So, if I quit, you become richer, can we share your extras?
Inge
----- Original Message -----
From: [log in to unmask]
To: [log in to unmask] ; [log in to unmask]
Sent: Wednesday, May 27, 2009 7:08 PM
Subject: Re: [TN] humidity test failure
Hi Inge,
"Why do we repeat all failures that have been known for decades?"
Easy, most companies get rid of their experienced people. And the new hires simply do not have the time to learn about anything that happened previously.
——sure keeps consultants busy, though.
Werner
-----Original Message-----
From: Inge <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, 27 May 2009 1:02 pm
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 beginnin g' . 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 n eed to conformal coat!
Wayne Thayer
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