On Thu, 24 Jan 2013 09:38:34 -0500, Tom Gardner Mars@Tacks wrote:

On 1/24/2013 1:12 AM, RogerN wrote:
"Tom Gardner" wrote in message
...

I got me Pro-Face PLC back from the manufacturer today. I had posted about
the failure a few weeks ago. It turns out that there was some sort of
corrosion on the main board and the video board. The PLC is in a gasketed
enclosure with three Greenfield cables coming in. The machine or the
control box never got wet and there is nothing in the air. The heat goes
from 65 day to 55 night with overhead gas furnaces.
Could just the Greenfield be letting moisture in? How do I prevent it from
happening again? A light bulb running all the time? A vent fan? Maybe the
thing shouldn't be that well sealed?

Here are some pix of the PLC, and enclosure.

http://i954.photobucket.com/albums/a...pse545a784.jpg

http://i954.photobucket.com/albums/a...psb1238b22.jpg

http://i954.photobucket.com/albums/a...pscbe73616.jpg

Where I work, we had a problem with corrosion on the Siemens Microbox PLC's.
Siemens sent one of their engineers in from Germany, he went around doing
air samples trying to figure out what was going on. Air samples showed no
problem. We also had the same problem with hard drives, not sure why the
problem was with MicroBox PLC's and Hard drives, other electronics didn't
seem to have the problem.

What we ended up doing is spraying the circuit boards with a coating we got
from the local electronics supplier, IIRC they called it conformal coating.
I don't know if it was a certain flux or solder, but some boards had this
problem, others didn't. McMaster Carr used to sell an insulating varnish
for electric motor coils, I would think it would work good too. Coating the
boards changed the life of the boards from months to years.

Good Luck

RogerN




Thanks Roger, I'll take your advise. The flux thing is the only thing
that makes sense.
Many years ago I worked on flight hardware for NASA. I recommend using
190 proof Everclear to remove any visible dried flux on circuit boards
before applying conformal coating to a board. Don't conformal coat any
contacts or socket interiors.