FR-4 laminate

FR-4 is the stiff material most circuit boards are made of. It's glass fabric bound in epoxy, with a flame retardant so it won't keep burning if a part overheats. The name is a grade: FR stands for flame retardant, and 4 is its number in an industry list of laminate types. On its own it's pale yellow; the familiar green comes from the solder mask on top.

It starts as rolls of woven glass fiber cloth. The cloth runs through a bath of liquid epoxy resin, soaking it up, then through an oven that partly cures the resin, leaving a dry, stiff sheet called prepreg. The epoxy has bromine built into its molecules, usually from a brominated version of bisphenol A. In a fire, the bromine is released and interrupts the chemical reactions that keep a flame going. Sheets of prepreg are stacked, often with copper foil on the outside, and pressed between steel plates at around 180 °C. The resin flows and hardens for good.

The result is a strong, flat board that doesn't conduct electricity and keeps its shape through soldering. It's the base of every printed circuit board here. Halogen-free versions, using phosphorus instead of bromine, are increasingly common.