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