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May 2009

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Subject:
From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Wed, 27 May 2009 07:15:14 -0500
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Hi Wayne - I recommend you determine just how much flux and/or flux 
residue remains under the component after processing. Your solvent 
cleaning process may not be removing all or enough flux residue to allow 
for the specific pwa circuit to function in a high humidity environment. 
Carefully, without using flux, remove several other capacitors in that 
region to see what is underneath. In some high humidity environments, 
conformal coating is necessary due to circuit spacing and functionality. 
Good Luck.

Dave Hillman
Rockwell Collins
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"Thayer, Wayne" <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
05/26/2009 09:01 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Thayer, Wayne" <[log in to unmask]>


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Subject
[TN] humidity test failure






Hi Technet Gurus!

I have attached a picture of a part responsible for excess current draw 
during an extended humidity/temperature cycling test. (I copied Steve so 
he can hopefully post!)

The board was carefully solvent cleaned, but no SIR test validating the 
process strips all ionic contamination.  The board is in an enclosure 
where plenty of ionic contaminants are available.

Humidity was supposed to not exceed 90% and was to be non-condensing 
during cycling.  There were a handful of components which displayed 
similar deposits on the same circuit board.  These were the only parts 
biased during the humidity test.  I believe the evidence suggests at least 
some condensation occurred.  To me the deposits look like salts.   But 
what do "the experts" think?

Is there any rule of thumb as to humidity levels where we are nuts to try 
to survive without conformal coating?  Part of the adjacent circuitry 
contains RF devices and those guys get panic-stricken when told we 
probably need to conformal coat!

Wayne Thayer

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