Not detracting from Steve's needs but these very large high layer count
boards have been around awhile. Not blowing my horn, but did Amdahl MLB's
first in the early 70's as Multi Chip Carrier (MCC) types for the first main
frames by Gene Amdahl but later replaced by 10"x10" ceramic substrates
supporting chip carriers with cooling towers and manifolds carrying water as
the coolant.

Cray used 52 layer boards early on in their super computer development and
beyond. The density years ago and today, as I marveled working there, is
almost beyond belief especially considering the incredible interconnect, to
the outside world, system using current induced flex connections on 10 mil
centers. A really good engineer did that and is now a damn manager but
still highly capable. Bare boards cost $100K. Assembled a bit more. Yikes!

Involved in recent Cray 42 layer development boards (we had to have a rework
plan as well) for MCM's with 3400 I/O's and "normal" 1200 I/O CCGA's. The
MCM's are cooled using the fruoronert stuff used in previous designs but now
manifolds deliver coolant as vapor much as in older vapor phase reflow
processes. You should see the new system plans. They're almost like the
"chunnel" connecting Britain and the continent.

Did a 60 layer type but all through hole designed to replace 24 layer boards
with wire wrap pins and wire above surface. The attempt was to eliminate the
wire wrapping by putting it all internal to the board and eliminate the wire
wraps. Missles still flying - only when fired of course.

Conversion from through hole to SMT rare for such large high layer count
boards. Usually start out and stay surface mount.

Has to be done considering high I/O count and all the electrical
performance, impedance and all, requirements to be met. Actually somewhat
simple but requiring lots of layers and careful design, fab process, and
assembly management. Just a few doing it. Damn fun, what?

Moonman

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