What is FR-4?
Short answer: a fully cured, stock shape, form of composite material from continuous filament fiberglass cloth (e-glass) with a non-brominated epoxy resin system.
Longer answer: FR-4 is a grade designation primarily controlled by National Electrical Manufactures Association (NEMA) NEMA LI-1 “Industrial Laminated Thermosetting Products” standard. This standard was revised in 1999 to mirror MIL-I-24768 with the intention of replacing this Military Specification. ASTM D-709, 1997, “Standard Specifications for Laminated Thermosetting Materials” matches, fairly closely, the grades that are in NEMA LI-1 and was re-approved in 2001.
FR-4 is, at its most basic level, a standard for materials used in electrical applications. See the article to the right (or below) that explains why FR-4 is a distinct designation from it’s non flame retardant cousin G-10.
The NEMA Standard provides generic category recognition of grades based on their construction, reinforcement, and resin. Typical properties and performance requirements for each category are defined and published along with standard dimensional tolerances for standard forms of the product.
FR-4 materials do not have strict controls on the details of the inputs. Manufacturers are free to chose different weight of glass, different chemistries, different filler types and levels, and different processing methods so long as they achieve the requirements as detailed in the standard. Manufacturers “self certify” to the various industrial standards.
FR-4, like FR-5, G-10, and G-11 are interesting to designers because they are one of the few composite materials that have an established set of performance requirements. This makes them easier to work with in the initial design phase where many composite materials require expertise to judge their potential performance characteristics.
FR-4 is a great starting point, a baseline, from which improvements to the material can be made to meet specific design objectives.
The minimum glass transition temperature (Tg) is an important design criteria for many structural applications. But, the NEMA LI-1 standard does not have a minimum Tg requirement. FR-4 is different from FR-5 in that FR-5 is intended for higher temperature applications. Many performance characteristics are the same, but there are some notable differences.
Norplex-Micarta manufactures NEMA FR-4 materials on a weekly basis. NP510A and NP130 are two of the available sheet versions, while RT130 is one of the tube versions. It is important to note, as detailed above, that NP510A and NP130, while both meeting the specification for NEMA FR-4, do not use the same chemistry.