G7 Silicone Glass Sheet

G-7 is not an epoxy laminate at all - it is woven glass cloth in a silicone resin, and it behaves differently enough that it should not be treated as a hotter G-11. It meets NEMA G-7 and MIL-I-24768/17 Type GSG. Two properties are why it exists. Maximum service temperature in air is 220C, with one maker publishing thermal resistance up to 250C. And arc resistance is 200 to 240 seconds by ASTM D495, the highest in this family - against 130 seconds for G-10, around 140 for FR-4 and about 180 for G-11.

The trade-off is mechanical and it is severe, so read this before substituting. Flexural strength is published from 18,000 psi to 30,000 psi, between about a quarter and a half of G-10 or G-11, and tensile strength 18,000 to 20,000 psi, about half. Compressive strength holds up better at 45,000 to 50,000 psi, and notched Izod impact is 17.0 ft-lb/in lengthwise. Do not swap G-7 in for G-11 on a structural part because the temperature rating is higher - it will not carry the same load.

Electrically it is good: dielectric constant 4.3 to 4.5 and dissipation factor 0.017 to 0.018 at 1 MHz, dielectric strength perpendicular 350 V/mil at 1/8 inch, breakdown voltage parallel 55 kV dry, water absorption 0.10 to 0.20% and hardness Rockwell M105. On flammability the sources disagree - one maker publishes UL 94 V-0 and another publishes HB for the same grade - so if the rating matters, ask us for the certificate rather than relying on the grade name.

It is machined into arc shields, barriers and arc chutes, high-temperature coil and terminal insulation, heat-platen and press insulation, microwave and RF components, and aerospace parts with a thermal duty. Standard colour is white to cream.

Machining and handling. The glass abrades tooling, so carbide is the floor and diamond-coated or PCD cutters are what hold tolerance in production; run high spindle speeds with secure workholding and design in generous internal radii. Threaded holes are normally milled rather than tapped and external threads turned or thread-milled, since the plies can delaminate. Treat the cutting dust as hazardous and work to the safety data sheet for the lot - the sensitiser classification published for epoxy laminate dust comes from the epoxy resin and does not necessarily apply to a silicone laminate, so do not assume either way. Use local exhaust, a dust respirator, eye protection and gloves. It comes off as powder rather than chips, so vacuum it up rather than blowing it airborne. These are our shop notes, not a manufacturer specification.