Good thorough response Adam! Thanks and Hello. (Again)


        Hadco Printed Circuits
Tech Center Two / Watsonville                


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Dave Hoover              (408) 728-6677 
Senior Process Engineer  (408) 728-1728 Fax 
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______________________________ Reply Separator _________________________________
Subject: Re: Burp Cycle!
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    10/18/95 2:09 AM


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