Insulating Backbone
PCBs consist of two basic parts: a substrate (the board) and printed wires (the copper traces).
Multi-layer boards require substrates that separate the various layers.
The substrate provides a structure that physically holds the circuit components and printed wires in place and provides electrical insulation between conductive parts.
A common type of substrate is FR-4, which is a fiberglass-epoxy laminate. It is similar to older types of fiberglass boards but is flame resistant.
Substrates are also made from Teflon, ceramics, and special polymers.
PCB Substrates are specialized materials that do not conduct electric currents.For this purpose, they serve as a laminated electrical insulator between circuitry,An electrical insulator is a material whose internal Electric charge do not flow freely, and which therefore does not conduct an under the influence of an Each layer of circuitry, consequently, is connected through plated through holes to connect traces on opposite layers.
Substrate Types
Substrates can be classified into five types, each having a unique set of characteristics for specific applications.
FR-2
This lower grade of substrate is a made of impregnated paper, also called Phenolic, that is easy to machine over a fiberglass material substrate. The “FR” refers to the term Flame Resistant. This type of substrate is usually found on more inexpensive consumer electronics.
FR-4
Fiberglass substrates are made up of a woven fiberglass material and impregnated with a flame resistant material. The material is rigid and can also be drilled cut or machined but due to the abrasive nature of the fiberglass, tungsten carbide tools are needed. An FR-4 is a stronger substrate compared to an FR-2 and is more resistant to cracking or breaking and are usually found in higher end electronics.
RF
RF substrates are comprised of low dielectric plastics and used in printed circuit boards for applications in high power radio frequencies. Although the substrate has poor mechanical properties it has exceptional electrical performance properties.
FLEX
Not all circuit boards use rigid core materials. Some are designed to be very flexible or slightly flexible called flex circuits. Thin and flexible plastics and or films are employed as substrates. The manufacturing process is more difficult than using rigid substrates, but offers benefits that cannot be achieved with rigid substrates such as saving space by bending the circuit board to fit a particular space or where repetitive movement requires a flexible layer. METAL
Power electronics demand a low-thermal resistivity substrate. A ceramic core or metal core substrate provides the necessary characteristics to handle larger copper tracks and the high electrical currents used with these type of circuit boards.
