Hi Mike,
I have experience with both Thermount® and PTFE (Teflon®) substrate material 
in PWBs.
Thermount is an aramid reinforcement combined with a thermoset resin matrix 
such as epoxy.  The x,y CTE values are in the range of 14-18 ppm/C° driven by 
the aramid structure.  The z-axis radial expansion of the aramid fiber is 60 
ppm/C°.  The particular resin system selected will also drive the z-axis 
value upward at its glass transition temperature.  So for the aramid case the 
substrate will expand during processing where the glass transition 
temperature is approached or exceeded.  In addition, the moisture absorption 
of the raw aramid fiber is in the range of 3-4% but in the cured resin 
matrix, it is  less.  

For the PTFE case, the substrate is a thermoplastic subject to pressure flow 
(but restrained somewhat by the glass or other reinforcement that might in 
the substrate), therefore this substrate with expand or grow in the x,y 
direction.  The moisture absorption of the PTFE is insignificant.  

Hope this helps

Lyle Wallig

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