Your boards are fairly dimensionally stable to begin with because of the
low layer counts. Beyond that, the following are basics:

1) Using balanced constructions with respect to reference plane
locations (equally distributed in Z axis) and features (openings in
metal equal on all planes, etc. to assure equal mass). If not, you have
a bi-metal strip situation resulting in potato chips instead of PCB's.

2) Using low resin to glass ratio prepregs and core materials
(7628,7629, 2116, as examples) - the more glass the more dimensional
stability in all axes with tradeoff being reduced innerlaminar bond
strength for the first two glass styles.

3) Using like, or homogeneous glass styles (60's yarn as an example
using 106, 1080, 2113, and 2116 materials) so yarn bundles "nest"
together and effect little relamination induced stress (later being
relieved thermally during soldering operations causing excessive
warpage).

4) Using unbalanced constructions with respect to higher layer count
MLB's wherein a resin rich glass style (106 or 1080) is laid up with a
less resin rich material (2113 or 2116) to provide the best of both
worlds as constructions with higher relative dimensional stability and
high bond strength. As an example for a 5 core construction, use one ply
of 106 with one ply of 2113 instead of using two plies of 1080 and so
on.

Earl Moon (also at [log in to unmask])

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