I would like clarification on IPC-TM-6012, procedure number 2.4.22 for Bow
and Twist.

Procedure number 1 for Bow uses the formula as follows:

              R1 - R2
Percent Bow = -------  x 100
                 L

Procedure number 2 for twist uses the formula as follows:

                R1 - R2
Percent Twist = -------  x 100
                 (2)L
(Why is the length multiplied by 2 in this procedure and not in procedure
number 3?)

Procedure number 3 for twist uses the formula as follows:

                R1 - R2
Percent Twist = -------  x 100
                   L
(The IPC manual incorrectly identifies this formula as percent bow under
para. 5.3.6.2)

Procedure number 4 for bow and twist is for production testing and does not
refer to any particular formula.  This method has been explained as follows:

1.  Maximum allowable bow and twist from customer spec is .7%.

2.  Multiply .7% (.007) by the length of the board (diagonal for twist,
edge for bow) to obtain the plug or feeler gauge size used as a go/no go
standard.  (ex. 13" board X .7% spec = .091 plug gauge)

3.  Three corners are held down against the datum surface for twist (four
for bow) and the inspector attempts to insert the plug gauge under the
raised portion of the board.   If the gauge will not go under the board, it
is good.

4.  This method requires no multiplication of the length by 2 when checking
for twist.

I am trying to clear up some confusion about determining the percentage of
bow and twist.  Any insight would be appreciated.  Thanks in advance.


Jay Breeden
Quality Engineer
Automata, Inc.
Voice: (703) 450-2600 Ext. 240
Fax:   (703) 450-7586

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