Genetlemen,

The definition of FR-5 is a laminate base material that will
retain 50% of its flexural stength at 150xC (it may be 125xC).  This
defintion was coined by NEMA before we had such base materials as polyimide
or BT.  The orginal FR-5's were white in color and had a much different
curing agent than any FR-5 available today.  FR-5 sold today is nothing more
than an enhanced FR-4.  The glass transition temperature is an indirect
measurement of a material being an FR-5 but the original flexural strength
defintion is the one that counts.

Doug Sober
IPC Base Materials Chairman