Peter,

We are designing the board to IPC specifications for scientific
instrumentation.
There is no special case for these designs. I don't have a problem with bow
and twist
as I haven't designed the boards yet. My question is more directed to how
to avoid
the issue if I want to design a large PCB.

Apart from stiffeners and the like I am specifically wondering how thick I
need to make
a PCB if I am going to be placing fine pitch devices and/or PBGAs on them.
If I were
to go with 2.4mm I have EMI stackup and aspect ratio concerns, but feel
comfortable
the board will be more rigid. I could and would prefer to step down in
thickness but do
not have a feel for how the board would fair during handling and assembly.

I am talking about 14 to 15 x 9 to 10 inch board sizes for processor
designs. An
obvious solution is to split the PCB up into two boards, but I do not have
that luxury.

I was just curios if there are any guidelines or figures fro bow & twist
with respect to
size versus thickness?



At 02:44 PM 19/07/2001 +0800, you wrote:
>Much of the bow and twist these days is down to the choice of material used
>for the substrate coupled with 'balance' in the board design and ultimately
>on the capability of the fabrication process.
>
>If you have heavy planes in the board, make sure they're distributed evenly
>throughout the layers, otherwise any good PCB Fab worth his salt will keep
>any bow and twist within the bounds of the usual specs. It used ot be that
>the thicker the board was, the more rigid it was and the less inclined it
>was to warping. These days, more than 20 mils bow or twist on a 63 mil
>thick board measuring 9 x 6 inches is unusual.
>
>Are you designing boards to be made to MIL-PRF-55110, to an IPC spec or to
>something more commercial?
>
>Pete Duncan
>
>
>
>
>                     <colin.weber@VARI        To:     [log in to unmask]
>
>                     07/19/01 12:18 PM
>
>Folks,
>
>Is there a method of determining the maximum permissible size of two board
>types?
>a) A SMT PCB containing Fine pitch QFP devices
>b) A SMT PCB containing PBGA 388pin 1.27mm pitch & Fine pitch QFPs
>
>with respect to Bow and Twist.
>
>I am interested in how the thickness of the board relates to maximum bow
>and twist.
>I'd like to determine this before I design the layout, rather than find out
>it is wrong
>after performing the tests of IPC-6012A,  IPC-TM-650 2.4.22.
>
>Regards,
>
>Colin Weber


Regards,

Colin Weber

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