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New for July 2010! FR-4 is a low-cost printed circuit board material, manufactured from fiberglass cloth embedded in epoxy resin. The "controlling authority" on FR-4 is National Electrical Manufacturers Association or NEMA.
The "FR" in FR-4 stands for "fire resistant". It has generally replaced the (flammable) board material G-10 because of this property.
Many engineers write "FR4" instead of "FR-4". The correct version is "FR-4", but it's not worth an argument!
It is possible to create low-cost RF circuit cards on FR-4, but the losses will always be much higher than on PTFE-based boards from reputable suppliers. You can expect significant performance variations from one assembly to the next. Because of the high loss tangent, don't try to make any filter structures!
Properties of FR-4
ER=4.7 (has been reported between 4.35 and 4.8, and is slightly frequency dependent and varies by manufacturer and lot-to-lot) When it comes to delectric constant, FR-4 is an example of an anisotropic material.
tanD=0.018 (which really stinks! and varies by manufacturer and lot-to-lot)
FR-4 is rated to 140C by Underwriter Laboratories.
There is more FR-4 data available in Wikipedia.