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 (Flame Resistant 2) is a NEMA designation for synthetic resin bonded paper, a composite material made of paper impregnated with a plasticized phenol formaldehyde resin, used in the manufacture of printed circuit boards. Its main properties ...

– FR-2 is rated to 130°C, FR-1 is rated to 105°C.Effects Building Techniques Version 1.2
By R.G. Keen, 1999, Website about amateur electronics prototyping

References

Electronics manufacturing Synthetic paper {{Electronics-stub