Dave
It sound like there just isn't enough free resin to fill the .025"
cavities. Thats laminating 18 oz copper! I would not expect 6 pieces
of prepreg to fill 18 oz copper.
If the volume of AVAILABLE resin was calculated and compared to the
volume of the cavities which are trying to be filled, I predict that the
cavities would have the greater volume.
Other factors, how much AVAILABLE resin gets squeezed out the edges,
converting it to UNAVAILABLE resin? AND How far does the resin have to
travel to fill the .025" cavities, does it have enough time to travel
that far, ie the time between melting and curing?
I used to have a "rule of thumb" on the number of prepreg sheets needed
to fill different copper thicknesses, but left it in a past life.
Perhaps, others have developed similar layup aids?
George Franck
Raytheon E-Systems
Falls Church Va
These are the opinions the author, and do not reflect on Raytheon
E-Systems, which is a pretty good place to work.
On Jan 23, 3:53pm, David Bergman wrote:
> Subject: FAB:
>
> 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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>-- End of excerpt from David Bergman
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