Most PCBs are composed of between one and sixteen (or even more) conductive layers separated and supported by layers of insulating material (substrates) laminated (glued) together. Layers are connected together through drilled holes called vias. Either the holes are electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are visible only on one surface, or buried vias, which are visible on neither.
Paper impregnated with phenolic resin
Low-end consumer grade PCB substrates frequently are made of paper impregnated with phenolic resin, sometimes branded "Pertinax". They carry designations such as XXXP, XXXPC, and FR-2. The material is inexpensive, easy to machine by drilling, shearing and cold punching, and causes less tool wear than glass fiber reinforced substrates. The letters "FR" in the designation indicate Flame Resistance.
High-end consumer and industrial circuit board substrates are typically made of a material designated FR-4. This consists of a woven fiberglass mat impregnated with a flame resistant epoxy resin. It can be drilled, punched and sheared, but due to its abrasive glass content requires tools made of tungsten carbide for high volume production. Due to the fiberglass reinforcement, it exhibits about five times higher flexural strength and resistance to cracking than paper-phenolic types, albeit at higher cost.
Microwave & RF
PCBs for high power radio frequency (RF) work use plastics with low dielectric constant (permittivity) and dissipation factor, such as Rogers RO4000, Rogers Duroid, DuPont Teflon(types GT and GX), polyimide, polystyrene and cross-linked polystyrene. They typically have poorer mechanical properties, but this is considered an acceptable engineering tradeoff in view of their superior electrical performance.
Not all circuit boards use rigid core materials. Some are designed to be very or slightly flexible, using DuPont's Kapton polyimide film, and others. This class of boards, sometimes called flex circuits, or rigid-flex circuits, respectively, are difficult to create but have many applications. Sometimes they are flexible to save space (PCBs inside cameras and hearing aids are almost always made of flex circuits so they can be folded up to fit into the limited available space). Sometimes, the flexible part of the circuit board is actually being used as a cable or moving connection to another board or device. One example of the latter application is the cable connected to the carriage in an inkjet printer.
Low-thermal resisivity substrates
Power electronic applications require low-thermal resisivity substrates, with thick copper track to carry high currents. The main technologies are ceramic-based substrates (Direct Bonded Copper) and metal-based substrates (Insulated Metal Substrate).