Hello,

We assemble a six-layer two-sided SMT board which has been troublesome
lately because of increased bow/twist problems.  The increased bow/
twist appears to coincide with our recent changing of suppliers for this
PWB.  Unfortunately, I cannot say this definitively because I don't have
historical bow/twist data - only qualitative perceptions from our SMT
line operators.

Another difference we noticed between the old supplier and the new
supplier is the old supplier's boards tended to be on the high end of the
thickness spec (.065"-.067") whereas the new supplier's boards tend to be
on the low end of the thickness spec (.057"-.059").  This led us to
speculate whether there might be some correlation between the material
chosen (core laminate and pre-preg) and the bow/twist properties.  Our
documentation doesn't specify what to use for core laminate or pre-preg
so it's quite possible the two suppliers chose different core-laminate and
pre-preg material.

I'm looking for some feedback:  What build-up would you use to fabricate a
six-layer .062" board with 1 oz copper inner layers and 2 oz copper outer
layers (after plating)?  Do you think the build-up could have any
influence on bow/twist?  (Also, the copper distribution on all layers appears
fairly balanced.)

We're going to cross section some of the old supplier's boards to determine
their build-up.  The new supplier has the following build-up:

     1 oz copper foil
     7629 B-stage
     .015 copper-clad core
     7629 B-stage
     .015 copper-clad core
     7629 B-stage
     1 oz copper foil

I believe the 7629 B-stage is .008" thick.  Would there be any advantages
to using two sheets of 1652 B-stage which is .005" thick per sheet?  Since
the new supplier's boards are on the low end of the thickness spec we
could add the additional .006" per board, I think, without a exceeding our
board thickness specification.

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Thanks!

Mark Lettang