David,

Metal Core Printed Wiring Boards are composed of three subassemblies (SA). Two SAPWBs and the prepared Metal Core.  The designs of SAPWB-A and SAPWB-B must be conservative due to the inability to electrically test prior to the final lamination set.  The two SAPWBs should be symmetrical and the layer facing the MC needs to be predominately copper; if it is a plane all holes must have pads... if a signal flood with copper.  The will greatly improve the SA to MC bond line.  Buried vias within the SAPWBs need to be avoided as the number of lamination sequences will begin to deteriorate dieletrics; especially Polyimide.  The MCPWB configuration is excellent for Via In Pad and I recommend using it right from the beginning.  SAPWB layer to layer separation needs to be .005 nominal.  External layer line widths and spacing facing the MC side need to be .005 while MCPWB external line and spacings need to be .007 and .007 due to heavy copper.  In! ner layers can be .004 and .004, preferably .004 and .005.
The Metal Core itself needs to be prepared prior to lamination.  Pre machine so the MC edges are .060 in of MCPWB edge.  I suggest EDM depending on core type reducing induced stress, followed by re-annealing to eliminate bow and twist; oil canning will kill the MCPWB.  After drill aggressive etch to deburr and reduce stress points.  Design of the core needs to allow minimum web and large clearance to the MCPWB PTH.  MCPWB PTH that contact MC can be quite challenging and should not be electrically Mission Critical. 
Lamination of the set varies from Suppliers.  Two significant camps are: Single lamination including resin fill for MC holes and cavities and; (Second Camp) prefill holes and cavities of MC, plannerize, then laminate.  A critical component of reliability is the oxide to MC, without it delamination will occur.  A very nice non destructive method of final lamination integrity is submerged water ultra sound.
MCPWBs require strict adherence to design rules.  The cost is very, very expensive and thus is regulated to smaller volume, high reliability types of usage.  Assembly will be costly as well.  Design in "spare" holes for future modifications and wiring.

Reexamine the two single sided assembly boards that can be bonded to a cold frame it is the best packaging approach.  The advantage of MCPWB is the thermal management, some CTE control (depending on MC material and SAPWB thickness), and the direct I/O to the SAPWBs.  In the cold frame approach signals must migrate to the "over the edge" connector.  This "down the drain" signal flow is reversed and spreads across the adjoining SAPWB.  An MCPWB PTH eliminates this effect very nicely.  Multiple stacked Metal Core Boards interfacing are ideal candidates for "Z axis" only I/O, such as Elastomerics or compression type configurations which saves much cubic area and eliminates half of mechanical I/O and associated cabling.

I have only touched the surface of parameters... Let me know if you want me to continue because I can go on and on.  Good fortune in your endeavor.

Boston Brad

Bradford Saunders
PCB Product Engineering
www.CORETEC-Inc.com
781 858 0783