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.
When ordering a copper clad laminate board, the FR-4 thickness and the copper foil thickness must be specified separately.
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).
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).
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.
