In addition to the other responses you have had on the subject of foil
lamination versus core lamination, I have one more piece of information.  If
you are using a single-sided core as the "cap" layer, the properties of a
single-sided thin core laminate are not always the same as the corresponding
double-sided core laminate.
Most laminate vendors produce single-sided core laminates back to back i.e.
with their unclad sides together with a release film in between them.  The
purpose of the release film is to provide a textured surface to which the
prepreg can bond adequately during the final lamination step at the customer.
 Two things happen when pressed in this fashion. First, some of the
plasticiser from the release film can penetrate the surface of the resin
lowering the effective Tg especially if the core is very thin.  The second
issue is that the mat surface has a greater surface area and moisture can
penetrate more easily.   

For these reasons, foil lamination is the preferred lay-up over the use of
single-sided caps.

Doug Sober
IPC Base Materials General Chairman