I have had no experience of pressing solid planes with drilled out holes
but have processed buried vias although these are much smaller.

I would like to ask if you are pulling the vacuum well before you close
up the press. We use vacuum frames and have at least 20 minutes under
vacuum before loading into the press. The problem is that the vacuum
itself seals the pack making it more difficult for the air to seap out.
Once the resin starts to flow it is too late to extract any more air. I
am not familiar with the 106 pre-preg, is this more resin rich than
1080? If the surface of this pre-preg is very smooth, the air will have
trouble escaping. Is the copper surface smooth, if so try roughening it
with a coarse grade brush.

Assuming there is sufficient resin volume present to fill the total
number of holes, the resin flow characteristics are important and I
would suggest you need a rapid heat up rate to achieve maximum resin
flow. If you can speed it up by removing Kraft paper or using hot start
that might help.

One other worry not connected with hole filling is that there could also
be a danger that the surface of the 1080 is very smooth after pressing
and may give you adhesion problems with the final bonding. Perhaps 7628
would be better or you could bond copper foil and etch it off to be
certain you have the best surface topography instead of using tedlar.

Hope this helps and look forward to hearing the solution

Paul Gould
Teknacron Circuits Ltd

>Statement of Process Problem
>
>We are fabricating a 10 layer multilayer PWB with a 0.025" thick solid copper 
>heatsink (12" x 18") in its center, with 5 layers on each side.  This PWB has 
>been previously fabricated using a conventional multilayer layup methodology 
>with all layers assembled and then laminated in one package.  We experienced a
>low yield rate.  It was attributed to shorts caused by bubbles that formed 
>within the drilled holes of the heatsink.
>
>We are currently experimenting with changing the production process to 
>laminate the PWB in two stages - first completely fill the holes in the 
>heatsink with prepreg, inspect and Second - layup the layers as usual.  The 
>heatsink undergoes pattern hole drilling, sanding and black oxide surface 
>preparation treatment.  The heatsink is assembled in the following package in 
>the Multilayer Lamination Room:  steel (12" x 18") lamination plate, aluminum
>sheet, 1 sheet tedlar release, 1 sheet each of 1080 prepreg, 2 sheets each of
>106 prepreg, heatsink, 2 sheets each of 106 prepreg, 1 sheet each of 1080 
>prepreg, 1 sheet tedlar release, 1 aluminum sheet, steel lamination plate.  
>The package undergoes lamination in a sealed vacuum "turkey" bag attached to 
>a vacuum bag pulling 30" hg, in a Wabash single opening lamination press for 
>30 minutes at 375 deg. F & 4500# total force.  Upon visual inspection, the 
>heatsink/pregreg package exhibited bubbles in every drilled hole.  It this the
>result of resin starvation in the holes? 
> 


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