Phenolic paper is a paper composite material often used to make printed circuit board (PCB) substrates (the flat board to which electronic 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 â FR-2 is rated to 130ðC, FR-1 is rated to 105ðC.