Hi Earl
The attached was certainly a timely post.....
I'm being asked to consider the integration of a CIC (or CMC)
"stiffener" within the structure of a very complex PWB. The board would
have SMT components & BGAs on both sides, so a "bonded" plane stiffener
approach would not work. A traditional mechanical designed "spacer" type
stiffener will not work because of space constraints. The heat sinking
and CTE tailoring attributes are also attractive but not the overriding
factor for trying this.
Like you, I have had to deal with using very thick copper (i.e. 0.060")
innerlayers. However other than the thick copper innerlayers, those PWB
applications were relatively less complex, i.e. big lines, spaces, PTHs,
etc....
As for complexity, the board currently looks like:
1) Approximately 26 layers
2) 15-20 mil Via diameter
3) 4/5 lines/spaces
4) Approximately 140 mils thick.
5) Etc.....
Mechanical folks are calculating the needed thickness for this
stiffener, but I'm assuming 0.060".
Question: Is it possible to laminate 60 mils of CIC or CMC into this
thing and then drill & plate the PTHs? Is this pig really gonna fly?
I'm thinking this is not possible technically/realistically. If I am
mistaken someone out there please enlighten me....
Regards
Gerry
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Friday, November 27, 1998 3:32 PM
To: [log in to unmask]
Subject: Heat Sinks and CIC on PWBs
Michael,
In the good old days. Jees, I'm getting tired of saying this. Anyway, in
the
military/aerospace industry, during warring times, we "heatsank" in so
many ways.
First came heavy copper, or was it aluminum, followed by CIC core
material. Considering heavy
copper or aluminum, we were able to either buy heavy weight clad
material or attach etched
(chem milled) heat sinks to outer layers (over .060" we machined
mechanically). As copper is
a pretty good heat conductor, often this was used. The weights were in
the range of 8-11
ounces. Sometimes, we even attempted to use only one sided multilayer
metal planes - very
exciting.
Copper Invar Copper (CIC) was developed and used primarily as a core
material (.060" thick in
the MLB center with clearance holes, epoxy fill, re-drill, plate, etc. -
first) to achieve
TCE matching characteristics close to ceramic in the leadless ceramic
chip carrier days
(LCCC's).
Then came CIC not as a constraining/heat sink core (.060" thick), but as
a tailoring core
(electrically and thermally) that was .010" thick placed very cose to
the MLB's surface
providing all the thermal and TCE matching attributes as CIC core
without the headaches
associated with multiple processing requirements.
Many others have used heavy copper cores (up to 10 oz.) as heat sinks
placed in the MLB
center. I've done all this and it's all exciting - especially the part
when you laminate and
expect to achieve heavy plane resin fill, high dimensional stability,
and required laminate
integrity.
Which works best? So many have tried and so many have failed or
succeeded. We succeeded most
with matched weight heat sinks bonded to MLB outers while remembering
these were the through
hole only days - mostly.
I have done internal copper heat sink cores for SMT as early as 1982. To
effect efficient
heat transfer, "heat pipes" or relatively large through holes plated to
the "sinks" were used
as it was often difficult to drill and plate small diameter holes in the
same way.
I could go on, but I'm getting tired of trying to remember all I forgot.
If you would like to
discuss this more off line, contact me. Of course you could hire me as a
consultant, but I'm
not very good though very cheap. Oh yes, I have some pretty/ugly
pictures as cross sections
you may wish to see - no charge but have plenty of time and disk space.
Earl Moon
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