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January 1997

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From [log in to unmask] Fri Jan 24 09:
03:32 1997
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[log in to unmask] (Ian Wylie)
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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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