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 seam plates. Additional materials include conductive cements and caulking
materials, usually silver filled. Some cheaper varieties use tiny glass beads
coated with silver, or copper flakes coated with silver. These conductive
materials are usually utilized in penetrations, or as a retrofit fix at bolted
seam leaks or bolted air vent leaks. A basic limitation to the gasket and
tape materials is that metal to metal contact is made with the mating surfaces
at high points without much surface pressure available (less than that under
bolted seam plates to shielding). When surfaces in contact are new and
bright, the contact resistance is low and shielding effectiveness measured is
high. Oxidation and dissimilar metal to metal corrosion rapidly deteriorate
the bonds and degrade shielding performance. The best long term shielding
design uses no gasket, tape, or conductive cement materials, and has all
penetrations continuously welded or brazed. The conductive cements and
caulking materials are available in hard and brittle form when set-up, (this
type provides the highest conductivity) or can be obtained in a pliable mix
when set up. The brittle material cracks easily with motion of the mated
parts where it is applied, to result in leakage. The pliable material allows
for flexibility but provides minimal shielding. Another disadvantage to
conductive cements is that they are expensive. Some tape materials are
available with an advertised conductive adhesive backing. Some have small
metal particles embedded in the adhesive that make contact between the metal
tape and surfaces where applied. Others are embossed so that the metal tape
makes contact through a raised ridge to the surfaces where applied. The use
of metal tapes with conductive adhesives is a Band-Aid or temporary measure at
best. There is insufficient metal to metal contact pressure available to
inhibit moisture or retard oxidation of the metal to metal contact points for
long periods of time, and their use should be limited to laboratory
measurements or temporary applications.
2.12.1
Mesh Gaskets. The mesh gasket usually found under the clean
compartment lid of power filters does not degrade as fast as other gasket
applications. The hot filter case, compared to the surrounding air, tends to
inhibit moisture penetration into the gasket. When gaskets are required, they
should be chosen first to provide the same coating material as the parent
shielding where they are installed. Of the coatings that are available, tin
plate seems to provide the best long term conductivity against typical
shielding material finishes. The mesh gasket materials are not typically
resilient like beryllium-copper fingerstock used in door closures, and once
compressed in a seam or penetration closure they can take a set. Subsequent
cycles of opening and closing will then require greater pressure to break down
surface oxidation. Brightening of contact surfaces and frequent replacement
of gaskets are recommended in order to retain necessary shielding
effectiveness.
2.13
Fingerstrips. Door closures with the exception of the pneumatic-
expanding, sliding door, utilize combinations of rows of fingerstrips for
achieving an electromagnetic seal between door leaf and frame surfaces.
Fingerstrips are typically made from microprocessed beryllium copper, about 4
mil thickness, and with available plated finishes of such metals as gold,
silver, rhodium, nickel, tin, zinc and chromium. The material provided in
most shielding manufacturer's doors is uncoated beryllium copper, unless
otherwise specified. If the door closure mating surfaces are brass or bronze,
the beryllium copper finish is a good choice for galvanic compatibility.
Against aluminum or zinc, tin plating would be preferred. The fingerstrips
are available with an adhesive backing strip to allow for installation without
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