Thermount is a registered trade name.  A web search of Thermount will give
huge blocks of info.  The following is my experience.

Thermount advantage is XY CTE in the 7-9 range verses FR4 of 13-15.  Unlike
the "fiberglass weave" or E-glass  FR4 Thermount dielectric (B & C stage)
uses Kevlar.  The Thermount differs in the "Ole Kevlar" configuration in that
it is non woven.  [Non woven is an odd word, it is akin to referring to
wooden particle board as "non woven."]
The reason for the Kevlar "bits" is to eliminate or greatly reduce the large
Z axis expansion of the woven configuration.  There is the ability to have
PWBs built in either Polyimide or Epoxy Thermount configurations.
Two cautions from my own experience; Thermount is high in moisture absorption
and being non woven the CTE is effected by copper percentage (adjacent to
plane will exhibit higher CTE numbers).  A lesson learned for me is increase
the worst case dielectric distance between outside diameter of plated through
hole barrel and clearance of plane.  The small quasi spheres of Kevlar can be
dislodged at or after drill and (be it ever so small) the Z axis expansion is
still present albeit relative to itself.
I am a Apps Eng for Coretec and we build Thermount all the time
(www.coretec-inc.com) as well as other shops, it is not that exotic.
Alternatives to Thermount are the good ole CIC which is a Texas Instruments
Tradename for a trimetal (Copper Invar Copper) contained inside the PCB that
acts as a constraining layer or layers with dual function as electrical
planes (also manufactured by Coretec at our SAS facility).  My favorite
method to "manage" these pesky LCCC's... never use more than a 28 pin device
(okay, maybe 32... but that's upper limit) AND when you do go higher than 28
attach leads (I like Penn, they do a nice job).  LCCC's (especially high pin
count) are usually thermally hot, make sure your thermal plan is a systems
approach...

Sorry for the shameless company plugs...

Boston Brad
781 858 0783

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