FR-4
FR-4 is the NEMA grade designation for glass reinforced epoxy laminate sheets, tubes, rods and printed circuit boards (PCB). FR-4 is made of woven fiberglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing).
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 and FR5. 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
Typical physical and electrical properties of FR-4 are as follows. LW (length wise, warp yarn direction) and CW (cross wise, fill yarn direction) refer to fiber orientations in the board that are perpendicular to one another.
| Parameter | Value | Alternate units |
|---|---|---|
| Specific Gravity/Density | 1850 kg/m³ | |
| Water Absorption | -.125" < .10 % | |
| Temperature Index | 140 °C | 284 °F |
| Rockwell Hardness | 110 M scale | |
| Bond Strength | > 1000 kg | 2200 lbs |
| Flexural Strength-LW-A-.125" | > 448 MPa | 65,000 psi |
| Flexural Strength-CW-A-.125" | > 345 MPa | 50,000 psi |
| Tensile Strength (.125") LW | > 310 MPa | 45,000 psi |
| Izod Impact Strength-LW | > 54 Nm/m | |
| Izod Impact Strength-CW | > 44 Nm/m | 8 ft-lbs/in |
| Compressive Strength-Flatwise | > 415 MPa | 60,000 psi |
| Dielectric Breakdown-A | > 50 kV | |
| Dielectric Breakdown-D48/50 | > 50 kV | |
| Dielectric strength | 20 kV/mm | |
| Permittivity-A | 4.8 | |
| Permittivity-D24/23 | 4.8 | |
| Dissipation Factor-A | 0.017 | |
| Dissipation Factor-D24/23 | 0.018 | |
| Dielectric constant permittivity | 4.70 max, 4.35 @ 500 MHz, 4.34 @ 1 GHz | |
| Glass Transition Temperature | Can vary, but is over 120°C |
Applications
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 etched into copper layers to produce printed circuit boards. More sophisticated and complex FR-4 printed circuit boards are produced in multiple layers, aka "multilayer circuitry".
Specifications
In the USA, copper foil thickness is specified in units of ounces per square foot (oz/ft2). One ounce copper foil has a weight of one oz/ft2. This works out to a thickness of 34.1 µm (1.34 mils). Some PCB manufacturers refer to one ounce copper foil as having a thickness of 35 µm this may also be referred to as 35μ, 35 micron, or 35 mic. Two ounce would be 70 μ and half ounce would be 17.5 μ or thereabouts.
1/0 - denotes 1 ounce (28.3 gram) copper 1 side / with no copper on the other side. 1/1 - denotes 1 ounce copper on both sides. Similarly: H/0 or H/H - denotes half ounce copper on 1 or both sides. 2/0 or 2/2 - denotes 2 ounce copper on 1 or both sides.
Other applications
FR-4 is also used in the construction of relays, switches, standoffs, busbars, washers, arc shields, transformers and screw terminal strips.