Another suggestion would be to decrease the number of sheets of 
     prepreg used in each opening. The higher resin content of the 1080 
     materials could be a contributor to the problem. Try maybe a 2116 and 
     2313 to get the same (approx.) thickness.
     
     
______________________________ Reply Separator _________________________________
Subject: Re: FAB(Shifting Layers)
Author:  [log in to unmask] at INTERNET_GATEWAY 
Date:    3/20/97 5:19 PM
     
     
In message <[log in to unmask]>, 
[log in to unmask] writes
>We are experincing a reaccuring problem with our lamination process on 10 
>layer projects with the following stackup: 
>
>1.  1 oz copper on all 5 FR4 5 mil sheets;
>2.  3 sheets of PGFK1080 2.5mil prepreg in each gap (total of 7.5mils each 
>gap); 
>3.  Grain of prepreg is in the same direction; 
>4.  finished thickness 69 mils;
>5.  layers 2-4 & 7-9 are signal layers; 
>6.  layers 5&6 are ground/VCC planes.
>
>Problem:  We are consistently seeing layers 2 and 9 shift 3-4 mils more than 
>the other layers (3:4, 5:6, & 7:8).  Layers 3:4, 5:6, & 7:8 all align well 
>with each other and the outermost sheets are shifted to the right by this 3-4 
>mils.  Once layers 1 and 10 are imaged onto the drilled panel, they also line 
>up well with the innermost layers (3:4, 5:6, & 7:8).  We are losing panels due 
>to annualar rings less than 2 mils on layers 2 & 9 (Mil-P-55110).
     
It seems you are getting good alignment with the inner panels and only 
the outer ones are shifted so the more common technique of using copper 
foil for the outer layers should solve your problem. Four double sided 
panels will make your inner 8 layers with pre-preg and copper foil on 
the outside and you should also find it cheaper to produce this way.
     
I can't explain why you are getting this slippage unless your platens 
are not quite parallel and the tooling pins are unable to stop some of 
the slip.
     
If you have buried vias or other reasons for the construction, then 
consider single sided panels for layers 2 & 9 and use foil for the outer 
layers. Don't use single sided laminate but etch off one side to get the 
best bond strength.
     
Good luck.
Paul Gould
Teknacron Circuits Ltd
     
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