John,
     
As a packaging engineer, this is my advice on your situation.  It does not 
directly answer your question on how to interpret the spec, but provides insight
as to the intent of the requirement and how I've handled similar problems with 
much larger boards.

        1) As a rule of thumb, I typically have warp and twist measured on the 
boards in their application orientation.  Typically when a board is laid flat, 
especially if it is populated, gravity will remove some if not all of the warp 
and twist.  If the customer is using the board is this orientation, then it may 
be acceptable depending on spacing and other mating components including 
packaging restraints.  If the board is going into a card cage then warp and 
twist can cause definite problems.  These problems can range from arcing between
boards to the boards physically jumping the card guides.  As an end user, it is 
very difficult to define and measure the amount of warp and twist in a populated
board.  In the past, I've taken measurements on boards vertically on edge from a
known flat surface at a known distance, e.g. a rigid metal plate. 

        2) If the end user is having problems with the boards in their system, 
yes it is a concern.  Depending on the environment that your customer is 
subjecting the boards to, the warp and twist can cause failures during testing. 
You'll need to work with your supplier and explain to them the concerns of your 
customer and the dissatisfaction in their quality and that the product does not 
meet its intended use.

I hope these thoughts from my experiences prove helpful in resolving your 
dilemma.

Pam Morrison
Lockheed Martin 
Information Systems

______________________________ Reply Separator _________________________________
Subject: Bow and Twist
Author:  [log in to unmask] at ORL-SMTP-G
Date:    5/8/96 12:17 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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