium-Copper Gaskets 06-XX14-XXXX-XX Brass Honeycomb Shielded Vents 164 81-XX-XXXX-XXXX SPRING-LINE 126-132 06-1010-XXXX-XX (fingerstock, card cage and D connector gaskets) ® 06-1014-XXXX-XX 82-XX-XXXX-XXXX SOFT-SHIELD 5000 93-97 Low Closure Force Foam Gaskets 06-11XX-XXXX-XX OMNI CELL® Shielded Vent Panels 162 83-XX-XXXX-XXXX CHO-SORB® EMI Ferrites 176-182 06-12XX-XXXX-XX 86-XX-XXXX-XXXX CHO-BUTTONTM EMI Grounding Contacts 156 06-15XX-XXXX-XX VIP Shielded Air Filters 165 CAD-XX-XXX-XXXX CHO-FOIL® EMI Shielding Tapes 146-149 06-16XX-XXXX-XX CCD-XX-XXX-XXXX POLASHEET® and POLASTRIP® 07-XXXX-XXXX Composite Gasketing 118-120 CCE-XX-XXX-XXXX 08-XXXX-XXXX PORCUPINE METALASTIC® EMI Gaskets 97 CCH-XX-XXX-XXXX 10-00-XXXX-XXXX Conductive Elastomer O-Rings 64 CCJ-XX-XXX-XXXX 10-01-XXXX-XXXX Conductive Elastomer D-Rings 63 CCK-XX-XXX-XXXX 10-02-XXXX-XXXX Conductive Elastomer Flat Washers 66 CBL-XX-XXXX-XXXX EMI Shielding Laminates 150-151 10-03-XXXX-XXXX CHO-EMI-TAPE-BOX EMI Shielding Tapes Kit 149 10-04-XXXX-XXXX Conductive Elastomer Extrusions 35-58 CFT-XX-XXX-XXXX CHO-FABTM EMI Shielding Fabric Tape 146-149 10-05-XXXX-XXXX (see detailed index on pages 87-90) CHO-MASK® II EMI Foil Tape with CMT-XX-XXXX-XXXX 144-145 10-06-XXXX-XXXX Peel-Off Mask ® 10-07-XXXX-XXXX CJ-XXX-XX CHO-JAC Cable Jacketing 149 CWA-XX-XXXX-XXXX SOFT-SHIELD® 10-08-XXXX-XXXX 4000 Low Closure 98-100 CWF-XX-XXXX-XXXX Force Foam Gaskets 10-09-XXXX-XXXX E-01-XXXXX EmiClareTM GP 70 EMI Shielded Windows 171-172 14-XXXX-XXXX-X ZIP-EX-2® Zippered Cable Shielding 184-185 STREAMSHIELDTM EMI Shielded 17-03XX-XXXX-XX METALKLIP® Clip-On EMI Gaskets 123 FPCV-XXXXX-XXXXXX 158-160 Vent/Airflow Panels 19-04-XXXX-XXXX Conductive Elastomer Extrusions 35-58 WIN-SHIELDTM Shielded Window (see detailed index on pages 87-90) G-01-XXXXX 172-173 19-05-XXXX-XXXX (Glass Assembly) 19-06-XXXX-XXXX L-XXXX-XX CHO-STRAP® Grounding Straps 153 19-07-XXXX-XXXX WIN-SHIELDTM Shielded Window P-01-XXXXX 172-174 19-08-XXXX-XXXX (Plastic or AgF8 Film Assembly) 19-09-XXXX-XXXX