Bravo, Peter, for the obvious answer. I have been thoroughly opposed to
prebaking ever since I first started using G-10 PTH boards in the 1960s.
Not only should it not be necessary, it is, as you say, an extra and
unnecessary thermal cycle AND it causes sufficient oxidation of the
finish on the pads (including the nickel under the porous gold with ENIG
or electrolytic) that you are giving the flux a lot of extra work to do.
Unfortunately, not all PCB fab shops realise that they, too, sometimes
ship boards with built-in blow-holing etc., so proper packing (rare) is
not going to help in these cases. However, this is one of those cases
where prevention is better than cure.
IMHO, purchasing contracts for bare boards should include a clause,
especially for value-added multilayers, to the effect that any boards
that are up to n months old that are removed from their original
packaging and blowholes occur during soldering within, say, 24 hours
shall be replaced free-of-charge. This will place the ball fairly in the
PCB fab's court and we'll get better boards which are correctly packed.
This will mean the demise of polyethylene as a packing material, because
it is as porous to humidity as a kitchen sieve. To boot, it will make a
small contribution to a better environment because if you add up all the
energy required to heat up x million boards per day throughout the world
some 80°C for an hour, it will represent so much less CO2 (and reduce
costs).
Voila, Brian can now get off his soap-box :-)
Brian
Rick Howieson wrote:
>
> What is the recommended pre-bake of multilayer boards prior to insertion and
> wave. 221F for 1hour??? Or would this be a little extreme?
> Thanks,
> Rick
>
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