The properties of the manufactured PCB depends to a great extent (electrically, mechanically and chemically) on the application and the environment it has to be used. The materials employed in the manufacturing process play a key role in determining the properties of the resultant PCB. Therefore, selection of suitable materials for manufacturing of PCB is quite important as they are responsible both to the expected performance of the PCB as well as the equipment made.
A common PCB needs generally a conductive ink and a substrate on which the ink has to be deposited to design conductive paths on the substrate on which components can be placed as per will and need suitable for the application. Broadly, they can be laminates, copper clad laminates, or resin impregnated materials.
The popular approaches use additive method of depositing a conductive ink onto the substrate to minimize material wastage and achieve greater resolution.
These inks are classified as fired high solid systems or PTF polymer thick film systems. These inks allow print circuit designs on a variety of substrate materials ranging from teflon to paper. The conductive flakes generally consist of conductive materials like powdered silver or carbon.
Silver inks have seen multiple uses in technologies like (Radio-frequency identification) RFID, keyboards and windshield defrosters.
Laminate materials are particularly the substrates used for depositing the ink and binding different layers of the PCB in case of multilayer PCB.
The materials prominently used as laminates are:
1. BT-Epoxy: It is a group of thermosetting resins. It is obtained by mixing the epoxy and BT (Bismaleimide-Triazine) resins. This material has high glass transition temperature up to 300 degrees C and and has a very low dielectric constant.
2. Composite Epoxy Material (CEM) : This is a class of materials where woven glass fabric structures with non woven glass core is combined with epoxy resin. There are different types of CEMs :
1. CEM1 : Single layered woven glass fabric with cellulose paper based laminate. It is of low cost and is flame retardant.
2. CEM2 : Woven glass fabric and cellulose paper core.
3. CEM3 : It's similar to FR4; flame retardant and white colored.
4. CEM4 : It's similar to CEM3 but not flame retardant.
5. CEM5 : Polyester woven glass core.
3. Cyanate esters : They are bisphenol and novolac based chemical substances. They have the property of getting treated many times in a varied range of high to low temperatures. They have a high glass transition temperature up to 400 degrees C and posses a very low dielectric constant. They are smoke, fire and toxicity resistant and their toughness gives them an edge over epoxies in space applications.
4. Flame Resistant-2(FR-2) : Basically, it is a synthetic resin bonded paper made by impregnating paper with a plasticized phenol formaldehyde resin. It's used in low end consumer electronics.
5. Flame Resistant-4 (FR-4) : These are glass reinforced epoxy laminate sheets made by combining glass fibers with a fire resistant epoxy resin. It's a widely used laminate material. When a thin layer of copper is deposited on both sides of the laminate, it's known as copper-clad laminate.
6. Polyimide : It is made by polymerizing imide monomers. They are flexible, high heat resistant and thermally stable. Used in producing flexible PCBs.
7. PTFE (Teflon) : Polytetrafluoroethylene (PTFE ) is commonly known as Teflon and is a polymer of tetrafluoroethane. It is water resistant and has a high temperature bearing capacity, which makes them ideal for the use in equipments like microwaves and furnaces.
Need for advanced materials
With the structures getting smaller with the time and the utilities and circuits getting more complex, electromagnetic interference problems arise. They obstruct the development of 3D printing. This is due to the difference in fabrication material used for different components and parts of the PCB.
This problem can be overcome by looking for anisotropic materials and materials with spatially variant lattices.
Material used for the purpose of PCB fabrication still has a long way to go in order to improvise their structure to suit the diverse applications.