A circuit board or PCB connects components placed on the board through the conductive thin copper tracks, vias and pads. In a raw form PCB sheet the copper is coated on an insulating sheet of 1 to 2mm thickness. The copper thickness ranges from 17µm to 70µm. The tracks are formed by removing the spare copper by etching process. The fundamental property, the resistance of the copper, is determined by copper-thickness, track width and end to end length. This property should be as allow as possible otherwise the track resistance leads to a considerable loss of signal. In high frequency signals like microwave and RF applications the capacitive and inductive properties alongwith the signal absorption properties are also paramount concern and special effort is offered during PCB design. The other PCB material properties are expansion coefficient, dielectric constant, tensile strength, glass transition temperature etc. The copper mainly connects the components electrically whereas the base material supports the components mechanically. The PCB materials are developed for low cost as well as low noise applications which are much more costly. The low cost PCBs are intended to use in home appliances and commonly used low priced electronic gadgets.
The FR-4 material is most widely used for PCBs. Other commonly available materials are Teflon, FR-1, FR-5, CEM-3, CEM-4, CEM-5 and the Roger material. Sometimes circuit signals dictate usage of material such as in high speed digital signals the board is manufactured with controlled impedance parameters that requires the controlled pre-preg thickness and uniformity.
The high current circuits like power supply circuits need thicker and wider copper tracks whereas high frequency and low power signals need thin copper tracks. The copper thickness is measured in ounces per square foot of area, 34µm (≈ 35µm) of copper thickness means ounce per square foot. Common thickness on FR-4 materials is 35µm other options like 70µm and 17µm thickness materials are also available with the PCB manufacturers.
The PCB finishing, coating, and tinning materials also enhances life of PCB, solder-ability and re-work. The flex PCBs, one of the latest techniques of circuit boards fabrication, are the PCBs which are created over flexible laminates and are moldable which are replacing the PCB wire cables. The flexPCBs and analog applications circuits mostly are made by polyimide PCB materials.
Figure 1 and 2 show different types of PCB materials.
Figure 1:- Rigid flex PCB material for high end Applications
Figure 2:- Arlon PCB material for microwave applications