FR-2 (Flame Resistant 2) is a NEMA designation for synthetic resin bonded paper, a composite material made of paper impregnated with a plasticized phenol formaldehyde resin, used in the manufacture of printed circuit boards. Its main properties are similar to NEMA grade XXXP (MIL-P-3115) material, and can be substituted for the latter in many applications.
FR-2 sheet with copper foil lamination on one or both sides is widely used to build low-end consumer electronic equipment. While its electrical and mechanical properties are inferior to those of epoxy-bonded fiberglass, FR-4, it is significantly cheaper. It is not suitable for devices installed in vehicles, as continuous vibration can make cracks propagate, causing hairline fractures in copper circuit traces. Without copper foil lamination, FR-2 is sometimes used for simple structural shapes and electrical insulation.
| Property | Value | |
|---|---|---|
| Dielectric constant, or relative permittivity | 4.5 @1MHz | |
| Dissipation factor | 0.024–0.26 @1MHz | |
| Dielectric strength | 29kV/mm (740V/thou) |
FR-2 can be machined by drilling, sawing, milling and hot punching. Cold punching and shearing are not recommended, as they leave a ragged edge and tend to cause cracking. Tools made of high-speed steel can be used, although tungsten carbide tooling is preferred for high volume production.
Adequate ventilation or respiration protection are mandatory during high-speed machining, as it gives off toxic vapors.