Phil

Depending on the components you use, I would be extremely concerned at
your drying time of 20 min @ 60°C. If you have components with
stand-offs of < 0.25 mm and you rely on this time/temp for evaporative
drying, then I'm afraid you are suffering from a delusion, even with
forced air. If you take a board and hit sharply your hand edgewise with
it, do not traces of water show from under the components?

In the machines I used to make (you can see a video including the drying
process at http://www.protonique.com/video) we had rotary airknives
running at c. 200 km/h which blew off the excess water and took the
temperature up to c. 100°C in a few seconds, but, with difficult boards,
10 minutes could be barely sufficient, even though the heaters were
about 15 kW for a well-insulated volume of about 1/8 m3. I never did any
tests at temps lower than 80°C, but I would guess that, even under these
violent conditions (watch how the water is blown off the boards), it
would take much longer than 20 minutes to dry at 65°C.

Obviously, if you wish to coat, you must have a dry board, free of any
superficial water. IMHO, it does not matter if the board has absorbed
humidity within it, provided it does not interfere with the curing
process, causing a whitish blush. Even if it is perfectly dry throughout
at the moment of coating, it won't be a few days later.

If you don't want to take your boards above 65°C, there is a way: vacuum
drying BUT it is very energy-intensive and you must have good traps
between the oven and the pump a) to prevent moisture from reaching the
pump and b) to prevent pump fluid vapours from refluxing into the oven.
I don't really recommend it, although it is a possibility.

Other than polystyrene components, there are very few that will suffer
for an hour or more at 100 to 120°C. Check the manufacturers' data
sheets if in doubt (or ask them, if they are lax enough not to put their
max core temp thereon). Temperature shock is more killing than
relatively slow rise and fall, such as is caused during drying.

Brian

Phil Nutting wrote:
>
> Good day, Technet
>
> After reviewing past threads regarding baking boards before conformal coating I come up with quite a varied selection of ideas.
> I found 1 hour @100°C, 4 hours at 150°C, and 4-5 hours at 125°C.
> My design engineer is concerned about the boards being exposed to this temperature for an extended time. Yes, in the wave solder it sees much higher temperatures... for a short period of time.  Our aqueous wash system dries at about 65°C for 20 minutes.
> Our product normally is tested to 40 - 50°C depending on the customer needs.
> The past threads suggested raising the temperature above 100°C to "boil off" any remaining water.
>
> Can this time/temperature be narrowed down any?
>
> And if we use a lower temperature are we possibly trapping moisture?
>
> How will the reliability be affected by these elevated temperatures.
>
> Thanks in advance,
>
> Phil Nutting
>
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