Mail*Link(r) SMTP               FWD>Bow and Twist

John:

Per the intent of the specifications and test methods, you and the printed
board (manufacturer) supplier are correct.  The test specimen is placed flat
and unrestrained on the reference surface plate.  The residual bow/twist you
have are within the normal "unless otherwise specified" acceptance
requirements.

You concern is that your customer did not adequately document the requirements
for the printed board and it's assembly.  I suspect what has happened is your
customer had "hidden" requirements that were not documented and place on the
product definition drawings (master and assembly drawings).

My suggestion if for you to have your customer document the final acceptance
requirements for the product, and then establish (and document) the
requirements for the printed board and it's assembly.

Ralph Hersey
e-mail:  [log in to unmask] 

--------------------------------------
Date: 5/8/96 12:35 PM
From: John Loveluck
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]


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