In agreement with Tom's comments, the major difference is in the thermal properties of the laminate, not only Tg, but max operating temperature. Typical value for FR-4 maximum operating temp is 130C, while FR-5 is 170C. Electrical properties are nearly identical, and some physical properties are significantly different. A solder float test value for FR-5 is about 2 times the tolerance of FR-4, and dimensional stability of the FR-5 is superior to FR-4. FR-5 can meet many of the tough requirements for military and commercial high density SMT applications. David Mander Sun Coast Supply