PCB Blog - Flexible PCB Material Analysis

Flexible pcb, also known as flexible circuit board, is an electronic component with high reliability and excellent flexibility. It is widely used in a variety of electronic devices, such as mobile phones, tablet computers, wearable devices, etc., responsible for connecting the various electronic components to achieve the normal operation of the device.The reason why flexible pcb have such a powerful function, and its unique composition of the material is inseparable.

The main materials of flexible pcb

1. Base material

The base material is an important part of the flexible pcb, usually made of polymer materials such as polyimide (PI) or polyester (PET). These materials have good high temperature resistance, electrical insulation and mechanical strength, to meet the needs of FPC in a variety of harsh environments. Among them, polyimide (PI) is widely used in high-end electronic products due to its higher heat resistance and stability.

Polyimide has excellent high-temperature resistance and chemical stability for demanding electronics applications, while polyester materials have lower cost and better flexibility for general consumer electronics applications.

With excellent chemical stability and mechanical properties, polyimide film is a high temperature resistant, high strength, highly insulating material. In flexible pcb, polyimide film is mainly used to make circuit lines and cover films.

The thickness of polyimide film is usually between 12.5-50μm. Different colours of polyimide film have slight differences in performance, for example, yellow polyimide film has a slight advantage in high temperature resistance.

Polyester film is a polymer material with excellent transparency, insulation and mechanical strength.

In flexible pcb, polyester film is mainly used to make circuit lines and cover films. The thickness of polyester film is usually between 5-25 μm. Different colours of polyester film have slight differences in performance, for example, white polyester film has a slight advantage in insulation performance.

PI, as a special engineering plastic with excellent comprehensive performance, is mainly characterised by the presence of an imide ring in its main chain, which is usually produced by the low-temperature polycondensation reaction of diamine and dianhydride monomers in a non-protonic polar organic solvent to produce a polyamidoacid solution, which is then dehydrated and cyclised.

Due to the diversity of the two monomers, so that PI has more synthetic pathways and processing methods, often can give PI more excellent performance, so that it can be widely used in more fields, in a large number of polymers to occupy an advantageous position, PI broad application prospects make it become an important part of the polymer research.

Classification of PI

Thermoplasticity

In addition to the excellent properties of conventional PI, thermoplastic polyimide (TPI) has special thermoplastic melt flow, outstanding oxidation resistance and heat resistance. According to the structure of the different dianhydrides used, they can be classified as homophthalic anhydride, ether anhydride, ketone anhydride and fluorine anhydride.

It is generally synthesised by a two-step process and can be moulded by injection and extrusion. Conventional PIs are often difficult to process and have a single product form, while TPIs can be melt moulded to effectively solve this problem.

Thermoset

Thermosetting PI has good heat-resistant performance, non-melt and non-soluble, non-thermoplastic, has been widely used as a base material for the preparation of composite materials. Depending on the different capping agents and preparation methods, they can be divided into bismaleimide resins and PMR (in-situ polymerization of monomer reactants) resins.

Bismaleimide is synthesised from diamine and maleic anhydride, and its properties are similar to those of aromatic PIs, with a maximum service temperature of generally less than 250 °C. It has the advantage of fewer synthesis steps and lower cost, but its cured products are more brittle in comparison.

PMR is the polymerisation of monomer reactants, with excellent heat-resistant and mechanical properties, can be used for a long time in the temperature of 260 ~ 288 ℃, up to 316 ℃ still maintain good mechanical properties, mainly used in aerospace and aviation, if combined with quartz or organic fibers, can also have excellent dielectric properties, in the electronics and electricity and other high-tech fields are also popular.

flexible pcb

2.Copper Foil

Copper foil is the conductive layer on flexible pcb, responsible for the transmission of current and signals. Factors such as the thickness, purity and surface treatment of the copper foil affect the conductivity and stability of the FPC. Generally speaking, the thinner the copper foil, the better the flexibility and bending performance of the flexible pcb; while the higher the purity of the copper foil, the better the conductivity. In order to improve the adhesion between the copper foil and the substrate, the surface of the copper foil is usually also subjected to special treatment, such as roughening, nickel-plating and so on.

3. Reinforcement materials

Reinforcement materials are mainly used to enhance the strength and stability of flexible pcb, common reinforcement materials include glass fibre cloth and polyimide film. Reinforcement materials are rich in variety, common PI reinforcement, PED reinforcement, FR4 reinforcement, steel reinforcement, as well as glass fibre cloth reinforcement, etc., the thickness of the reinforcement layer can be achieved by adjusting the thickness of the PI and glue according to customer demand.

303 stainless steel is a kind of easy-to-cut and wear-resistant austenitic stainless steel containing sulphur and selenium, which is suitable for occasions with high requirements on cutting performance and surface finish. In the flexible pcb manufacturing process, the steel reinforcement is mainly assumed to enhance the strength and stability of the role of the circuit board, while the steel reinforcement can also be processed through the etching process into the desired circuit pattern.

Special attention needs to be paid to the choice of steel reinforcement should be considered when its processing performance and cost factors to ensure that the production of efficient and economical.

With its unique combination of materials, flexible pcb achieve excellent flexibility, reliability and the ability to adapt to diverse application environments. In the future, with the continuous progress of materials science, FPC materials will continue to optimise, bringing higher performance and wider application space for the electronics industry, and promoting the development of electronic products in the direction of thin, light, efficient and intelligent.