JIm,

By the look of your message, you must be a quality eng/mgr or
sales eng/mgr.  No harm if i am wrong.  Qual/'sales eng/mgr's
usually like to go by the book and process/mfg eng's like to go by
what work and what not work.  To get to my points, IPC-275 Bow
& Twist spec should be in a permanent storage.   Boards that
are closer to 0.01"/inch spec causes some problems in smt
manufacuring and  th wavesolding.

They  introduces printing registration inaccuracies, uneven
solderpaste depositions and solderpaste skips.  They go thru
bumping rides during p/p placement cycles and off-placements
of plcc's, qfp's and bga's are visible.  They wont transfer thru
edge conveyors without an operator assistence.  Some chip
components get disloged due to the springness of boards and
slight vibrations.  They cause more open solder joints.

They allow flooding of solder on comp side during
wavesoldering and prevents solder wetting on the highest area
of warped boards.  It becomes more important to have flat
boards with advancement of finer qfp's and bga's.

Bow & Twist spec of 0.002"/inch or less is better suitable for SMT
assy.  I know a lot of board fab shops won't guarantee with hasl
boards.

regards
Matthew Park
Automation Engineer
NII Norsat International Inc.

>>> Jim Mathis <[log in to unmask]> February 18, 1998  2:27
pm >>>
I spec'd out .01"/inch, also, which used to be the spec.  IPC-275
section
5.2.5.2 now specs Bow & Twist in terms of percentages.  If you
don't have a
copy of IPC-275, let me know and I'll fax you the appropriate
pages.

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