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From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Thu Nov 18 13:32:52 2004

>> The trick is to chase the water afterwards [...]
> "chase the water" - can you explain? does that mean after the
> dishwasher spray the top of it liberally with isopropanol?

No, not quite.

Now, I haven't doine this with electronics myself. But I think I
understand the issues to an extent.

Thing is, you don't want to just let the water dry on the device. You
can do that if you are using _very_ pure water and have rinsed the
device _thoroughly_, because then the water will just evaporate and
that's it.

But if you're using tap water, it contains dissolved salts and
suchlike. (Even soft tap water does - just in lower quantity, and to
some extent different salts.) And if you just air-dry electronics,
these salts get left behind in a layer on the surface. _Sometimes_
this causes no trouble, but if the salts happen to be conductive enough
(for values of "enough" that depend on factors beyond the scope of this
discussion), or in some cases if they're insulative enough (for example
if they're being deposited on bare conductor pads used by a switch),
they will interfere.

So you want to eliminate the water. This can be done mechanically, but
it is quite difficult to get rid of enough of the water mechanically in
most cases. So instead, you do a rinse with a compound that (a) is
itself highly volatile, (b) does not come with dissolved salts and such
the way tap water does, and (c) is miscible with water and thus will
remove the water that's left on the surface.

In the chemistry lab, when washing glassware, we used acetone for this
(maybe they still do - I don't know). I wouldn't want to use acetone
on electronics, because too many of the plastics and other compounds
used in electronics are too soluble in acetone. But isopropanol also
satisfies the conditions I sketched (though slightly less well) and is
a far less effective solvent for things like chip housings and PC board
resin.

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Received on Thu Nov 18 2004 - 13:32:52 GMT