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January 1997

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Sun, 26 Jan 1997 21:57:06 -0500 (EST)
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Dave Bergman

Perhaps you will forward following info to the member who needed data on
getting the holes filled in his center located heatsink.  

It appears that the six sheets of prepreg are a bit short when it comes to
filling 0.25 inch.  I know that voids are often experienced when I have
triedt to fill 0.010 with 4 sheets.

You can play with the press cycle and try to close the press at just the
right time when the outer edge resin is still a bit stiff and the center
section is at a low viscosity; hopefully pushing the trapped resin in the
holes   A safer method is to squeege a thixotropic filled resin into the
holes of the oxide treated heat sink and it  will fill them about 80%.  Then
let the filler resin at least harden and partially cure at room temperature
so it will not run all over during the final cure.  Then you can follow your
previously described  process or you can finish the hole filling with the
preprg during the multilayer lamination process.  A small amount of the
filler material may be left on the surface by squeegee drag, but if it is a
good insulator and will not create any problems.  If you are later drilling
thru the holes and plating them through the filled resin will have better
resistance to cracking than the unreinforced resin that came from the
prepreg.  Arlon and several other companies sell hole-fill material that it
used for the same purpose that you require.

Phil Hinton 
Hinton "PWB" Engineering        

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