Ted,

Yes, there are multiple other causes that will cause out-of-flatness boards.
These are generally random and related to the fabricator or his materials and
are not caused by a "designed-in" unbalanced construction or unbalance
circuitry.  I probably will not remember all of them, but here are a few of
the other causes.

1. The supplier used two different glass style construction on his internal
layers.  For  example, one core made from 7628 and an opposite core made from
2116/1080.

2. Unsymmetrical crossplying  of the layup of either the prepreg or the
laminate.  The warp and fill directions change/shrink differently during
curing.

3. A heater burnt-out or overheating in one section of the platen.

4. The platens being out of parallel so that one side or one corner gets
excess pressure..

5. Lagging material insufficient so that the external layer on the outside
boads gets a faster heatup and cure than the rest of the board.

6.  Different cure state of the cores or the prepreg used in the layup.

7.  Warped or dished press plates.

8.  Putting in the oven to dry on a non-flat surface or on in a rack without
support.

9. Hot air leveling without a balanced pressure or partially clogged air
passages.

10. Fusing the tin-lead plating with unbalanced heat from side to side.

11. Shipping without support in big stacks.

These are not usually continuous conditions and do not appear over several
suppliers.  If I see a continuing condition from several suppliers,  I go look
at the artwork to make sure that a layer called '"Signal" is really not a
large copper plane that is being used to provide controlled impedance etc.
Another possibility is an unbalance within a single layer, for example, a
plane section on one end of the board and fine conductors on the other end.
Also look at inplant storage or prebaking (drying) prior to screening.

Phil Hinton

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