-----Original Message-----
        From:   Jim Mathis [SMTP:[log in to unmask]]
        Sent:   Monday, February 16, 1998 3:53 PM
        To:     [log in to unmask]
        Subject:        [TN] PCB warpage

        What causes PCBs to warp?  How can we prevent this from
happening?
        (Thanks to all responses in advance!)

In general warpage is caused by unrelieved stresses built up in the
fabrication process.  The most common cause  for these stresses is an
imbalance of copper from layer to layer (i.e. a single plane layer on
the third layer of a 4 layer PWB.)  Other things that can contribute to
this are an odd number of layers, and imbalanced stacking of dielectric
between layers.
        To control this you need to do three things: 1) design PWBs with
an even number of layers, 2) make the stack-up of the PWB symmetrical
about the center of the boards thickness, and 3) balance the copper to
etch from layer to layer.  Following these guide lines should allow your
fabricator to supply you with PWB that meet level C requirements for Bow
and Twist as outlined in 5.2.5.1 of IPC-D-275.


Jeffrey McGlaughlin
[log in to unmask]
Columbus Ohio

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