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 MIL-HDBK-419A
3.4.3 Equipment Enclosures. The shielding effectiveness of the enclosure depends on the structural material
and on the mechanical design, construction, and installation of the equipment. The choice of material depends
primarily on the ambient field and on the degree of shielding required (see Volume I, Section 8.3). The main
problems with shielding, however, are usually not with the material but with the control of leakage through
openings at seams, apertures, and wire penetrations. The following guidelines are suggested as aids toward
controlling this high leakage.
3.4.3.1 Seams.
Hold mechanical discontinuities to a minimum.
a.
Bond equipment enclosures at every seam and discontinuity.
b.
Recognize that the poorest electrical joint will determine the shielding effectiveness of the
c.
enclosure.
Obtain clean metal-to-metal contact at seams to prevent leakage and radiation of energy.
d.
Where possible, seams should be welded, brazed, or soldered such that the joint is continuous;
e.
however, satisfactory results for some applications can be obtained with closely spaced rivets, spot welding, or
nuts and bolts.
Provide as much overlap as possible and closely space fasteners to minimize the tendency of the
f.
joint to buckle.
Ensure that the fastening method exerts sufficient pressure to hold the surfaces in contact in the
g.
presence of deforming stresses, shock, and vibrations associated with the normal operation of the equipment in
its expected environment.
h.
Use gasket or finger stock material where seam unevenness is encountered or where removable
panels, drawers, etc., are used. The gasketing material should fill gaps and uneven places to provide continuous
electrical contact between the mating surfaces.
i.
Attach removable covers and panels with closely spaced screws and apply conductive gasketing
around the periphery.
Choose gaskets with properties of high resilience and high conductivity.
j.
Provide the minimum gasket thickness and width which will allow for the expected surface
k.
discontinuities of the joint.
l.
Provide the pressure necessary to achieve an rf-tight seam.
3-27
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