Well, it depends.
You say that the casing seals the electronics from the external atmosphere.
How well? Is it a hermetic seal? Are you using an elastomeric gasket? Do
you package with an inert cover gas? What is the design life of the
product and what are the consequences of failure?
In my experience, even those enclosures assumed to be "hermetic" are not,
and that usually means moisture will eventually get in and will have a hard
time getting back out. So it all depends on how well insulated your case
keeps the electronics throughout the design life. Elastomeric gaskets
willl wear out. Field repair is often less than optimal.
If moisture gets inside the case, it will condense on your boards when you
go through dew point. Can it grow dendrites? Yes. What is the risk? It
Depends. Dendrites have been observed to grow in as little as 3.5 volts
DC. There is no "safe" level of humidity because the degree of risk is
dependent on both humidity and the amount of ionic soil. The dirtier it
is, the less humidity it takes. Some papers have suggested 70% RH as a
threshold, but this is usually SIR type of testing, not field trials. In
addition, if your product is functioning in some of the more polluted areas
of the world, and the enclosure is not hermetic, then the pollutant gasses
also get into the enclosure and do nasty things.
Doug Pauls
"Tempea, Ioan"
<itempea@POSITRON
.QC.CA> To
Sent by: TechNet [log in to unmask]
<[log in to unmask]> cc
Subject
11/29/2005 10:43 [TN] Humidity and enclosed RF board
AM
Please respond to
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; Please respond
to
"Tempea, Ioan"
<itempea@POSITRON
.QC.CA>
Hi Technos,
application: one RF board and one antenna, no conformal coating, both
sealed inside of a casing; outdoor unit, will see temp and humidity
variations.
question: assuming that the boards are well insulated from the exterior,
should I worry about long term rel of the boards? Otherwise said, due to
temp variations, can the residual humidity inside the casing condensate on
the boards and cause damage? Or can it make grow dendrites?
And a general question: For dendrites we need a potential difference,
humidity and dirt; is there any threshold for the humidity level, below
which dendrites will not grow?
Thank you,
Ioan
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