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.

Ive had pretty good luck with Krylon transparent spray for
distributors or similar. Its a very thin spray "paint"

Some guys swear by this. Ive not used it..but have had pretty good
reports on it.

http://www.corrosionx.com/corrosionx.html

it has the benifit of killing existing corrosion on circuit boards

"conformal" coatings are good..but make future service problematic.
They basically varnish the boards and glue everything together.

If there is something ON the boards that is creating a problem..you
have just sealed it under the coating..and it will spread under it.

Gunner

Gunner

The methodology of the left has always been:

1. Lie
2. Repeat the lie as many times as possible
3. Have as many people repeat the lie as often as possible
4. Eventually, the uninformed believe the lie
5. The lie will then be made into some form oflaw
6. Then everyone must conform to the lie