I must disagree a bit on this one, from my viewpoint as an assembler I
see the manufacturer of the board as having a lot of influence on the
flatness.  I have in the past used boards from three different fab
houses, exact same board data.  From two suppliers (including one that
was a very quick turn for the prototype build) the boards supplied are
nice and flat, from the other supplier the boards are shaped a bit like
a hockey stick.  All involved were reputable fab houses who occasionally
post to this forum.

Even more annoying are boards that are flat enough to use on arrival,
but warp during the first reflow pass.

regards,

Graham Collins
Process Engineer,
Litton Systems Canada
(902) 873-2000 ext 215

> -----Original Message-----
> From: Hint pwb1 [SMTP:[log in to unmask]]
> Sent: Tuesday, December 16, 1997 10:43 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Board Flatness
>
> Technetters
>
> I am in agreement with Dave that the greatest cause of board bow and
> twist is
> board design.
>
> Also if you look in IPC-6012 Para. 3.4.4, you will note that the
> flatness
> requirement for all Classes is 0.75% max. for surface mount and 1.5%
> max. for
> through-hole.
>
> The design specification, IPC-2221 series, has the same numbers and
> this spec.
> which replaces IPC-D-275 will be issued in the near furture
>
> Phil Hinton
>
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