Phenolic paper is a material often used to make printed circuit board substrates (the flat board to which the components and traces are attached). It is a very tough board made of wood fibre and phenolic polymers. It is most commonly brown in colour, and is a fibre reinforced plastic. These PCB materials are known as FR-1 and FR-2 – FR-1 is rated to 130 °C, FR-2 is rated to 105 °C.
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Phenolic paper Wikipedia(Text) CC BY-SA