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May 2009

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From:
Werner Engelmaier /* <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 26 May 2009 22:40:43 -0400
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 Hi Wayne,
Have not seen the pictures yet, but from your description you got condensation.
Humidity/temperature cycling is a tricky proposition, On heating, you need to heat first before introducing humidity, but even more importantly on cooling you need to first dry the chamber volume before dropping the temperature. Many of the parts within the chamber will lag seriously behind in the temperature changes and you et condensation.
In some past life I did some CAF testing, and setting up and running the humidity/temperature cycling test was a huge challenge needing constant attention. Just making sure that the wet bulb was wet was not simple—you need to gravity feed the water because the pressure in the chamber is elevated.
Werner


 


 

-----Original Message-----
From: Thayer, Wayne <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, 26 May 2009 10:01 pm
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=2
0parts 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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