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Protecting and Packaging
Electronic Equipment
Part One of Two Parts
Fig. 1: Encapsulated
vacuum tube
Recent experiences of the military in hot, humid
climates has focussed attention on the need for protecting
electronic circuits against fungi and moisture, as well
as mechanical shock and stress. Protective measures
to accomplish this goal are described here.
By W. H. HANNAHS
THE traditional enemies of electronic
equipment are water,
moist air, and heat, in about this
order. Recently it has been necessary
to acknowledge also cold, bacterial
agents, and the absence of air.
Deterioration of performance by any
of these agents is through the deterioration
of insulation. While all
things electrical depend upon insulation,
electronic equipment is particularly
sensitive because of fundamental
dependence on circuits involving
high impedance, high resistance,
and high voltages. Some
circuits of course can tolerate much
lower insulation resistance than
others but, in general, lowering of
the insulation resistance to the
neighborhood of 1 megohm impairs
the performance of equipment. Sensitive
spots may require insulation
values of higher than 10 megohms.
A lesser causation also exists in the
failure of conductors by direct intervention
of corrosion.
The longevity of insulation is
primarily a function of its thermal
properties. To insulate our electronic
equipment we use very
largely organic polymers: unfilled
like polystyrene, polymers like
phenol formaldehyde containing fillers
of paper or cloth, flexible copolymers
like vinyl chloride- acetate,
highly compounded and filled elastomers
of the rubber family and innumerable
resinous coatings from
the types which gel or set by drying
and oxidizing. We depend also on
some inorganics, on pressurized
nitrogen, on vacuum, and on hydro-.
carbons such as waxes or liquid
oils. All of these insulations respond
somewhat differently to the various
environments. Furthermore, many
equipments, as for example, radar,
may contain examples of all types.
Background
Only since the early 40's, upon
the occasion of the precipitate buildup
of military strength in the tropical
South Pacific, has the true import
and prime necessity of protecting
equipment been brought into
full play. Just prior to the 40's, of
course, there had been a tremendous
increase in the complexity of strategic
and tactical military electronic
equipment. When this equipment
began to reach Guadalcanal and
Guam, nature's rebuttal to our
ignorance of tropical conditions was
an additional catastrophe to a long
list. Communications sets and
walkie- talkie sets arrived inoperative
or functioned sporadically for a
Fig. 2: Equipment
mounts should provide
convenience in
operating
week, only to expire. Only heroic
maintenance effort kept communication
links open.
Then came the awakening. Examination
showed cables, baked by
unremitting heat, age- hardened and
fissured. Vent holes in cases admitted
moisture that accumulated to
the point of dripping. Plasticizers
had migrated from wire coverings,
moisture combining with impurities
in insulation had become electrolytes
between conductors, fine wires
had been eaten away, heavy conductors
had shed hydroscopic
powders whose conductivity bridged
terminals. Supposedly undigestable
phenolics were covered with fingers
of aspergillus, the organics in wiring
harness had provided a banquet
for the penicillin. Even ceramics
when untreated were found to contain
their growth of fungi. In equipments
failing from obscure causes
only after months, plastic laminates
of the standard varieties were found
to have delaminated and blistered
under the combined impact of
fungus and moisture.
W. H. HANNAS, Automatic Production Research,
195 S. Columbus Ave., Mt. Verson.
(This article was prepared while the
author was employed at Sylvania Electric
Products Inc., Physics Labs., Bayside,
N. Y.)
84
Tele -Tech & ELECTRONIC INDUSTRIES June 1956