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.+