Frank,

Aramid fibers (Kevlar TM) do have a negative CTE.  The CTE of a board
made with them and a resin system (epoxy or polyimide) is a function
of the percentage of each in the resulting board.  I have seen plots
of board CTE (from experimental measurements) as a function of the
number of layers in a Kevlar board.  A 2 layer board (1 core, no
prepreg) has the lowest CTE.  As you add layers and prepreg (that
typically have more resin than fibers) the CTE goes up.

Other materials can be used to lower a board surface CTE.  There are
two main techniques that I have seen:

1) Reduce the CTE of the core materials.  There are two subcategorizes
to this:
1a) Reduce the CTE of the fibers.  Kevlar is one method as well as
Quartz (small positive CTE).  Kevlar fibers come in two forms, woven
fabric and random orientation paper (AKA Thermount TM).
1b) Reduce the CTE of the electrical conductors.  TI makes a
Copper-Invar-Copper foil material to use for electrical conductors.
Contact Jill Cusson, Phone: 508-236-3212,
mailto:[log in to unmask] for more information on CU-Invar-Cu.

Arlon http://www.arlonmed.com (among others) makes PCB materials using
both techniques.

2) Add low CTE material layers between conducting board layers.  There
are two subcategorizes to this:
2a) Laminate with prepreg.  This effectively makes a thicker board
with integral controlled expansion layers. This technique can allow
PTHs to connect between layers. If you pick your materials wisely, you
can also get better thermal conductivity (K) as well.  I have seen
this done with a wide range on materials some with low CTE (e.g.
Invar), some with high K (e.g. Aluminum) and some with both (e.g.
AlSiC, Be, BeBeO, Carbon-Carbon, Cu-Invar-Cu, Cu-M-Cu, etc.)

See http://www.dyconex.com/html/technologies_products/applications.htm
for some examples of this.

2b) Laminate two PWBs to a center "heatsink" with adhesive.  This
technique uses two PWBs with components mounted on one side of each
board.  The heatsink can be any of the materials described above and
the adhesive can either be rigid or flexible, depending on the desired
result.

A design that I have worked on recently uses 2 kevlar PWBs (Arlon 55NT
Epoxy Thermount) bonded to a center Al heatsink with Arlon Thermabond
(a very flexible silicone elastomer adhesive).  This combination gives
low board surface CTE, high thermal conductivity, good vibration
damping  and a reasonable cost.

There are an almost infinite number of combinations and permutations
that can be explored based on the above mix of materials and assembly
techniques.  I hope this helps answer your questions.
--
Karl Sweitzer
Eastman Kodak Company             voice: 716.47.77546
800 Lee Road                        fax: 716.47.77293
Rochester, NY 14650-3118         mailto:[log in to unmask]

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