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April 2000

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Subject:
From:
"Olson, Jack" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 26 Apr 2000 09:03:24 -0700
Content-Type:
text/plain
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text/plain (76 lines)
I was hoping you would get some answers, because it is a question I have
never heard before (just when you think you've heard them all, eh?)

I am no expert, but my first thought was that laminate bow would be very
hard to control because the weave is all going in the same direction. In
multilayer boards they are interleaving laminates with the weave one way and
then the other so they "balance out" when they all get squished together...
BUT... the laminate thicknesses you mentioned are the thickest I've ever
heard of, which doesn't leave a lot of room for cross-weaved layers bonded
together.

When you say laminate do you mean just the B-stage material that separates
the core copper clad stuff? Maybe the copper clad is more stable, and the
laminate being pliable just conforms?

                Anyway, the real question you asked was "should we require
the laminate vendor to
tighten the bow and twist specification?" and my answer would be "I doubt
it" since we are getting good boards that meet spec. If you are having
trouble getting flat boards, either talk to your fabrication vendor about
better construction (balanced stackups) or look for a better fabricator

best wishes,
Jack

                -----Original Message-----
                From:   Shirley Xiao [mailto:[log in to unmask]]
                Sent:   Tuesday, April 25, 2000 5:20 PM
                Subject:        Resend: Bow & Twist Specification

                Sorry to bother everyone again. I guess few response
                to this topic maybe becasue of Easten Holiday.
                Shirley Xiao


                Dear All:
                  What I have on hand is a old revision of
                IPC-6012(1996). section 3.4.4 Bow and twist: "The
                printed board shall have a maximum bow and twist of
                0.75% for boards that use surface mount components and
                1.5% for all other boards. Panels which contain
                multiple printed boards which are assembled on the
                panel and later separated shall be assessed in panel
                form."
                  From the PCB user point of view, what is the maximum
                bow and twist percentage that the placement machine
                can  tolerate?
                  While in IPC-4101 section 3.8.4.3 Bow and twist
                laminate materials (table 8 bow and twist, mm per 30.5
                cm), 3.0 mm per 30.5 cm which is equivalent to 1.0% of
                bow and twist (for laminate thickness of 0.79-1.67 mm
                double sided with panel size from 200 mm to 300mm).
                 My question is the requirement for laminate is even
                loose than that of printed circuit board. Is it
                reasonable or should we require the laminate vendor to
                tighten the bow and twist specification?
                  Any comments will be appreciated.

                  Shirley Xiao



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