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From: Paul E. Schoen on 14 Feb 2010 22:58

"MooseFET" <kensmith(a)rahul.net> wrote in message
news:f92f49c7-e07f-40a2-b384-b721a558efb4(a)u19g2000prh.googlegroups.com...
On Feb 14, 1:50 pm, Joerg <inva...(a)invalid.invalid> wrote:
>
> I use the thinnest Kester No-Clean there is, 15mils. Leaded, of course.
> Gets the job done and I don't have to clean up the flux.

Never use "no clean" flux. The stuff is just nasty because it
only takes a little exposure to any hydrocarbon to make the ions
get loose and start causing trouble.

----------------------------------------------------------------

I have a roll of no-clean flux but I have not used it. I don't like the way
it works (or not), and it still leaves a visible residue. I have been using
a spool of 60/40 0.025" wire solder that does not mention any flux, so I
assume it is just solder. I use GC liquid solder flux and apply as needed.
I clean the flux off the board with Isopropyl alcohol and a stiff horsehair
acid brush, and then scrub it with spray detergent and a rinse with hot
water. A final dry-off with a heat gun finishes the job.

I've had problems with ionic contaminants that cause leakage and noise in a
humid environment, and I've seen a build-up of white crystalline deposits
on the solder of boards that I have not cleaned properly, after sitting
around for a few months. The ideal final touch is probably a rinse with
deionized or distilled water and a thorough drying.

I've had mixed success with conformal coatings to reduce leakage in humid
environments. Sometimes test equipment is stored in an outbuilding or
transported in a truck and then brought inside where condensation sometimes
occurs and there is almost no way to avoid some problems. But removing any
trace of salts on the board is really important, and usually any leakage
problems soon disappear. In some cases I have also added a power resistor
as a heater to reduce and remove condensation .

Paul



From: MooseFET on 15 Feb 2010 12:15
On Feb 14, 7:58 pm, "Paul E. Schoen" <p...(a)peschoen.com> wrote:
[....]
> I've had mixed success with conformal coatings to reduce leakage in humid
> environments. Sometimes test equipment is stored in an outbuilding or
> transported in a truck and then brought inside where condensation sometimes
> occurs and there is almost no way to avoid some problems. But removing any
> trace of salts on the board is really important, and usually any leakage
> problems soon disappear. In some cases I have also added a power resistor
> as a heater to reduce and remove condensation .

The best solution is to put the board in an air tight metal housing.
Plastic
isn't good enough because the water sneaks through over time.

Conformal coatings are usually far more trouble than they are worth.
They slow down the water getting into the PCB but they don't stop it
so
the PCB ends up seeing what the average of the environment is. If it
is nearly always wet. That is what it will see.

To dry out boards, a few days in the hot box is needed. You need to
allow
enough time for the water that has made its way into the FR4 to get
back
out. Once the board is clean and dry, putting it directly into the
housing
means that it doesn't get time to absorb anything.

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