Hi Gerry,
I wonder what your primary need is for the board. Is it for thermal
relief or is it for the CTE constraint, or a combination of both?
I worked with the pioneering effort at TI on the constrained core approach as
well as some high powered boards which needed extensive thermal relief ca. 95
watts. We started off with 40 to 50 mil CIC and later used 35 mil CMC to
lighten things up. One thing you must bear in mind; if you need the CTE
constraint, the stress at the CICor CMC/board interface gets to be quite high
due to the differential CTE's. You must have a very good bond and don't bump
the boards around much. If you on the other hand only need the thermal relief
or path, the Copper is much more forgiving because of the better CTE match. In
the past, and we still do, we built and build balanced construction boards of 8
layers on each side of the CIC or CMC core, yielding a 16 layer constrained
core board. As you begin to get further away from the core you begin to lose
the 6 ppm CTE that is required for the LCCC match. Most of our designs have
been in the 5-6 mil L/S areana and 8 layer per side has sufficed. I'm not sure
how your 26 layer boards will fit. We were able to obtain good reliability on
these boards up to 84 I/O LCCC's. Smaller device packages, such as mini BGA'a
might still be ok.
If I can be of assistance, feel free to ask.
Regards,
Les
> From: "Gerald G. Gagnon" <[log in to unmask]>, on 12/11/98 11:03 AM:
> 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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