FR-4
From Wikipedia, the free encyclopedia
FR-4, an abbreviation for Flame Resistant 4, is a type of material used for making a printed circuit board (PCB). It describes the board itself with no copper covering. The FR-4 used in PCBs is typically UV stabilized with a tetrafunctional resin system. It is typically a yellowish color. FR-4 manufactured strictly as an insulator (without copper cladding) is typically a difunctional resin system and a greenish color. FR-4 is similar to an older material called G-10. G-10 lacked FR-4's self extinguishing flammability characteristics. FR-4 has widely replaced G-10 in most applications. Some military appications where destruction of the circuitboard is a desirable trait will still utilize G-10.
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[edit] Explanation
A PCB needs to be an insulator to avoid shorting the circuit, physically strong to protect the copper tracks placed upon it, and to have certain other physical electrical qualities (see below). FR-4 is preferred over cheaper alternatives such as synthetic resin bonded paper (SRBP) due to several mechanical and electrical properties; it is less lossy at high frequencies, absorbs less moisture, has greater strength and stiffness and is highly flame resistant compared to its less costly counterpart. FR-4 is widely used to build high-end consumer, industrial, and military electronic equipment.
FR4 as a material for PCBs faces a number of limitations. These are due to the treater-based manufacturing process used to produce the substrate from which PCBs are manufactured. Specifically, treaters tend to produce material which suffers from inclusions (air bubbles) and striations (longitudinal air bubbles) as well a deformation of the glass cloth. These imperfections can lead to CAF, problems with consistent dielectric constant (dK) (due to resin-rich resin-poor areas) and yield issues (broken connections on the PCB) when a PCB and its components is re-flow soldered. Given these issues, FR4 could be considered more as a mechanical carrier rather than a homogeneous electrical insulator. To address these issues new types of epoxy-glass material are becoming available (e.g. Monolayer) which whilst chemically identical to the "old" material do not suffer any of the problems currently associated with FR4.
[edit] Description and specifications
FR-4 is a composite of a resin epoxy reinforced with a woven fiberglass mat. It is a material from the class of epoxy resin bonded glass fabric (ERBGF).
| Property | Value |
|---|---|
| Dielectric constant (Permittivity) | 4.70 Max, 4.35 @ 500 MHz, 4.34 @ 1 GHz |
| Dissipation Factor (Loss tangent) | 0.02 @1 MHz, 0.01 @ 1 GHz |
| Dielectric strength | 20 MV/m (500 V/mil ) |
| Surface Resistivity (min) | 2x10^5 MΩ |
| Volume Resistivity (min) | 8x10^7 MΩ*cm |
| Typical Thickness | 1.25 mm - 2.54 mm (0.049-0.100 inches) |
| Typical stiffness (Young's modulus) | 17 GPa (2.5x10^6 PSI; for use in PCBs) |
| Density | 1.91 kg/L |
[edit] Applications
FR-4 is acceptable for signals up to around 10 GHz, although Rogers 4003 board provides superior electrical characteristics.
FR-4 is also used for manufacturing insulating or structural components.
