PCB generally consists of four layers, which are heat laminated together into a single layer. The material used in PCB from top to bottom includes Silkscreen, Soldermask, Copper and Substrate.
The last of those layers, substrate, is made of fiberglass with the FR letters standing for "fire retardant." This substrate layer provides a solid foundation for PCBs, though the thickness can vary according to the uses of a given board.
A cheaper range of boards also exist on the market that don't utilize the same aforementioned PCB materials, but instead consist of phenolics or epoxies. Due to the thermal sensitivity of these boards, they tend to lose their lamination easily. These cheaper boards are often easy to identify by the smell they give off when being soldered.
PCB second layer is copper, which is laminated onto the substrate with a mixture of heat and adhesive. The copper layer is thin, and on some boards there are two such layers - one above and one below the substrate. PCBs with only one layer of copper tend to be used for cheaper electronics device.
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