Fr-4

FR-4, an abbreviation for Flame Retardant 4, is a type of material used for making a printed circuit board (PCB). It describes the board itself with no copper covering. FR-4 meets the requirements of Underwriters Laboratories UL94-V0. The FR-4 used in PCBs is typically UV stabilized with a tetrafunctional epoxy resin system. It is typically a yellowish color. FR-4 manufactured strictly as an insulator (without copper cladding) is typically a difunctional epoxy 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 applications where destruction of the circuit board is a desirable trait will still utilize G-10.^{[citation needed]}

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. It is also ultra high vacuum (UHV) compatible.Rouki, C.; L. Westerberg (2003). "Ultra-High Vacuum Compatibility Measurements of Materials for the CHICSi Detector System" T104: 107–108. 

FR-4 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 conductive anodic filaments (CAFs) (caused by electrochemical migration), 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, FR-4 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 FR-4.

Advanced epoxy-glass materials with fillers (i.e.: phenolic-filled materials) typically have higher associated costs and increased difficulty in drilling/metalizing. Choosing FR-4 is often a balance of cost and performance.

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)

Applications

FR-4 is acceptable for signals up to around 2 GHz, depending on the application. (Loss and crosstalk will increase, especially if the same FR-4 board carries multiple high-frequency signals.) Other materials, such as the Rogers 4003 board, provide superior electrical characteristics at higher frequencies.

FR-4 is also used for manufacturing insulating or structural components.

See also

• FR-2
• Veroboard
• IPC 4101 Printed Circuit Board Materials Specification for Rigid boards

References

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