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January 2004

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Subject:
From:
Robert Sefton <[log in to unmask]>
Reply To:
Robert Sefton <[log in to unmask]>
Date:
Tue, 27 Jan 2004 09:33:19 -0800
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I'm a design engineer, not involved the PCB industry other than as a
consumer. I want to understand the mechanisms of bow and twist in
multi-layer PCBs so that I can specify stackups that will stay flat
during fabrication and assembly. I want is to understand the mechanisms
well enough to be able to get away from the constraint of always
building perfectly balanced PCBs with symmetric stackups. There are
times when unbalanced stackups have advantages, and I'd like to have the
flexibility to use them with confidence.

I've heard a lot of different opinions on how to prevent warping. Copper
balance, balance of laminate and prepreg thicknesses and types, copper
thieving, solid copper ring around the perimeter of each layer, etc.
Others say there is simply no way to predict whether a board will warp
or not. There has to be some science to this. Have there been any
detailed studies of the causes and prevention of PCB warping? Are the
stresses that lead to warp always created during board fabrication, or
can stresses be introduced later on during assembly or rework? Lots of
questions. Any info or pointers to more info would be much appreciated.

Thanks,

Robert Sefton

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