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December 1999

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Subject:
From:
Alain Savard <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 2 Dec 1999 10:57:40 -0500
Content-Type:
text/plain
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text/plain (56 lines)
I do not quite agree with the baking of boards to remove warp. Baking =
can be
used to flatten a board... temporarely. As soon as the board meets =
another
heat cycle above Tg, the stress in the board will be relieved again ,
bringing the board back into the warped/twisted  position... Since this =
may
accour during relow or wave soldering, it may slightly shift coponents =
at
that time. If you find this may be a problem be very careful. I have =
not
tried the SLOW cool down method however and it might work.

Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.
e-mail: [log in to unmask]

-----Original Message-----
From: Bob Dube
<SNIP/>
If however, a construction is largely well balanced
and some warpage is still for whatever reason noted a bake cycle may =
very
well help. The trick would be to apply light pressure to flatten (we =
use
steel weights), bring the temperature of the panels up past the Tg =
(glass
transition temp) hold at temp for several hours, and then allow for a =
very
slow cool down under weight back through the Tg. A cool down of less =
than 4
=B0 F per min. should help in avoiding re-introducing the sort of =
stresses
which will lend to warpge.

This is a key component in the lamination process of multilayer boards.
Slow, gentle cool down.
</snip>

Bob Dube

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