FR-4 (or FR4) is a grade designation assigned to glass-reinforced epoxy laminate sheets, tubes, rods and printed circuit boards (PCB). FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing).
Contents
"FR" stands for flame retardant, and denotes that safety of flammability of FR-4 is in compliance with the standard UL94V-0. FR-4 was created from the constituent materials (epoxy resin, woven glass fabric reinforcement, brominated flame retardant, etc.) by NEMA in 1968.
FR-4 glass epoxy is a popular and versatile high-pressure thermoset plastic laminate grade with good strength to weight ratios. With near zero water absorption, FR-4 is most commonly used as an electrical insulator possessing considerable mechanical strength. The material is known to retain its high mechanical values and electrical insulating qualities in both dry and humid conditions. These attributes, along with good fabrication characteristics, lend utility to this grade for a wide variety of electrical and mechanical applications.
NEMA is the regulating authority for FR-4 and other insulating laminate grades. Grade designations for glass epoxy laminates are: G10, G11, FR4, FR5 and FR6. Of these, FR4 is the grade most widely in use today. G-10, the predecessor to FR-4, lacks FR-4's self-extinguishing flammability characteristics. Hence, FR-4 has since replaced G-10 in most applications.
FR-4 epoxy resin systems typically employ bromine, a halogen, to facilitate flame-resistant properties in FR-4 glass epoxy laminates. Some applications where thermal destruction of the material is a desirable trait will still use G-10 non flame resistant.
Properties
FR-4 does not specify specific material, only a grade of material, as defined by NEMA LI 1-1998 specification. Typical physical and electrical properties of FR-4 are as follows. The abbreviations LW (lengthwise, warp yarn direction) and CW (crosswise, fill yarn direction) refer to the conventional perpendicular fiber orientations in the XY plane of the board (in-plane). In terms of Cartesian coordinates, lengthwise is along the x-axis, crosswise is along the y-axis, and the z-axis is referred to as the through-plane direction. Keep in mind that the values for the parameters listed below are an example for a certain manufacturer's material. Each manufacturer will have slightly different values for the parameters listed below. It's better to check the datasheet of the specific material being used. Verifying the actual values is very important for high frequency designs.
Printed circuit boards
FR-4 is the primary insulating backbone upon which the vast majority of rigid printed circuit boards (PCBs) are produced. A thin layer of copper foil is laminated to one or both sides of an FR-4 glass epoxy panel. These are commonly referred to as copperclad laminates.
FR-4 copper-clad sheets are fabricated with circuitry interconnections etched into copper layers to produce printed circuit boards. More sophisticated and complex FR-4 printed circuit boards are produced in multiple layers, also known as multilayer circuitry.
Specifications
When ordering a copper clad laminate board, the FR-4 thickness and the copper foil thickness must be specified separately.
In the USA, FR-4 thickness is specified in thou or inches, and common thicknesses range from 10 thou (0.010 in, 254 µm) to 3 inches (76 mm).
In the USA, copper foil thickness is specified in units of ounces per square foot (oz/ft2), commonly referred to simply as ounce. Common thicknesses are 1 oz/ft2 (300 g/m2), 2 oz/ft2 (600 g/m2), and 3 oz/ft2 (900 g/m2). These work out to thicknesses of 34.1 µm (1.34 thou), 68.2 µm (2.68 thou), and 102.3 µm (4.02 thou), respectively. Some PCB manufacturers refer to 1 oz/ft2 copper foil as having a thickness of 35 µm (may also be referred to as 35 μ, 35 micron, or 35 mic).
Other applications
FR-4 is also used in the construction of relays, switches, standoffs, busbars, washers, arc shields, transformers and screw terminal strips.