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
                    <colin.weber@VARI        To:     [log in to unmask]
                    ANINC.COM>               cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
                    Sent by: TechNet         Subject:     [TN] Bow & Twist
                    <[log in to unmask]>


                    07/19/01 12:18 PM
                    Please respond to
                    "TechNet E-Mail
                    Forum."; Please
                    respond to Colin
                    Weber






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

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