I would like to ask the TechNet community to share their knowledge on the
subject of bow and twist and how it relates to copper balancing on a given
layer and copper balancing on the stack up.

If you have fabricated or designed a board that bowed or twisted what was
the problem?  Did the board have a non-symmetrical construction, i.e. odd
number of planes, only part of a layer with a plane?  Was there a signal
layer with a larger amount of copper then its symmetrical layer?  In other
words, what did you find to be the problem?  Can you describe or quantify
the problem?

Is there a rule of thumb as to the percentage of copper per square unit
about the center of the board, or percentage of copper on symmetrical layers?

IPC 2221, section 5.2.4 Bow and Twist, talks about the amount the board can
bow and twist before it can be rejected and about the "Proper board design"
to prevent this from occurring.

IPC 2222, section 10.1.2 Balanced Conductors talks about "conductors should
be balanced within an individual layer."

In the IPC PWB Advanced Designer Certification Study Guide, section 2.4
COPPER BALANCED EFFECTS ON PRINTED BOARD FABRICATION, page 101 states "The
main idea of balanced construction is to start at the center and move out
toward the edges with an equal number of planes and circuit layers on
either side of the center."


Thanks in advance

Donald Kyle C.I.D.+

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