If the boards were built flat in the past, something changed when
manufacturing the board.  There's a good chance the board was
cross-plied (the grain in the cores is different from the grain in the
prepreg).  If you know the type of prepreg used (7628, 1080, etc.)
you can burn the board using a torch to expose the weave.  Once
you've burned the resin, you peel layers off until you get to the core
and prepreg.  You then count the bundles in one inch.  7628 for
example has 44 bundles in warp (grain) direction and 32 in the fill
direction.  If the core and prepreg contain 7628, the number of
bundles should be the same in the one direction.

There could also be problems in the process of manufacturing the
board.  If there are heaters out on one side of the press platens,
the rate of rise is different from one side versus the other.  This will
cause different stresses in the material.  There's also some theory
to stress relieve the core material before lay up by baking at 250F
for 30-60 minutes.  Did your supplier change this?  Also some
laminate suppliers stress relieve their material before shipment.
Sometimes they don't if they get busy.



Rick Babyak
Process Engineer
Proto Circuit Inc.
7 Ascot Parkway
Cuyahoga Falls, OH 44223
330-572-3401  Fax:330-572-3434
[log in to unmask]

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