[SI-LIST] PCB material with er=2.5?

All,
   I'm doing a board that needs 8 stripline layers running edge-coupled
diff traces at circa 115 ohms, and a total of 22 layers.  Using Fr4 with
an er of 3.85 gets me 25 mils per stripline layer (ie ground plane to
ground plane).  Makes the board a tad thick to say the least.  Using an
er of 2.5 gets me 12 mils per.   Anyone know of a usable and ecconomic
material that can be used in this application, and which we can get UL
approval for?  
Thanks

Dave Instone. Compliance Engineer
 Storage Systems Development, MP24/22
 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
Tel: +44 (0)23-92-496862 (direct line)
Fax: +44 (0)23-92-496014
http://www.xyratex.com  Tel: +44 (0)23-92-496000
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