IPC-TM-650 2.4.22 is the test procedure for bow and twist.  It is found in
various documents, mine is in the back of my trusty IPC-RB-276.  In section
5.4 Procedure 4, Production Testing using a "go/no-go" pin gague, there are
two methods and measurements.  The first, in para. 5.4.1, (fig 6)  show the
bowed board place cup side down and the measurement is the gap under the
cup.  In 5.4.2, (fig 7) the board is turned over and the cup is up.  The
measurement is at the edge or corner of the board.

My question is this.  The cup side up measurement is about twice the cup
side down measurement.  Must both measurements meet the percent deviation
requirement?  Is there a different way of calculating the maximum
percentage deviation for the two?  I am confused.

You can see what I am talking about by taking out a business card, and
curling it a little.  Now put it cup side down, look at the gap.  Turn it
over and hold down two corners (across the edge which does not have the
curl in it), the gap is about twice as much.

Thanks and don't let your boss catch you playing with your cards at your desk!


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                           George Franck
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