The most common types of printed wiring boards include: FR-2 (a composite material made of paper), primarily used in basic consumer electronic equipment; FR-4 wiring boards, made of flame-retarded epoxy resins, often called self-extinguishing, which meet high flammability UL standards; and high and ultra-high performance FR-4 wiring boards, used in battery-powered consumer electronics that need to be highly reliable, and sophisticated applications such as computer servers and aerospace applications.
The standards affecting printed wiring boards are, for the most part, set by Underwriters Laboratories UL, an independent, not-for-profit safety testing and certification organization. Such standards are often harmonized with the international standards set by the International Electrotechnical Commission (IEC). The IEC has developed a family of standards (IEC 61188) that apply to printed wiring boards, under the general designation of printed boards and printed board assemblies.
The key UL standard used for printed wiring boards is UL 796, which requires the use of the UL 94 flammability test. UL 94 measures the burn behavior of materials and the rate at which a vertical test specimen burns (with either flaming or glowing combustion), how quickly the flame self-extinguishes after the ignition source is removed and whether the specimen drops flaming particles that can cause ignition. UL 94 includes a number of different classifications.
FR-4 wiring boards, for example, are most commonly used as electrical insulators because of their physical properties (near zero water absorption and good mechanical strength). Both FR-4 wiring boards and high and ultra-high performance FR-4 wiring boards, which have complex electrical requirements, must meet strict flammability standards. These wiring boards must fall within the testing requirements set by UL-94 V-0, one of the strictest UL-94 classifications. UL 94 V-0, indicates, among other measures, a sample stopped burning within 10 seconds after the source of ignition was removed. FR-2 wiring boards also must meet UL 94 testing requirements, but they are not as strict as those for FR-4 boards. Typically, flammability requirements are met by the addition of flame retardants in the material formulations. Learn more about how flame retardants are used in printed wiring boards.
Additionally, retailers usually require electrical and electronic equipment, including equipment that includes printed wiring boards, to be “listed” by an organization. This means the organization assures each component complies with the safety requirements of the listing organization, including applicable fire safety requirements, and that all materials (including printed wiring board materials) are approved by the listing organization. The two major U.S. listing organizations for EEE are UL and Intertek.