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Fig. 3: Hot components are mounted on underside of chassis
Fig. 4:
Humidity and temperature problems arise with printed ckts
Loss of insulation through loss of
air became important with the introduction
of pressurized airborne
radar, and elevated temperature became
significant when equipment
was compressed into micro- miniature
dimensions.
What engineers have done since
1942 toward packaging of equipment
is indeed a commendation of single -
minded effort. Indication of confusion
in an earlier day is the fact that
there was active disputation as to
whether fungi actually ingested
ceramics on which they were found.
The list of materials found to be
most degraded by fungial action,
however, witness the fact that the
elemental need of the fungi is carbon,
particularly in the form of cellulose
with perhaps a little nitro -
geneous matter for variety. We may
conclude, then, that all military
equipment should have protection
for textiles, paper, wood, casein, felt,
etc., whenever these cannot be
omitted.
Not the least enemy of electronic
equipment is microscopic debris,
U
eo
15
I
GLASS WOOL
Is u. cu. FT.,
both organic and inorganic. Besides
providing insulation where it is not
wanted by slipping in between
metallic contacts, it also furnishes
a media on inorganic surfaces in
which bacteria and fungi may live
and attract moisture. The combined
coincidence of moisture, warmth,
cellulose, and lots of debris leads to
generally riotous living among the
yeasts and fungi. We might mention
the aiding circumstance of darkness,
too, but given only the favoritism of
a tropical environment and the
limited food supply of an old -
fashioned paper label adhered with
animal glue, nature can contrive an
awful mess in an under chassis.
Environmental testing of materials
for mildew resistance with prescribed
organisms is rather difficult,
but a reasonably good estimate can
be made by burying samples 14 days
in a box of rich garden soil maintained
at 98 °F. and 95% R.H.1
If all components and wiring in an
equipment are separately enclosed
so as to be "dust tight" the outer enclosure
may be limited to the cate-
THERMAL CONDUCTIVITY OF MATERIALS
SOFT GLASSES
100
,II
:o'...4:.11 issiXl
,ELLOS
LIGFT FILLS.
50
`
WASS
LAMINATES
~
STTWEFE
PLASTIC `
CC.GLESEFS
\°:.f F[4
, F-CG.SIGS--I
SILVER
LL y EEI6L.SS 1
'
o
THERMAL
u.
s
CONDUCTIVITY WATTS /IN2 /C /IN
Fig. S: Thermal effects with various commonly used materials
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gory of "dust proof" which means
that dust may enter provided that
it does not prevent normal operation.
Air Force tests- define dust as
the mixed oxides of silica, iron,
aluminum, titanium, and magnesium
present in 5 micron to 325 mesh size,
and equipment is generally required
to operate in hot dry and humid
conditions after 6 hrs. exposure to
heavy turbulent concentrations.
Generally considered, protection
can be supplied not only by boxes
and cans but by plastic embedding,
wax potting, dip coating, over -varnishing
normal insulations, immersion
in oil, pressurized cases or high
vacuum.
TV and Radios
The protection applied to domestic
radio receivers is probably representative
of the minimum order.
Aside from the decorative function,
the cabinet supplies only protection
from meddling and shock. In inexpensive
sets even the protection
from dust is almost incidental to
having the electrically hot components
on the underside of the
chassis (see Fig. 3). At this cost -
conscious level, the major consideration
is the bedrock requirements
of the National Board of Fire
Underwriters. Whether or not the
underwriters acknowledge such
policy, it is rather generally recognized
that their labs largely approve
specific complete appliances and
prefer to do so because specification
of individual parts and materials
are constantly subject to
modification in the face of technical
progress. Therefore, in applying the
basic standard for Power Operated
Radio Receiving Applicances supplied
by Underwriters$ and approved
by the ASA, these individual
(Continued on page 362)
Tele -Tech & ELECTRONIC INDUSTRIES June 1956 85