More information is needed :
How are you storing your prepreg?
 Do you dry nitrogen or vacuum pull before lamination?
Are you using vacuum lamination?
If not how much pressure, are you getting all the air out?
What does your boarder pattern look like, and how thick is the copper? Both 
can affect non vacuum lamination air removal
The idea of heating the prepreg could affect how it bonds the board at 
lamination  and probably should be avoided
 ----------
From: [log in to unmask]
To: [log in to unmask]
Subject: FWD>Laminate Voids
Date: Thursday, May 23, 1996 6:10PM

Mail*Link(r) SMTP               FWD>Laminate Voids

<[log in to unmask]> inquired about "laminate voids" with the
following:
 >We are looking for some help with a processing anomaly which surfaced > ~ 
six months ago. We began to see laminate voids in our epoxy boards. 
>Polyimide does not seem to be affected. We have verified lamination 
>cycles, investigated layer clean, oxide thickness and pre-clean. >Solutions 
seem to remain random. We are running low on ideas.    >Has anyone out there 
experienced the same problems? Solutions? Riston >residues? Conversion 
coatings?

>From: [log in to unmask]

You probably have volatiles absorbed (moisture or solvent(s)) in the base
(core) material or in the prepreg.

If you have subsurface voids, in your cross sections you should be able to
tell if the voids are in the core or prepreg

If the voids are in the core material, then it is a supplier problem.

If the voids are in the pregreg layers of the printed board then it can be
either a core or prepreg material problem.  In both cases, volatilize-out 
the
materials prior to lamination using some form of desiccation such as heat
and/or vacuum.
If the voids are surface and you are foil laminating, you most probably have
volitles in the prepreg and need to desiccate as above.

Ralph Hersey
e-mail:  [log in to unmask]