Hello There:

The chemical composition of FR-4 epoxy and G-10 epoxy resin is very close. I wish I could draw the structures here. FR-4 is brominated, which means you have  4 bromine attached on the benzene ring. Aromatic C-Br bond will break and  form of flame-retardant gas during combustion. I would guess that is why it is called FR-4. Replacing  bromine with hydrogen will be G-10 resin. The mechanical and electrical properties of these two system  are almost identical, as far as I know. Since they are coated on the same reinforcement material (borosilicated E-Glass). 

You can check with your laminate suppliers for details. 

Regards


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