Brian,

I believe you are referring to a "kiss" cycle which some shops utilize as
part of their multilayer panel lamination cycle.  

A kiss cycle is contact pressure (usually about 50 psi) in the hot press for
a set time, until "kick-over" to "high pressure" (usually 150-225 psi with
vaccuum assist hydraulic presses) for the remaining time of the hot press
portion of the cycle.

If a "kiss" cycle is thought to be necessary, the time duration of the kiss
portion of the cycle is best determined via a fluidity curve done for you by
your b-stage supplier.  The fluidity curve will be based upon the resin
system used, and your average heat rise (measured via thermocouples) at the
center and outer panel(s) in a book.  The optimum kick over time ("x" minutes
into the hot press cycle) will be the "between point" for the minimum
viscosity curves of the center and outer panels, on the time axis of the
curve.

In my experience, the intention of the kiss cycle is to apply pressure when
the resin, throughout the stack of panels, is in a state of flow, with the
theory being that this will help to ensure complete encapsulation of all
circuit features.  

Forcing air and volatiles from the panels is more effectively achieved with a
properly charaterized press cycle, matched to the resin system. And vacuum
assist is of course the ultimate addition to ensure complete evacuation of
volatiles.

Some Practical (I hope) Comments On Usefulness & Applicability:
1.  Some of the people who have been in the industry longer than I may have
further comments on this, but I believe that "way back when", when very high
lamination pressures were used, and there was no (or little) vacuum assist,
and most people used a steel separator between each panel, and there were
heavy (and not very well controlled black oxide coatings) - a  "kiss" cycle
was used because there was concern that applying high pressure to the boards
prior to resin flow could shear or crush the oxide growth that was supposed
to enhance the bond, thus resulting in a poorer bond.

2.  I believe that some resin systems are formulated, via scale flow, with
the intention of having them be the best resin systems to use in processes
that utilize a kiss cycle.  I also believe that in todays "controlled and
monitored" raw material manufacturing processes, and board fabrication
proceses, that kiss cycles are often not necessary, but are more something
that provides an extra level of comfort for those who use them.    

3.  One situation where a kiss cycle is very necessary, is when laminating
with aluminum separators and using prepreg with what I have heard referred to
as "resin bumps".  (My experience is that resin bumps is a rare occurence -
except on the resin-rich "thick plys", such as the 7628's that yield 9+ mils.
 In the field, I have seen quite a bit of this material with resin bumps.)
In a "straight high pressure cycle", with aluminum separators, the aluminum
is forced to form to the shape of these bumps prior to the resin flowing, and
then when the resin flows and cures the outer surfaces of the panels can have
an "orange peel" texture that is quite ugly and may impact outer layer
processing.
In this scenario, adding a properly characterized kiss cycle will eliminate
the surface condition caused by the resin bumps, because the resin bumps will
flow prior to application of high pressure.
 
Another situation where I will recommend that customers try a kiss cycle is
when they have an intolerable level of image transfer.  Depending on the high
pressure being used, and the copper weights on the inner layers, a kiss cycle
will sometimes help to further reduce the image transfer evident on the outer
layers after lamination.



Sorry for the long winded answer, I hope this is helpful,

Adam Davies
Product Manager, CACr Lamination Foil
JJA (Johnson & Johnston Associates)
Office - (603) 329-5691
Voice mail - (603) 329-7215, Ext 108