Actually BT suppliers provide lower CTE values (5-6 ppm -x-y) above
Tg.
Yuan
Dee
I am not a board supplier, but work with many suppliers and
users to see that they get what they need in the Printed Board industry.
A few notes:
If you are operating below the glass
transition temperature (Tg), of the epoxy in epoxy-glass boards, you do not
need polyimide to prevent high thermal expansion changes CTE change.
Polyimide has a Tg of 220-250oC depending on the resin system and high
temp epoxy has a Tg of about 170oC. Epoxy-glass and Polyimide-glass both
have a CTE of about 16 ppm/oC in the X-Y directions below the TG temperature.
To reduce the X-Y expansion characteristic of polyimide and
also epoxy boards the industry uses planes of copper clad Invar (CIC)
internally in the multilayer board, these may be either passive or functional
power/ground planes. This will reduce the CTE to about 8-10
ppm/oC. We use this technique mainly in the oil drilling industry
where operating temperature for the electronic units that go downhole are in
the 180-200oC range. They typically use polyimide material with a
nickel, palladium, gold surface finish.
A couple of suppliers in tyhe
US make good quality polyimide CIC boards for this industry in short orders, I
can give you their names, if you so desire.
If you have any questions
give me a call. I am located in Tucson at 745-1013.
Phil
Hinton
520-745-1013
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