This is a fairly in depth subject however the main causes of warp are usually
design related in terms of the internal construction of the board being a
balanced layout. Bonded heatsinks also tend to cause an imbalance. When
dissimilar materials are joined and subjected to heat processes they expand and
contract at differing rates resulting in deviation from flatness. It would
worry me if your supplier is "flattening" these boards since they will tend to
retain a "memory" of the initial warp and your reflow process is relieving
stresses and this explains why they are warping.
In summary, the root cause is probably board design however there are other
possibilities such as base laminate quality which, although rarer in occurence,
can cause the same problem.

Regards

Bryan Kerr
Alenia Marconi (Scotland)

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