John,
     
     I would be concerned if I were you (customer viewpoint).  We would not 
     accept boards in that kind of shape regardless of how they have 
     flattened out under weight.  The degree of twist indicates that 
     additional stresses have been placed on the material which could 
     affect fit after assembly.  When placed under additional stress on the 
     reflow/wave solder line the board could twist up even more (or it 
     might flatten out).  I would be scared to try and place any high 
     density QFP's on that board (my QA and MFG departments would kill me 
     if I tried to send them something like that).  We have had 
     circumstances where even if a board is heavy enough to flatten itself 
     (or is screwed down by ~20 screws per side to a stiffener) then the 
     board still won't fit with other boards in the assembly unless it was 
     flat to begin with.
     
     Currently, with some of our suppliers, we are having them bake our 
     boards after final test to ensure there will be no warp when received 
     (based upon past problems).
     
     I don't really care what the accepted (or industry standard) specs 
     might be, if there is an obvious problem, I will reject it.
     
     Regards,
     
     Roger Held
     Manager of Electrical Component Engineering
     Hitachi Computer Products (America), Inc.
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______________________________ Reply Separator _________________________________
Subject: Bow and Twist
Author:  [log in to unmask] at Internet-HICAM-OK
Date:    5/8/96 6:17 PM


A question on interpretation of the IPC method IPC-TM-650, 2.4.22.
     
I have a batch of boards 400mm square, which are quite clearly twisted when 
held free in my hands. When I lay a board horizontal, it straightens under 
its own weight. All corners touch the plane and I am left with a small 
residual bow of 1 to 2mm, say 0.25 to 0.5%. 
     
My supplier says it is in specification, but my customer (who cannot carry
out the prescribed test on the populated board) comments on the degree of twist.
     
Questions:
     
1) Is this the correct interpretation of the test and specification?
     
2) Should I be concerned?
     
Regards
     
John Loveluck
Tadpole Technology
Cambridge
United Kingdom
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