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Date: | Wed, 16 Jul 1997 13:40:10 -0700 |
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One thing that works relatively well to lower the amount of expansion
on the ground plane is to create a mesh plane rather than a solid
ground plane. Mesh density would depend on current loading and
whether you have impedance lines depending on that ground plane.
Frank Hinojos
Watkins-Johnson Co.
______________________________ Reply Separator _________________________________
Subject: Assy: Rigid-Flex Thermal Processing
Author: "Ploughman; Mark" <[log in to unmask]> at INTERNET
Date: 7/16/97 12:05 PM
We have a rigid-flex assembly that consists of 4 modules (4 layer -
polyimide/FR-4) linked by flex (1 layer polyimide). Layer 2 on the
modules is a ground plane. When the assembly enters reflow, it buckles
in response to the heat. We have confirmed that this action is in
response to the thermal expansion of the copper ground plane which has
an expansion coefficient significantly higher than the surrounding
material. There is inadequate mass in the surrounding layers/substrate
to constrin this buckling (we call it a "bimetallic strip effect"). We
have kicked around a few alternatives rangining from modificati9ons in
design (ie. eliminate ground plane) to better constraining the board in
the assembly fixture. We were somewhat concerned about constraining the
board further due to potential shear loads on the vias.
Does anybody have any ideas/solutions which I can add to the list????
We're still pretty green with this technology.
Thanks,
Mark Ploughman
Production Manager
Lotek Marine Technologies Inc.
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