cbyxbee reported having occasional "problems with moisture in the
soldering operations" of boards and wondered whether the problems
might be due to packaging them in paper instead of something that
would protect them from moisture.
I thought that I'd see a response by now; since I didn't, I'll try,
but before sticking my neck out too far, I'd like to have a better
idea of what constitutes "problems with moisture in the soldering
operations". In a previous posting I described how blow-holes are
caused by moisture that is present in improperly drilled holes as part
of the crystal structure of occluded copper salts; this moisture
cannot be driven off by any baking you'd be willing to consider, but
the blow-holes are rarely so severe as to threaten the reliability of
the connection.
The condition called "measling" is also caused by moisture: previously
dissolved water molecules in the resin of the board are evolved as a
gas during the sudden heating of soldering. This condition has also
not been shown to threaten reliability, but its occurrence can be
diminished by baking the board before soldering. I'm guessing that
the problems you refer to are one of these.
Given that a board must be exposed to air of 40% to 60% relative
humidity while parts are being installed, it will pick up some
moisture during this time if it was dried before. How hard is it
worth trying to keep the board dry during assembly, to avoid having to
bake it before soldering?
Incidentally, let me mention again that the use of vacuum to promote
drying, however much it may appeal to one's intuition, is worthless.
Vacuum cannot reach into the plastic to pull water molecules to the
surface, and that's the step in the drying process that takes the
time: waiting for water molecules to diffuse to the surface on their
own, driven by a concentration gradient, at a speed that depends only
on the temperature. In comparison, removing water molecules from the
surface takes no time at all in an ordinary oven.
To make it seem a little more reasonable that vacuum doesn't help, one
can calculate that if the relative humidity at room temperature is
50%, that corresponds to a partial pressure of water vapor of about
0.015 atmosphere. The partial pressure in the oven will be the same
(as long as there is any kind of venting), but since the temperature
is much hotter, the RH will be much lower: 1.5% at 100 deg. C, for
example. So the effect of the vacuum, no matter how good, is to
reduce the RH only slightly. Since the vacuum may make it more
difficult for the item to reach temperature, it might even slow things
down. The only case I have seen where a vacuum bake was worth doing
was where the item to be dried out had only a small orifice in the
wall through which all the water molecules had to pass. But in the
case of a board, there is no restriction to water molecules leaving
the vicinity of the surface.
Also, if a board is dried, putting it in a plastic bag won't help keep
it dry, because water molecules from the air will go right through the
bag, probably much faster than they will enter the board. You could
add desiccant to the bag, or store the board in a dry environment, but
is it worth it? If there is some special situation that I have not
covered, post again and I'll try again.
Gordon Davy
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