A PCB(full form:Printed Circuit Board) is a stiff reinforced copper laminated material that ensures enough support for mounted components, provides pin-to-pin connectivity and electrical signals conductance. It is also known as base material or copper clad material. On base material a thin sheet in the order of a few tens of microns thickness is laminated over either one side or both sides. The thickness of the copper and insulator resin composite determines the intended application of that material like power, RF, general purpose digital etc. The base material thickness is generally varies around to 1.6mm or 63mils. There are various reinforced materials used for PCBs whereas for conductivity the copper or aluminum is used. The cost and performance highly dictate the material selection both for conductivity and mechanical support.
According to different reinforced materials and their properties the PCB board materials can be classified into various categories:
♦ Based on mechanical rigidness the materials are classified as rigid PCB materials and flex PCB materials. For rigid and flex PCBs the rigid and flex board materials are used respectively while for rigid-flex PCBs mix of both materials are used.
♦ Based on thickness, commonly, the materials are available in 0.5mm to 3mm thickness. The copper clad thickness is excluded from overall thickness of the material. The copper clad or laminate is available from 17 to 70µm thickness. Special versions of thicker copper clad are also available by the manufacturers.
♦ Based on the resin or reinforced material used there are different materials for PCB boards like FR-1, FR-2, FR-3, FR-4, FR-5, arlon, CEM, Phenolic etc.
Fig 1: Copper clad board (left) and different parts of copper clad board
The performance of circuits and boards utilization heavily depends over the materials used:
Appearance and finishing of the copper clads: The copper clad should have smooth, clear, uniform thickness and scratch-less surface. Particularly a PCB made with scratched copper clad surfaces may result into failure of the PCB.
The PCB clad materials are manufactured and cut in standard size i.e., length and width. It is required by the manufacturers to fix the sheets into the machine fixtures and later on reduce the wastage of PCB material by replicating the design on the entire sheet.
The electrical parameters are very important and the utmost requirement of the user. The electrical parameters such as dielectric constant, conductor resistance, reinforced material signal absorption and resistance, electrical impedance control and dielectric breakdown voltage etc., are the important factors.
The PCB boards have higher flammability resistance because the electronic circuits are very prone to generate heat and very often to be exploded. A good fire retardant material lessens the chance of flammability.
The electronic circuits are deployed almost every field of daily life. So the materials are categorized by moisture absorption ratio also.
The multilayer boards are made with stacking the multiple layers of base material and copper separated with pre-preg insulation material.
Figure 2:- Rigid PCB fabricated over a slightly rough copper clad sheet that has open circuit in the circuit’s circled area.
Fig 3: A flex PCB circuit fabricated over flex PCB board material