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May 2009

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From:
"Thayer, Wayne" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Thayer, Wayne
Date:
Wed, 27 May 2009 16:10:38 -0400
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WOW!  A lot of good advice in all responses to this query, and some excellent suggestions for further test.  I wanted to wait until the main barrages stopped before replying:

Inge-Thanks much for all of the experiential data and pictures.  I like your rhetorical question at the end, which interestingly loses some of the rhetorical value because there are two likely answers which make very different significant points: 
1. Failures get repeated because we foolishly ignore past experience.
2. The problem is very hard and the markets have found no cure-all or universally applicable solution.
I like this because having more than one answer turns it into something I need to think about.
From your previous responses, you seem to be saying that Parylene C is likely to completely prevent any corrosion problem--but aren't ALL polymers able to pass water vapor to some degree?  In the past I have had very good performance with the DowCorning 1-2577, which is high in VOC, but can easily be put on thick and is nicely reworkable (but RF guys would kill me!)

Dave H--Thanks for the advice on removing adjacent components which weren't biased to look for residues.  I'll do that.

Kevin G--Thanks for pointing out that I will need to do board contaminant analysis anyway if I am going to CC this area.

Vladimir--More good recommendations on favorite analysis for surface contaminants.

Denny F.--Thanks for a great tip for analytic services!

Mike Fenner--I really like the idea of testing for contaminants using a DI water based test kit.  Thanks for the pointer.

George W.--Thanks for the tips on Telcordia compliance and trying to clean the residue.  Worth investigating.

Chris Mahanna--Your comments about growth from non-wetted surfaces is interesting.  The board finish ImAg, which it seems the consensus is that it won't have this kind of problem unless a lot of sulfur is present, and then it grows everywhere without respect to voltage bias.

ALL--The victim is very clean except for the affected voltage-biased parts.  We are quite confident that the growth is the result of moisture and voltage bias, although we will investigate the possibility of a rosin-contamination "seed" for the process.

Thanks to everyone!

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Wednesday, May 27, 2009 1:02 PM
To: [log in to unmask]
Subject: Re: [TN] humidity test failure

We had an awful lot of environmental test chambers, and we followed MIL-STD like a blind klinging to his stick. With time, we have realized the questionable relationship between those tests and the unpredictable mother Nature. The exact repetition of temperature up, temp down, dwell, temp down etc does not always correspond to the erratic reality with endless variations and combinations of condensation and vibration, heat and salt, power switching and deep freeze, sun irradiation and sand blast , very short exposure followed by extremly long one etc etc. I listened to a guy who was a successful design manager, whose motto was 'skip all testing, do the right design or process from the beginning' . Easy said, but still interesting topic. Am brought up with MIL-STD, so I'm little ambivalent, but I try to think like him. And, in fact, with all billion hours of testing everything on this planet, should we not have background enough for avoiding all disasters..like corroded ceramic caps, for instance. Why do we repeat all failures that have been known for decades?

Inge


----- Original Message -----
From: "Chris Mahanna" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, May 27, 2009 6:34 PM
Subject: Re: [TN] humidity test failure


Hi Wayne,

Agreed.  The DUT probably slipped close to, or below dew.  Close can be just 
as bad.  And, surely there is trapped ionics.  But, I suspect a correlation 
ranking might show those non-wetted solderable surfaces as your real 
culprit.

Ability to control the chamber and DUT is highly dependent on absolute 
humidity.  85/85 is very tough. Anything higher is crazy.  The common 
25-65C, 90% is quite difficult without good multi loop control.  If you're 
cycling a heavy assembly you will probably want to use the surface temp as 
an input to your controller.
IPC-9201A is a good source of info.  NPL has several papers.

Fully processed comb patterns can 'survive' 85/85 or 25-65/90 SIR/ECM tests 
for 1000 hours without cc, but it's not easy.

Chris





-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, May 26, 2009 10:01 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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