I have not run FR-4 for a long time but that sounds like excessive movement to 
me.  I once worked for a laminator that made thin core and prepreg most of which
was FR-4.  We also did mass lam.  I believe we usually had less than 0.5 mils 
movement per inch even on designs that had the worst movement.

To make the prepreg you have to drag and pull glass cloth through the treater 
and so there will be movement.  Talk to your material supplier about this 
problem, they might suggest another construction to reduce movement.  More resin
means more movement.

Good Luck
Chuck Brummer
[log in to unmask]

______________________________ Reply Separator _________________________________
Subject: FR-4 material movement
Author:  [log in to unmask] at ccUnix
Date:    2/14/97 4:58 PM


My previous posting did not yield any suggestions. I'm submitting again and 
hope to have responses.

>Date: Fri, 07 Feb 1997 10:25:09 -0500 
>To: TECHNET
>From: sam mccorkel <[log in to unmask]> 
>Subject: FR-4 material movement
>
>We experience up to .020" material movement on FR-4 materials.We ran some 
tests and measured after each operation and found that it began to move after 
etch, and moved a lot more after screen bakes and hot air leveling.We have 
not seperated panels by grain direction in our tests so far. We've tried 
baking before drilling and have not seen a significant difference from those 
not baked. The movement we experience causes major problems when we rout or 
score. We have to internally pin each piece to hold a =B1 .005" tolerance 
between a drilled hole and the edge of the part.We used to be able to pin 
externally with 4 pins to rout our boards and had no problems. Internally 
pinning adds a lot of labor with panels that have many parts on them. This 
problem cropped up on us about 10 months ago. Before that it wasn't a 
problem. Are other fabricators out there experiencing this same problem? Is 
there anything we can do to reduce the amount of movement? Should the 
material move this much with "normal" processing of 2-sided boards that are 
.062" thick? Do you have to internally pin each part on your panels?
>

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