Phenolic paper

The back of two circuit boards. The brown one is an FR-2 made of paper phenol.

Phenolic paper is a 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 – FR-2 is rated to 130 °C, FR-1 is rated to 105 °C.[1]

References

1. Effects Building Techniques Version 1.2, By R.G. Keen, 1999, Website about amateur electronics prototyping