On Wed, 16 Nov 2011 22:32:33 -0700, Tony Hwang wrote:

Paul Drahn wrote:
On 11/16/2011 2:45 PM, Michael Dobony wrote:
I have an all-in-one AC/furnace sitting outside. The circuit board was
replaced a very short time ago, about two months ago. It is already
showing
signs of degradation (rust on a diode lead). Is there any spray coating I
can put on this to prevent further degradation, assuming it is still in
operating condition?

This is a link to the type of unit I have
http://www.americanstandardair.com/P...ckaged+Systems


Mike D.
Mike,
Rust on diode leads is quite surprising as even the high current diodes
with steel wire leads have a solder coating (tinned leads). Perhaps the
leads were damaged during assembly or testing and the coating was broken.

If you can turn the power off and get to the leads, I would suggest
cleaning off the rust and retinning the leads with a soldering iron and
rosin core solder.

I had our heat pump serviced last week and watched the young man as he
did his work. The circuit board controller is like yours, outside on the
heat pump and and under a metal cover that protects it from all but
blowing snow. Quite a bit of dust, but no other problems. The board does
have three 2 watt resistors that have discolored the board from their
heat. They should have been placed further from the board material.

Such heat producing devices are one good reason not to conformal coat
the circuit board. Or they need to be masked so coating material is kept
away from the resistors.

My electronic assembly company is required to conformal coat several
different circuit boards. In most cases we use urethane applied with a
brush and carefully avoid the locations the customer wants to not be
coated. Connectors, switches, sensors, test points, and mounting holes.

Other customers have specified the silicon based coating. Both coating
material is rather expensive. Much of it is ONLY available in metal one
gallon cans because bottles can't be shipped UPS. Urethane is available
in spray cans, but most cans are plugged up and won't spray, even right
from the distributor. Shelf life is almost zero. There is no return on
conformal coating material.

Paul

Hi,
Orientation of board is important as well. Right side up, on vert. vs.
horiz. plane exposure to the element or under ventilated cover, etc.

Vertical orientation. The old board had rust running down from one
connection to the next, about five or six connections gone. It was in there
two years unused. Now I have freezing weather and can't wait for shipped
unit. If I have to replace it again it will cost double what I can find on
the internet.