Hi Michael,

Are there large plane areas that are not evenly distributed across the
board? 
Is it possible that you are seeing warpage due to uneven CTE of the
materials when applying heat?

Bill Brooks 
PCB Design Engineer 
DATRON WORLD COMMUNICATIONS INC. 
3030 Enterprise Court 
Vista, CA 92083 
Tel: (760)597-1500  Ext 3772  Fax: (760)597-1510 
e-mail: [log in to unmask] 
 



-----Original Message-----
From: Michael R. Forrester
[mailto:[log in to unmask]]
Sent: Friday, November 10, 2000 12:07 PM
To: [log in to unmask]
Subject: [TN] PCB Warp & Twist and then Flat again?


A batch of PCBs we received twisted during reflow.  We sent a few bare PCBs
from the same batch through the reflow cycle and as expected also
twisted.  The strange thing is that the PCBs are "untwisting" as time goes
on.  The twisted PCBs returned to flat after 20 minutes.  Other boards
twisted
during the board bake-out before assy.  On one board we originally measured
a bow & twist of .450 in. per IPC spec.  One week later it is down to under
.200 in. just sitting on a shelf.  The memory usually is in the twisted
mode not the flat mode.  The boards are 6 layer FR4, cap lamination
(cores on the outer layers) with a well matched construction and are all
from the same date code.  Our PCB house has never seen this before, and has
not explanation for it.  Since we do not have a resolution on the problem I
am concerned we may see it again on another date code.  Has anyone ever
experienced this or have a possible explanation for it?  Are there any
suggestions on what further testing can be done to come to an answer?
Thank you.

Best Regards,

Michael Forrester
Sr. Manufacturing Engineer
LeCroy Corp.

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