I agree with Mike's answer, but I wanted to add a little more detail.
Yes, board warpage (bow and twist) is the same as flatness.
The straightness spec is for an element or feature in a single axis.
Flatness controls all the elements or features for a given plane.
Therefore, flatness is a better specification to use to control or specify
max bow and twist.  (Note section 6.4.2.1 in Y14.5.  It allows double
tolerancing, in effect giving an overall maximum tolerance, while providing
for tighter flatness control for smaller areas of the board.)
IPC-6012, section 3.4.4 gives some info on bow and twist and what they
consider to be "okay;"  your customer probably doesn't.  It would also
probably be best to use Regardless of Feature Size for your calculations,
as a board at LMC would be allowed to warp greater that MMC, and it would
show too!
Glad I'm not in your shoes...

 Roger M. Stoops,       PCB Designer

 Spectra Precision Inc. Ph:     937.233.8921
 5475 Kellenburger Rd.     937.233.4574 ext
 Dayton, OH 45424-1099  288
 USA                    Fax:    937.233.7511

                        Member IPC
                        Designer's Council,
                        C.I.D.







                    "Mcmaster, Michael"
                    <[log in to unmask]        To:     [log in to unmask]
                    MERIX.COM>                 cc:
                    Sent by: TechNet           Subject:     [TN] Warpage, Flatness and Surface Profile
                    <[log in to unmask]>


                    06/13/00 12:02 AM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to
                    "Mcmaster, Michael"





I have a customer with a specification for bare board warpage, but the part
also has flatness and surface profile requirements on the print.  The
warpage requirement is in the customer's specification, which also says
that
any print requirements supercede the spec.

Are flatness and warpage the same thing?  Or are they additive?  If the
flatness spec is based on Maximum Material Condition do I get to include
the
warpage as separate MMC condition that's in addition to the flatness
requirement?

I've never been able to find anything in the ANSI Y14.5 for warpage, bow or
twist which are the terms most of us are familiar with.   Can anybody help
me out here?
Mike McMaster
RF Product Engineer
Merix Corporation
Forest Grove OR
503-992-4263

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