Conductive Gasket
How Gaskets Restore Electrical Continuity Across a Seam
A shielded enclosure works only as well as its worst seam. The metal walls themselves attenuate fields by tens to hundreds of dB, but every mating joint introduces a gap whose impedance is set by surface roughness, paint, oxide layers, and the inevitable warping of a bolted flange between fasteners. Surface currents driven by an internal source must cross that gap; where the contact impedance is high, voltage develops across the slot and the seam radiates like a thin aperture. A conductive gasket fills the gap with a deformable conductor that conforms to flange irregularities and maintains many parallel metal-to-metal contact points per centimeter, dropping the seam transfer impedance from ohms down into the milliohm range.
The mechanism that matters is contact, not bulk conductivity. The gasket must apply enough force to fracture surface oxides and establish asperity contacts, yet recover its height when the joint is opened and reclosed for service. This is why compression set, the fraction of deflection a gasket fails to recover, is as important as initial conductivity. A gasket with 5 percent set after thermal cycling still seals; one that has yielded to 25 percent set leaves micro-gaps that leak preferentially at the highest frequencies, where even a 1 mm discontinuity is an appreciable fraction of a wavelength. Fastener spacing follows the same wavelength logic: bolt centers are kept below roughly one-twentieth of a wavelength at the highest frequency of concern so the flange cannot bow into a resonant slot between screws.
Because the joint is two dissimilar metals bridged by a conductor in the presence of humidity, it is also a galvanic cell. Over months in a humid or salt-laden environment, an incompatible pairing corrodes, the corrosion products are insulating, and contact impedance climbs until shielding quietly degrades. Material selection therefore balances three competing demands at once: low contact resistance, mechanical resilience over the service life, and galvanic compatibility with the flange alloy.
Shielding, Transfer Impedance, and Contact Force
Two complementary metrics describe gasket performance. Shielding effectiveness rises with frequency for a good gasket but eventually rolls off as seam discontinuities approach a wavelength. Transfer impedance ZT is the gasket-level figure of merit: the voltage developed across the seam per unit of surface current flowing past it. Lower ZT means less coupled voltage and therefore higher SE, and it can be qualified per IEEE 1302 independent of the surrounding hardware.
SE ≈ 20 log10(1 + Z0 / (2 × Zgasket)) dB
Seam transfer impedance:
ZT = Vleak / (Isurface × L) Ω/m, Zgasket ≈ Rcontact + jωLgap
Maximum slot length before leakage (rule of thumb):
Lslot < λ / 20 → fastener spacing < c / (20 × fmax)
Where Z0 ≈ 377 Ω (free-space wave impedance), Rcontact = gasket contact resistance, Lgap = residual gap inductance, ω = 2πf, L = seam length, c ≈ 3 × 108 m/s. Example: at 10 GHz, λ = 30 mm, so the λ/20 limit is 1.5 mm. Bolting cannot space fasteners that tightly, which is precisely why a continuous gasket is needed: it shrinks the effective seam discontinuity to well below 1.5 mm so no resonant slot can form between screws.
Gasket Material Comparison
| Gasket type | SE (typical) | Frequency range | Working compression | Closure force | Best application |
|---|---|---|---|---|---|
| BeCu finger stock | 60 to 100 dB | DC to 40 GHz | 15 to 25% | Low (0.3 to 2 N/cm) | Frequently opened doors, RF chambers |
| Conductive elastomer | 40 to 90 dB | DC to 18 GHz | 10 to 20% | High (5 to 35 N/cm) | Sealed, environmental + EMI joints |
| Knitted wire mesh | 50 to 90 dB | DC to 18 GHz | 20 to 30% | Medium | General enclosure seams |
| Fabric-over-foam | 40 to 70 dB | DC to 10 GHz | 15 to 25% | Very low (0.1 to 0.5 N/cm) | Board-level shields, light closures |
| Oriented wires in silicone | 70 to 110 dB | DC to 40 GHz | 10 to 15% | Medium | High-performance flanges |
Frequently Asked Questions
How much compression does a conductive gasket need to seal properly?
Most gaskets are rated for 10 to 30% deflection of free height, enough to break surface oxides and establish low contact resistance without taking a permanent set. Conductive elastomers target 10 to 20% at roughly 5 to 35 N/cm closure force; BeCu finger stock works at 15 to 25% with far lower force. Under-compression leaves leaky high-impedance gaps; over-compression yields the binder and drives compression set past 20%, so hardware needs a compression stop or controlled gland depth.
Why do conductive gaskets fail from galvanic corrosion and how is it prevented?
The joint is an electrochemical cell: dissimilar metals in contact with condensed humidity or salt fog. When the galvanic potential gap exceeds about 0.25 V, the less noble metal corrodes and its insulating oxides raise contact resistance until shielding collapses. Prevention is material matching, for example Monel or silver-plated-aluminum filler with aluminum flanges, tin or nickel platings to narrow the potential gap, and adherence to MIL-STD-889 tables, with a fluorosilicone environmental seal for marine service.
What is the difference between transfer impedance and shielding effectiveness for a gasket?
Shielding effectiveness is a frequency-dependent dB ratio of fields with and without the shield, and it characterizes the whole enclosure. Transfer impedance ZT, in mΩ/cm, is the gasket-level metric relating seam voltage to surface current; lower ZT yields higher SE. ZT is preferred for qualification because IEEE 1302 measures it independent of enclosure geometry, whereas SE per MIL-DTL-83528 or IEEE 299 blends gasket and hardware behavior.