At 03:52 PM 2/18/98 -0800, Matthew Park wrote:
>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.

Hi,
I'm from Quality and I'm here to help you.  :^)

But seriously,  <fwiw> we typically spec .0075" max bow/twist on fine pitch
SMT assy's but never (rarely) see that much bow/twist.  Truth is we can't
live with any bow/twist greater than .005 in our conveyors or SMT process.
By the time it gets to .005 the sky is falling, yields are in the toilet
and assy's are either jamming conveyors or crashing to the floor.

How do you spell scrap?  .010 inch per inch.

(my $.02),
Dwight Mattix
Staff QE, Manager Interconnects Quality

 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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