Hi Wayne!
I understand the RF guys. If I as a RF guy (why does 'RF woman' sound odd?)have to make a choice between concoat and unprotected board, I would take the later. Now, this was a generalisation, because there are many things to consider, like frequency, voltage levels, leakage current tolerance etc. I can mention a typical RF board that we use without coating. It's a Teflon type board for the X-band, components are tantalytes, ceramic chip resistors, switch diodes, ceramic hight power transistors and some inductors designed directly on the copper. The boards are in aluminum enclosures standing in vertical position. The enclosures are dust protected, but open for the surrounding air to circulate through. When the system goes through varying temperatures, most of the condensation is taken care of by sort of air condition. But there will still be some condensation on inside of the RF enclosure(s). Any water droplets that develops on the board and its components will move slowly along the board and finally escape through a little hole at the bottom of the enclosure. When the boards are powered, the little water that condensed will fade away because of the dissipated heat. The critical moment is when humidity develops and the power is not on. In such case, we have a system that starts a temporary board heating by means of a couple of power resistors. All this may seem messy, but is in fact very simple...and works good. We have thousands of those units in use around the world.
I've seen various kind of salts too. They use to originate from the enclosure or its surrounding, and be introduced to the board following wire harnesses or other mechanical stuff, accumulated there and by time growing to visible salts.
I've been in the game for three decades, and never seen any thumb rules. I'm not surprised. RF is magics, and the bureaucrats and standardisation peoples seem to loose the enthusiasm when confronting with RF.
I hope this does not help
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: onsdag 27 maj 2009 04:01
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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