Hi, Franklin,
The first answer I would ask myself is "Is conductive epoxy filing really
necessary?". It's an axtra process, and not especially cheap, particularly
if you're filling blind vias after laser drilling.
Things to look at to determine the effectiveness of the process are:
-epoxy type - most people seem to go for DuPont's CB101, as it has a small
particle size, fills smaller holes well and doesn't suffer from out-gassing
problems in a properly established process..
- hole diameter and aspect ration that you're intending to fill - 6:1
aspect ratio is about the limit, with the minimum diamter being around 12
mils. I have had 24 mils diameter holes filled that are only 7 mils deep,
which is an aspect ratio of about 2:5. Bigger than that and the epoxy sags.
Don't know what the upper diameter limit is, but I doubt you would want to
be filling very large holes.
- CTE relative to your board material, though I haven't encountered any
problems with that so far.
Regarding the process sequence offered to you, I would say "OK" only as
long as you're not mounting anything on top of the holes (e.g. BGA
via-in-pad) or don't need a flat surface on the side of the board that's
opposite the squeegee. If you do, then you need one more process, so the
whole thing would be:
electroless copper
flash copper
3 mil resist cleared 5 mils around the vias to be plugged
squeegee the conductive epoxy (from one side till 'tails' form on the back
side)
tack cure
final cure
GRIND FLAT
clean
Final Image
Copper II
Done nicely, you would almost not know that there had been any holes there
at all. We initially used conductive epoxy to help maximise a thermal path
through the board, but actually a very small increase in hole plating
thickness gives a much better result, and removes one process/cost. The
grinding has to be done carefully so that the epoxy doesn't overheat and
smear.
Hope it works out for you.
Peter
Franklin D Asbell <[log in to unmask]> 09/01/2003 10:29 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Franklin D
Asbell
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Conductive Silver Epoxy
We're about to begin developing a process for plugging vias with conductive
silver epoxy. We've had the vendor's technicians here explainging the
processes, providing material and literature, and our experimenting will
begin later today...just wondering what all of your thoughts and/or
experiences are/were on this topic???
What (they've) we've planned out is;
electroless copper
flash copper
3 mil resist cleared 5 mils around the vias to be plugged
squeegee the conductive epoxy (from one side till 'tails' form on the back
side)
tack cure
final cure
clean
Final Image
Copper II
etc etc
Thanks,
Franklin
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