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.
     
     [log in to unmask]
     
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