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 MIL-HDBK-419A
atmosphere, recognizing that a final material heat treatment will be necessary. Temporary seams are usually
screwed or bolted together. Figures 8-24 and 8-25 indicate the change in shielding effectiveness of an
AMPB-65 seam at various frequencies as a function of screw spacing and lap joint width, respectively.
8.5.2 Seams With Gaskets.
Considerable shielding improvement over direct metal-to-metal mating of shields used as temporary bonds can
be obtained using flexible, resilient metallic gaskets placed between shielding surfaces to be joined. Clean
metal-to-metal mating surfaces and a good pressure contact are necessary.
The major material requirements for rf gaskets include compatibility with the mating surfaces, corrosion
resistance, appropriate electrical properties, resilience (particularly when repeated compression and
decompression of the gasket is expected), mechanical wear, and ability to form into the desired shape. On this
basis, monel and silver-plated brass are generally the preferred materials, with aluminum used only for
gasketing between two aluminum surfaces. Beryllium-copper contact fingers are also employed, with a variety
of platings available, if desired. Mumetal and Permalloy have been used when magnetic shielding effectiveness
is of concern.
Gaskets are manufactured with rubber or neoprene to provide both fluid and conductive seals, or to sustain a
pressure differential, as well as provide an rf barrier. They are also made using sponge silicon for high
temperature applications and are made with both nonconductive or conductive pressure sensitive adhesives. A
few of the gasket design approaches that have been employed are summarized in Table 8-15. Typical gasket
mounting techniques are given in Figure 8-6. The most frequently used gasket configuration is the knitted wire
mesh; the structure of this mesh is shown in Figure 8-27.
The necessary gasket thickness is dependent on the unevenness of the joint to be sealed, the compressibility of
the gasket, and the force available. The shape required depends on the particular application involved, as well
as the space available, the manner in which the gasket is held in place, and the same parameters that influence
gasket thickness. Gaskets may be held in place by sidewall friction, by soldering, by adhesives, or by positioning
in a slot or on a shoulder. Soldering must be controlled carefully to prevent its soaking into the gasket and
destroying gasket resiliency. Adhesives (particularly nonconductive adhesives) should not be applied to gasket
surfaces that mate for rf shielding purposes; auxiliary tabs should be used. A recommended pressure is about 20
psi.
8.5.3 Penetration Holes. One effective method of neutralizing the shielding discontinuities created by
planned holes (e.g., for air ventilation and circuit adjustment) in a shield is to use cylindrical and rectangular
waveguide-below-cutoff slots or tubes.
8-45
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