Ian,
Years ago I filled .040" aluminum cores and copper cores.. The method
you described using resin rich b-stage to fill always laked a
completeness for many reasons...not enough resin, air entrapment etc...
We used a core fill material that was a ground powder...It was called
"core fill" it had ground glass and resin that we applied to the cores
as follows:
1. Place drilled core on a sheet of tedlar greatly over size for
neatness.
2. Spread (we used a flour sifter) the core fill powder over the
surface filling the holes make an even coating and vibrate the core
slightly to assure the holes fill.
Note: The filler material needs to settle into the holes so the
vibrating of the prep table or some method of lightly shaking the work
is needed. (or you will have voids).
3. Carefully put the core in a press book and laminate (std. fr4 press
cycle).
4. Sand both sides (a milling operation is a better description) to
remove all the resin material and set the core thickness.
Note: The core should be thicker than final needs as the sanding will
have to take off some core material to completly remove the resin.
This operation was a mess and we did have a subcontrator for a
while...Lacking Sanding Service in a Chicago subburb..I don't know if
they are still in business as their two major customers are now gone...
I will dig thru my old files and try to find the supplier of the Core
Fill material..but it is basicly ground b-stage...I know when you got
the stuff on your hands it had glass in it because you could feel the
scratches...
The process was not perfect..we still found some holes with voids.. but
the corefill powder made it possible.. I don't think we ever made a
good board with the b-stage filling, even with vacuum bags and frames.
Note: do not use vacuum with the powder....or you will clog up
everything.
Doug Jeffery
Electrotek, Inc
[log in to unmask]
414-762-1390
You wrote:
>
>
>We received this via fax at the IPC office and are forwarding to
technet
>for comment. Can anyone help?
>
>Regards
>Dave Bergman, IPC
>
>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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>
>Ian Wylie
>Technical Systems Manager
>Exacta Circuits Ltd
>Printed Circuit Manufacturers
>Selkirk
>Scotland TD7 5EJ
>00 44 1750 21601 Ext 3219
>
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