Looks like whatever it is it may have started from residues leaching from
UNDER the component.  The voltage bias and environment would have only
accelerated this.   The solder joints exposed to the cleaning process look
fine.  You should review your cleaning procedures and perform testing
(IC/SIR) to determine if you are adequately cleaning, rinsing, and drying to
remove contaminants from under low-standoff components.  My suspicion is
that if that is in fact the case, and you decided to try to conformal coat,
the problem would have only manifested itself sooner with conformal coat
adhesion problems.  


-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]] 
Sent: Wednesday, May 27, 2009 8:00 AM
To: [log in to unmask]
Subject: Re: [TN] humidity test failure

Good Morning!

Half the week down already. That's another nice thing about 3-day weekends;
you also get a 4-day work week either prior to, or after the weekend.

I have Wayne's picture posted now. It's at:
 
http://stevezeva.homestead.com/files/C23_Humidity_failure.JPG

Definitely some kinda' junk there!

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, May 26, 2009 9:01 PM
To: [log in to unmask]
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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