A PCB or printed circuit board electrically connects all components through the thin copper (or aluminum) tracks, vias and pads. The conductive material either copper or aluminum is etched over the non-conductive and highly insulative material called the substrate material. The base material is substrate coated or laminated with very thin sheet of copper on one or both sides. The components are mechanically supported by the base material and are soldered over the copper pads. There are many different types of PCB base material where some are specifically developed for specific applications like low noise analog, microwave and RF circuits. A very commonly used base material in the electronics industry ranging from very ordinary household gadgets to airborne systems is the FR-4 base material used to fabricate single layer, double layer and multilayer PCBs. A base material makes easy to manufacture and assemble an electronics product rather than printed wiring board. This becomes even cheaper than a PWB. Another benefit of a base material is that the copper is etched that makes the routing and track more accurate.
FR-4 stands for Fire Retardant or Flame Retardant level 4. This is a good insulator material so does not conduct the current. This is a glass-reinforced epoxy laminate composite material or the woven glass with epoxy resin. A basic component of a PCB is an FR-4 panel sheet laminated with a thin layer (such as 17µm, 35µm or 70µm) of copper on at least one side or both sides. The FR-4 material due to its fire retardant properties replaced its predecessor G-10 material which lacks the self-extinguishing properties. The relative permittivity is 4.4, dielectric strength is 5MW/m and the electrical breakdown is 50kV. The glass transition temperature or Tg is another characteristic of a PCB material where the substrate changes its electrical properties abruptly, FR-4 has 140°C Tg. The Tg changes the substrate from glassy or resin substrate to softer and rubbery material. When ordering an FR-4 material or PCB fabrication the thickness of copper over this material should also be mentioned. As high current circuits need thick and wide copper tracks and the opposite is true for low current circuits. The default thickness of a PCB material is 1.6mm.The FR-4 material is used for low frequency signals whereas for high frequency signals such as antenna design, RF and microwave applications other materials with lower dissipation factor are used.
In figure 1, a double layer PCB base material is shown with copper laminated on both sides. In figure 2, a single layer PCB base material is shown.
Figure 1. Double layer PCB base material
Figure 2. Single layer PCB base material