Forwarded message:
From:     Self <Single-user mode>
To: [log in to unmask]
Subject: Re: FAB(Shifting Layers)
Date: Fri, 21 Mar 1997 08:45:45


> 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).
> 
> We would appreciate any suggestions from those who have seen similar 
> occurances.
> 
> Patty Causey / Marty W. Sheppard
> Process Engineering Section
> email address:  [log in to unmask] / 
> [log in to unmask]
> Phone: (912) 926-9473(9632)
> 



There's a couple of things you can do to fix this problem.
1. Go to a foil construction if you can. The all core construction 
with solid copper on the outer layers tends to"lock in" the outer 
most cores, where the inner cores will shrink as expected, 
unconstricted. This is seen more with the use of aluminum seperator 
plates than with steel plates.
2. If all core is the only option, etch off all the copper on the 
outer layers and replace with foil and a single ply of prepreg at 
lamination. This prevents the "lock in" effect.

In addition, 3 plys of the same glass style of prepreg should be 
avoided, especially a light glass like 1080. These tend to "swim" on 
each other during lamination. Try to stay with 2 plys, say 106, 2116 
or 1080, 2313.
Robert P. McGarry jr
Technical Sales Analyst
Proto Circuit Inc.
2035 Midway Dr.
Twinsburg, OH. 44087
216-425-4939 fx. 216-425-9704
[log in to unmask]

"QUALITY MANUFACTURED PRINTED CIRCUIT BOARDS"

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