We are an OEM that assembles boards utilizing multiple technologies (SMT
and Thru-hole) on FR4.  We are using both 2-sided and multilayer
boards.  I am a bit confused about what we should regard as acceptable
bow and twist of boards both from our vendors and after assembly.

Per IPC-A-610B, bow and twist after soldering shall not exceed .75% for
a board containing SMT components.  However, IPC-D-275 states that for
even a Level C "high design complexity", 2-sided board should only be
specified with a maximum bow and twist of .9%.  It has been our
experience that the boards do not get better after processing, but tend
to bow worse.

Is there something I'm missing as to why these two specs don't seem to
be in sync with one another?  Also, does anyone have a suggestion how we
could be specifying boards from our vendors to assure the proper
flatness through our manufacturing process?

Thanks,
Jeff Field

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