Eric/John,
     
     What I have seen here is that localized heat will have an impact on 
     the warp and twist of a board.  We have seen boards that we have done 
     localized rework on have more warp and twist then at the start.  We 
     also have found out that if those boards are placed in an oven at 
     moderate temperatures or placed in a card cage with heated air across 
     that the warp and twist reduces.  These observances have been made 
     with thick (~.077 in ) thick boards in both multilayer and multiwire.  
     My theory is that this is a function of the fiber orientation in the 
     material and the memory associated with the material.  I will also 
     agree that warp and twist will impact reliability and quality of 
     solder joints especially in fine pitch and ball grid devices.  We have 
     some standard repair procedures for these types of problems that I can 
     forward to you if desired.
     
     Pam Morrison
      (407)826-1424

______________________________ Reply Separator _________________________________
Subject: Re: Bow and Twist
Author:  [log in to unmask] at ORL-SMTP-G
Date:    5/9/96 1:32 PM


John\Pam,
     
          As an aside, one other concern I could forsee is the possibility
of fracturing solder joints.  This of course would be dependent on quite a 
few variables such as board size/thickness, how badly warped/twisted, 
assembly process(thru-hole/SMT, mixed technology), how flat the board is 
during wave/reflow & what type of application the board is being used in 
(i.e put in a card rack, torqued down in a case, etc.).  We have run tests 
comparing bow&twist measurements on boards by serial number both as raw 
boards and also after assembly/wave.  Some boards actually improved and some 
got worse, why? I'm not sure, maybe due to which way it was bowed/twisted 
before it hit wave.  Any way I thought you might want to take this info. 
into consideration on your mission.
     
Eric Dovenbarger
[log in to unmask]
 ----------
From: TechNet-request
To: jol; TechNet
Subject: Re: Bow and Twist
Date: Thursday, May 09, 1996 9:49AM
     
     
     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
[log in to unmask]