App. Note 28 - RF Case Seals
What types of RF seals are in general use?
The most common seals encountered on RF shielded enclosures are:
- The finger stock seal: --This type of seals employs a series of
thin spring fingers which are generally manufactured as a strip of material.
The finger stock is attached to one component of the closure, and makes
electrical contact with the other component. There should be sufficient
force to ensure that a positive contact is established and maintained,
ideally enough pressure so as to exclude oxygen from the joint.
- The knit or wire braid seal:--Utilizing a compressible element made
from either a wire knit or wire braid component resting in or clamped in
a groove, this type of RF seal is usually arranged so that the braid is
part of the cover and seals against the edge of the sheet metal forming
- The conductive elastomer seal:-- Here a molded or extruded strip
of conductive elastomer is used to establish contact between the two parts
of the closure.
What are some of the problems associated with these RF seals?
As is the case with most connectors, the two major problems are contamination
and corrosion, and the effect they have on RF leakage through the seal.
In fact some corrosion products exhibit semi-conductive action and the
rectification that take place in these areas may result in a significant
and unacceptable amount of the local RF field being demodulated and re-radiated
as either an audio frequency signal, a R signal or signals at a different
frequency or a combination of both.
The problem can be solved by maintaining sufficiently high contact pressure
so as t exclude oxygen etc. from the contacting areas, but the pressure
required (for ex ample, in an IC socket, approx. 3 oz. pressure per pin
is applied laterally) is so high as to make the total closing force unacceptable
for all but the smallest of RF enclosures.
The next step is one of securing a seal with acceptable pressures. At
times it ma seem that there is no hope of maintaining a reasonable continuous
seal. In fact in the design stage it may be better to accept the fact that
it is not possible to maintain a continuous seal and concentrate the design
effort on determining what spacing of excellent contact points would be
acceptable, and then designing the seal so that the spring force is concentrated
at these points.
We have not overlooked the fact that for many applications involving
a low power RF field of very uniform gradient, a capacitative seal may
be sufficient. Generally this is the exceptional case. In any event an
actual erratic contact here can destroy the value of the seal.
Where environmental conditions are less than ideal, such as the presence
of salt, moisture, pollution products, and even cigarette smoke, the problem
can range from thin films of material preventing perfect contact, to corrosion-product
encrustation of the contacts. Many persons do not realize that (in the
absence of ablative effects) the corrosion products may occupy much more
space than the metal parts from which they were formed; leading to physical
changes causing problems ranging from jamming of the cover to deformation
of the seals. Sometimes a very small amount of contamination can result
in a major problem; most designers of buildings set up to provide a "reasonable"
environment for electronic equipment forget that if you provide openings
for people to enter and exit, ventilation systems using outside air sources,
etc. there is going to be some entry of outside contamination even if it
is only the typical salt-air fog encountered in coastal areas.
Another problem is galvanic corrosion. This is caused by the use of
dissimilar metals and or dissimilar metal-component finishes on the two
surfaces forming the RF seal. Given the presence of any moisture, there
will be an electrical potential set up between the two surfaces which can
eat away one of the surfaces.
It must also be remembered that given an RF field of sufficient gradient,
there nay well be a potential difference (albeit it AC) between the various
components of the shield system, which will in increase as the RF seal
develops problems further aggravating that same problem.
Aluminum cabinets with anodized surfaces can be a major problem because
of the fact that the anodizing produces a thin coating of non-conductive
Silver plating is more desirable, silver oxide is conductive but some
of the other tarnish products are not so conductive and rectification can
Often Stabilant be used to solve these problems?
While not developed for this situation, the Stabilants can be
used to good advantage. Because of factors involving formation of corrosion
products within contacting areas of low pressure the Stabilants may not
stop the problem in the way they do with discreet connectors. At the worst
they will reduce the problem to the point where the interval between failures
is both extended and more highly predictable. This will allow for a maintenance
schedule to be set up for the treating of the RF seal areas.
Consideration must be given to possible galvanic incompatibilities caused
by the plating on the woven wire or braided wire if that is the type of
seal being used. Solder alloy plated or even tin plated copper or a mix
of solder or tin plated copper with solder or tin plated steel wore can
be especially troublesome when combined with an aluminum case.
Here there is almost always some hydrolysis and/or etching of the aluminum
with the formation of a non-conductive oxide. The Stabilants have
a strong surfactant action and part of their mode of protection lies in
their ability to lift much of the oxide and hold it in suspension. Even
a thin film of the concentrate will help in this situation But the material
will have to be periodically cleaned off with a solvent such a isopropanol
and the concentrate re-applied. The general limits of protection are about
six to nine months in a coastal area with thin films, and from nine month
to year or more with thicker films of the concentrate.
Elastomeric seals impose special problems.
Elastomeric seals often consist of a good compression-set-resistant
elastomer compounded with SAF carbon-black, a very finely divided
carbon-black used almost exclusively for this purpose. We would refer you
to our Technical Note # 21 "Compatibility of Stabilant 22 with Elastomers".
As the dilute form (Stabilant 22a) does contain isopropanol which
may increase the swell and degrade the compression-se properties of some
elastomers such as Buna S, we would suggest that where there is any doubt
as to the compound, the concentrate (Stabilant 22) be employed.
A possible problem can occur here which might reduce the initial application
of the Stabilants. If there is any unsaturated oils present on the
metal that is in contact with the elastomer, the curing agent and accelerator
used in the elastomer ma cause these oils to cross-link (or cure) either
making the elastomer stick to the metal or causing the oil to form a poor-varnish-like
coating on the metal's surface. This ca even happen with skin oils. The
situation can be aggravated where the elastomer seal has been itself cemented
to one component of the closure, as the cement and/or primer for the cement
may contain an isocyanate. In the confines of the seal this isocyanate
can promote cross-linking in any unsaturated material present. This is
not a problem with the Stabilants themselves, but be sure that any
residual patina is removed from the metal surface before they are used.