Properties of FR-4 Circuit Board Material

Written by daniel r. mueller
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Properties of FR-4 Circuit Board Material
FR-4 circuit boards have flame resistant qualities. (circuit board image by mite from

FR-4 is an industry-wide standard material format for consumer electronic circuit boards. The key property of FR-4 relates to the flame retardant qualities of the material, but there are a few normal operational ranges for the material between manufacturers beyond fire resistance.

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Electrical Properties

FR-4 circuit boards are mostly made of low to no-conductivity resin and fibreglass. Fibreglass is a type of glass based on elemental silicon, a perfect electrical insulator. The net effect of the materials used in the FR-4 circuit board substrate when it comes to electrical conduction, which means that electrons only flow along the copper conductor paths embedded in the board. This forms the basis for most modern micro-computing.

Flame Retardant Qualities

FR-4 circuit boards are composed mostly of fibreglass, a material that does not burn and are bound with flame retardant resin that self extinguishes once heat is removed. It is extremely unlikely that anything is able to set the board aflame in the first place because of the specially-formulated resin. Quality control lab testing, as documented by the AirBorn electronics website, shows that even in the event of a propane torch flame being used on the board, the board will not totally combust and self extinguishes as soon as the flames are removed. Batches that do not pass this kind of quality control test are not used in finished electronics that bear the FR-4 designation.

Future Outlook

The outlook in the short to midterm is still good for FR-4 circuit boards; but since the boards contain halogens, the board does give off toxic fumes if it burns. This is the main drawback to the existing FR-4 circuit board, and the main area that material engineers are looking to improve on. Eventually, it is possible that the resin composition will change, creating a new, safer circuit-board standard. The need for a new alternative material is exacerbated in regions where electronic waste is most often disposed of by incineration due to the toxic gas emissions.

A brief treatise on the subject entitled "Benzoxazine Chemistry: A New Material to Meet Fire Retardant Challenges of Aerospace Interiors Applications" by Rodger Titze and Philippe Christou of Huntsman's Advanced Material, Texas suggests that Benzoxazine-based flame retardant resins may be the basis for the next generation standard.

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