It also depends on the environment where the box will live. If it is
near the sea, there is a much higher probability of failure if any
moisture does enter from outside.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls
Sent: Tuesday, November 29, 2005 11:01 AM
To: [log in to unmask]
Subject: Re: [TN] Humidity and enclosed RF board
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
TechNet E-Mail
Forum
<[log in to unmask]>
; 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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