Isobel,

IMO, the one with 3 corners touching datum are meant to measure twist, typically
the board edges tend to buckle up more than the middle. 5.4.2 deals with board 
warping in the center when placed naturally on a flat surface. It is possible to
have the same board passing 5.4.1 (twist) and failing 5.4.2 (warp in the center)
or vice versa. At least that has been my experience dealing with this issue.

Teik M. Ng
PC World
Toronto, Canada
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______________________________ Reply Separator _________________________________
Subject: IPC-TM-650 - need clarification of 2.4.22
Author:  donb at PCWORLD
Date:    11/22/96 5:59 AM


Greetings.  
     
I would like to make inquiry on measurement for twist.
     
Please refer to section 5.4.2 of IPC-TM-650 number 2.4.22 rev B dated 
11/94.  In this section,
it is not mentioned if 3 corners are required to touch the datum surface 
when performing the
measurement for twist.  
     
(I have no problem interpreting section 5.4.1.  The method described 
clearly that both corners of
an edge are to touch the datum surface when performing measurement for 
bow.)
     
Please refer to section 5.2.2 of IPC-TM-650, no. 2.4.22.  "When the correct 
shim thickness is
used the three corners will be in contact with the datum surface" .   I 
interpret the statement has
an intend to ensure three corners are touching the datum surface when R1 is 
recorded.  If
positive, would it be appropriate to apply pressure on 3 corners when 
engaged section 5.4.2?
     
I appreciate your clarification.
     
     
Regards,
     
Isobel Tan
Tel. 65-750-4138
Fax. 65-751-6165
     
     
     
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