Michael:

Your description of the warp (boards flat when received, develop warp after thermal cycle,
then flatten after cooling) seems to describe a board with some type of copper mismatch.
Examination of the board should reveal either large copper areas that are not mirrored by
similar areas that are the opposite distance from the center of the board.  (Example: layer
pairs 3&4, 2&5, 1&6 should have large areas in identical geographic locations when the artwork
for those pairs are compared) or the lay-up is mismatched. (Example: the same layer pairs are
different Z-axis distances from the center of the board.)

Sincerely,

Don Vischulis
[log in to unmask]

"Michael R. Forrester" wrote:

> A batch of PCBs we received twisted during reflow.  We sent a few bare PCBs
> from the same batch through the reflow cycle and as expected also
> twisted.  The strange thing is that the PCBs are "untwisting" as time goes
> on.  The twisted PCBs returned to flat after 20 minutes.  Other boards
> twisted
> during the board bake-out before assy.  On one board we originally measured
> a bow & twist of .450 in. per IPC spec.  One week later it is down to under
> .200 in. just sitting on a shelf.  The memory usually is in the twisted
> mode not the flat mode.  The boards are 6 layer FR4, cap lamination
> (cores on the outer layers) with a well matched construction and are all
> from the same date code.  Our PCB house has never seen this before, and has
> not explanation for it.  Since we do not have a resolution on the problem I
> am concerned we may see it again on another date code.  Has anyone ever
> experienced this or have a possible explanation for it?  Are there any
> suggestions on what further testing can be done to come to an answer?
> Thank you.
>
> Best Regards,
>
> Michael Forrester
> Sr. Manufacturing Engineer
> LeCroy Corp.
>
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