EMI gasket design: groove, deflection, closure force and flange finish

The junction comes first

The handbook's opening rule is that the ideal gasketing surface is rigid, recessed to house the gasket, as conductive as possible and non-corrosive, and that where corrosion products cannot be avoided they should conduct or be easy to abrade through [S9]. It states that surface conductivity of gasket and flange is the single most important property of an effective seam, so flange-to-gasket resistance must be as low as possible, and that a gasket exists only because the junction is imperfect [S9]. Two rigid flat surfaces never meet at every point; the surface irregularities form leak paths for gas, liquid and high-frequency energy, and a resilient gasket that conforms to both faces closes the leak, electrically as well as physically when it conducts [S11]. The practical corollaries: make the flange of the enclosure metal with the highest conductivity available, put a caution on the drawing not to paint the mating faces, mask them before painting and clean after, and read SAE ARP1481 for aerospace flange finishing [S9]. The filler question that follows from the flange metal is on the galvanic compatibility guide.

Grooves versus flat flanges

Grooves are preferred because all rubber takes compression set, especially when over-compressed, and because flanges bow between bolts so a gasket on a flat flange over-compresses at the bolts; a groove limits deflection and also gives metal-to-metal flange contact that lowers junction resistance [S9]. A second parallel groove adds about 6 dB and more than two add little; fasteners belong outboard of the groove so no gasket is needed under them [S9]. Flat gaskets on sheet-metal or machined flanges are the alternative: bend the flange outward so the bolts stay outside the shielded volume, or seal the bolt path with close-fitting holes, seals under the heads, or flared inserts and blind nuts [S9]. Rectangular sections may share an O-groove if void is left; keep the height no greater than the base width, with a height-to-width ratio of one half as the optimum, because tall gaskets roll over [S9]. Wherever a groove does not limit compression, the handbook asks for a compression stop [S9].

Deflection windows by cross-section, with the edition

Deflection is the change in gasket height under the closing load, stated as a percentage of the free height, and the manufacturers publish it per profile rather than per material [S9][S11]. The two references disagree on the lower bound for a solid O and both are shown; where a third source publishes a figure it is in the last column.

The handbook's own figure of deflection by size gives, for example, a 0.070 in solid O deflected 0.007 to 0.018 in, a 0.125 in O 0.013 to 0.031 in, a 0.062 in flat gasket 0.003 to 0.006 in and a 0.250 in hollow P 0.030 to 0.125 in [S9]. For a gasket that is not contained in a groove, a first estimate of its thickness is four times the difference between the largest and smallest flange gap you expect [S9]. Manufacturers state a minimum compression-deflection per grade as a material property, 2.5 to 8 percent by ASTM D575 on one sheet and 3.5 percent minimum (5.0 for nickel-graphite) on another, which is a material test, not a design window [S11][S14]. One consequence of filler loading is that the compound is mechanically structured, so a harder grade can deflect more than a softer one under the same load, and hardness alone is not a guide to closure force [S9].

Groove sizing for a solid O, November 2000 procedure

The handbook gives two levels of guidance and this guide keeps them apart. The quick rule: solid O-rings are normally limited to 25 percent deflection, so groove depth is 0.75 times the free diameter (0.75 to 0.90 on the figure), groove width is 1.1 times the free diameter, and the groove must leave enough void for a maximum fill of 95 percent, because the elastomer behaves as an incompressible fluid and tolerance stack-up must never over-fill it [S9]. The detailed procedure on the solid-O pages: minimum deflection 10 percent of diameter after all worst-case gasket and groove tolerances, cover bowing and lack of conformity; maximum 25 percent under the same worst case; a recommended limit of 100 percent groove fill at worst case (largest gasket section into smallest groove section); design around a nominal 18 percent deflection, so nominal groove depth is 0.82 times nominal gasket diameter; groove width comes from dividing the maximum gasket area by the minimum groove depth and adding the width tolerance [S9]. Laird's recommended groove tables put its solid O at roughly 1.07 to 1.13 times the diameter wide and 0.80 times deep, and D profiles about 1.2 times the base wide; 3G prints groove width and height beside each of its D and round profiles [S11][S14].

The two fill limits are disputed within the same edition: 95 percent in the flange-design section and 100 percent in the solid-O procedure and on the molded-ring page [S9]. The handbook gives both limits; which one governs is the designer's call on the drawing. The worst cases the procedure checks by name are a gasket too tall for the minimum groove depth (over-deflection, damage or fracture), a maximum gasket in a minimum groove (over-fill, damage), and a minimum gasket in a maximum groove with cover bowing and non-conformity (under-deflection and eventual loss of contact) [S9]. Cover bowing is modeled as a uniformly loaded beam with fixed ends from bolt spacing, flange width, cover thickness and Young's modulus (10 million psi for aluminum), using the gasket's deflection force at 10 percent as the load, and lack of conformity is taken as 0.001 in for machined faces of 32 to 64 microinch RMS [S9]. The plain-O-ring gland on the groove design guide squeezes harder than this; use the conductive figures where they differ, and the conductive O-ring page carries the sizes.

Closure pressure and stress relaxation

Most applications need no more than 100 psi on the gasket; waveguide flanges often apply ten times that; hollow strips need under 10 pounds per inch of length [S9]. Laird's flat-sheet data give 40 to 115 psi at 5 percent deflection and 280 to 345 psi at 20 percent for 0.045 to 0.125 in sheets [S11]. The handbook's O-strip curves reach about 30 pounds per inch at the maximum recommended deflection for 0.125 to 0.250 in diameters, and hollow profiles are limited at the point where the hollow closes [S9]. Form-in-place beads run 4 N/cm, or 2.6 N/cm with a wave-pattern bead, in the handbook's example and 0.26 to 6.38 N/cm across 10 to 50 percent compression in Nolato's table; foam gaskets take 2 to 3 psi at 50 percent [S9][S16][S17]. Shielding varies appreciably with pressure, and plane-wave attenuation is the most pressure-sensitive of the three field types [S9].

Cured silicone relaxes: the load settles to about 70 to 75 percent of its initial value within the hour as the polymer rearranges [S9]. The handbook's performance section therefore recommends loading to 125 percent of the design stress so it settles at 100 percent, while its fastener section says the final assembly torque should be 133 percent of design to overcome relaxation [S9]. The two factors are disputed within the edition and both are quoted here; either way the torque is re-checked in cold service [S9].

Fastener spacing against the flange gap

The rules of thumb: fasteners no more than 2.0 in apart on stiff flanges and 0.75 in apart on sheet metal where high shielding is needed; cabinet doors run to 3 in or to single latches and pay for it in stiffness, gasket size or performance [S9]. The basis is a beam on an elastic foundation: pressure variation between bolts stays within plus or minus 10 percent when the foundation constant times the spacing equals 2, with a seal foundation modulus of 10,000 to 15,000 psi and a flange modulus of 30 million psi for steel or 10 million for aluminum, and the mid-span deflection is then 20 percent below that under the bolts [S9]. The flange between two bolts is also a slot antenna loaded with a lossy dielectric: when the spacing is much less than half a wavelength the seam performs as if loaded at the mean pressure, but when it approaches half a wavelength at the highest frequency of interest the point of least pressure governs and pressure must rise 20 percent, or 64 percent at a wider spacing [S9]. At 10 GHz the free-space wavelength is 30 mm, so a half-wave stretch is 15 mm, our arithmetic from the speed of light; the testing guide explains the slot rules.

Torque figures in the handbook's table for mild steel bolts include 80 in-lb for a 1/4-20 and 250 in-lb for a 3/8-16; bolt tension is estimated as torque divided by 0.2 times the bolt diameter, and that friction rule fails with lubricants [S9]. Excessive preload can itself cause RF leakage, stainless fasteners lose torque under shock and vibration so locking inserts beat split washers, and working threads in soft metals need inserts [S9]. Bolt holes in sheet flanges sit at least 1.5 bolt diameters from the edge, drilled holes no closer to an edge than the gasket thickness, and the minimum distance from the groove edge to any terminal edge equals the groove width [S9].

Flatness, tolerance and finish

The handbook's machining figures: groove tolerance 0.002 in; drilled holes 0.005 in on location and punched holes 0.010 in; sheet-metal bends plus 0.030 minus 0.000 in [S9]. A groove bottom used for pressure sealing needs a 32 to 64 microinch RMS finish without circumferential scratches or chatter, and a surface that is too smooth makes the gasket roll or twist [S9]. For form-in-place dispensing the housing must repeat its non-parallelism, non-flatness and warp within 0.012 in part to part or be fixtured [S9]. For a gasket that is opened and reseated, the change in gasket thickness must not exceed twice the maximum mismatch between the mating surfaces [S9]. Flat, molded and extruded gaskets all show free-state variation, so the handbook recommends inspection fixtures rather than free-state measurement [S9].

Flange finish by environment class

The handbook's finish table lists the minimum finish that arrests corrosion and keeps a conductive face by environment class: A controlled, B uncontrolled, C marine [S9]. The chromate class it names is MIL-C-5541 Class 3, now MIL-DTL-5541F dated 11 July 2006, which the handbook summarizes as under 200 milliohms at 200 psi after 168 hours of 5 percent salt spray [S9][S41].

Three flange metals show how the guidance works. On carbon steel the handbook lists cadmium, zinc or tin at 0.0003 to 0.001 in, with 0.003 in of nickel added for marine service [S9]. On 2000 and 7000 series aluminum it lists chromate to MIL-C-5541 Class 3, with a conductive epoxy or urethane added in uncontrolled environments and 0.001 in of tin in marine service [S9]. Corrosion-resistant steel can stay bare, or take a thin nickel finish against tarnish [S9]. For any other flange metal, the handbook's own table and the finish specification on your drawing govern.

Nickel plating is generally recommended for aluminum, tin is widely accepted, and zinc is the finish for steel; most metal oxides are hard and insulating, silver's are soft and conductive, and gold does not oxidize [S9]. NASA compared Type II Class 3 chrome-free coatings against a Type I hexavalent control on 5052-H32 and 6061-T6 aluminum with a nickel-aluminum fluorosilicone gasket, measuring shielding from 50 MHz to 18 GHz before and after salt spray, 85 C at 85 percent humidity and twelve months of beachfront exposure [S18]. The galvanic compatibility guide covers which filler survives on which finish.

Dual sealing and adhesive

A single conductive elastomer in a groove seals pressure and fluid as well as EMI, which is the first option both references describe [S9][S11]. Co-extruded strips put a conductive elastomer in parallel with a non-conductive one, with the non-conductive half outboard for corrosion protection and adhesive; mesh combination strips and seal-to-seal bonded gaskets on both flanges for aircraft are the other patterns [S9]. Laird's outdoor radio study set groove fill above 90 percent and compression of 25 percent or more to survive water flushing [S11]. Non-conductive adhesive applied to the conductive part of a gasket seriously degrades shielding, so the drawing must restrict adhesive to the non-EMI portion; conductive adhesives at 0.05 ohm-cm (silver-plated glass) and 0.01 ohm-cm (silver-plated copper) bond gaskets and caulk seams [S9]. MIL-STD-889D clause 5.11(d) requires all faying edges sealed, and clause 5.11(b) requires a compatible conductive gasket or plating of the cathode where dissimilar faying surfaces serve as an electrical connection [S5].

Design checklist

The handbook's rules above, in the order the handbook takes them.

1. Environment class and flange metal first, then the finish from the environment-class rows on this page and the filler from the galvanic guide [S9][S5].
2. A groove over a flat flange; solid O at 0.82 D deep and about 1.1 D wide, 10 to 25 percent deflection, the handbook's 95 and 100 percent fill limits [S9].
3. Bolt spacing within 2.0 in on stiff flanges or 0.75 in on sheet metal, checked against half a wavelength at the top frequency [S9].
4. 32 to 64 microinch RMS on sealing groove bottoms and 0.002 in on the groove [S9].
5. Compression stops wherever a groove does not limit compression [S9].
6. Adhesive only on the non-conductive portion, masking before paint, faying edges sealed [S9][S5].
7. Torque to the relaxation factor, 125 or 133 percent of design as quoted above, and re-checked in cold service [S9].

The material behind the figures is on the conductive elastomers page, the type letters on the MIL-DTL-83528 guide, and flat gaskets cut from sheet on the conductive silicone sheet page. Send the drawing with the groove and the free cross-section through the request a quote form.

Design figures on this page are the manufacturers' published values, cited with their edition; where two sources disagree both are shown. The compound datasheet and your flange govern the final design.