Colin,
I haven't come across any such figures, and I doubt that they would have
been very meaningful if I had, as there are too many variables in terms of
material choice and choice of fab house to pin numbers to. I suggest you
determine what flatness you need and the materials you want to use and then
go talk to fab houses for advice on how possible your requirements are.
My view is that, if you're using BGA's, I would prefer a thinner board to a
thicker one from a manufacturing point of view. It takes a much longer time
to get BGA contacts up to soldering temperature compared with most other
components where the leads stick out the sides. A thicker board means an
even longer dwell time at high temperature, which to me is a risky time for
a board. Peak soldering temperatures, especially where BGA's are involved,
are pretty close to the max case temperatures of a lot of components, and
if you increase the length of time components cases are exposed to high
temps just to heat up a thick board, you may induce internal damage.
You can put the boards in support jigs during processing until the
stiffeners are fitted.
Pete Duncan
Colin Weber
<colin.weber@vari To: "TechNet E-Mail Forum." <[log in to unmask]>, DUNCAN
aninc.com> Peter/Asst Prin Engr/ST Aero/ST Group@ST Domain
cc:
07/20/01 05:43 AM Subject: Re: [TN] Bow & Twist
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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