PCB Technical - FPCB material characteristics for functional design

The flexibility of FPCB allows for circuit connection for a variety of design purposes.

The use of FPCB in consumer electronics is increasing.In addition to the current market’s higher requirements for the appearance design of electronic products, the existing PCB and HDI multilayer circuit board material restrictions cannot meet the changing appearance structure.Adaptive design,even if the circuit density of FPCB fails to reach the PCB level,it has become a key material that cannot be reduced in most consumer electronics.

FPCB material,in terms of structural flexibility,allows it to adapt to a variety of bending angles without worrying about the carrier board rupture, and under the flexible design structure, it also allows FPCB to The design trend of electronic products plays an indispensable and key role.

FPCB is not capable of unlimited flexing.In order to avoid excessive flexing and pulling, the copper foil is usually affixed with a reinforcing patch.

FPCB can be used as a flexible board for connecting multiple functional carrier boards.

For special structure modeling that requires large deflection applications,FPCB can be elastically laser-cut,so that FPCB materials have better flexing capabilities.

FPCB material characteristics

The product characteristics of FPCB, in addition to the soft material,are actually light in texture and extremely thin in configuration. Very light structure,the material can be flexed many times without breaking the insulation material of the hard PCB.

The flexible plastic base material and wire layout of the soft board make the soft board unable to cope with the excessively high conduction current,Voltage, so it is almost impossible to see the soft board design in the application of high-power electronic circuits.On the contrary, in consumer electronic products with low current and low power, the use of soft boards is quite large.

Because the cost of the soft board is still controlled by the key material PI,the unit cost is high,so when designing the product, the soft board is usually not used as the main carrier board, but the key design that requires "soft" characteristics is partially applied.Above,for example,the soft board application of the digital camera electronic zoom lens, or the soft board material of the optical disc drive read head electronic circuit, are all due to the electronic components or functional modules that must move and run, and the hard circuit board materials are not compatible. Under the circumstances, adopt the design example of the flexible circuit.

PI is also known as polyimide.From its heat resistance and different molecular structure,PI can be divided into different structures such as fully aromatic PI and semi-aromatic PI.Fully aromatic PI belongs to the linear type.There are infusible and infusible and thermoplastic substances.The properties of infusible materials cannot be injection molded during production,but the materials can be compressed and sintered,and the other can be produced by injection molding.

Semi-aromatic PI belongs to this type of material in Polyetherimide.Polyetherimide is generally thermoplastic and can be manufactured by injection molding.As for the thermosetting PI, different raw material characteristics can be used for lamination molding of impregnated materials,compression molding, or transfer molding.

FPCB material has high heat resistance and high stability performance

In terms of the final formed products of chemical materials, PI can be used as gaskets, gaskets, and sealing materials, and bismale materials can be used as the base material of flexible multilayer circuit boards, fully aromatic materials, and organic materials in use. Among the polymer materials, it is the material with the highest heat resistance, and the heat resistance temperature can reach 250~360°C! As for the bismale type PI used as a flexible circuit board, the heat resistance is slightly lower than that of the fully aromatic PI, generally around 200°C.

The bismale type PI has excellent mechanical properties, extremely low temperature change, and can maintain a highly stable state in high temperature environments, with minimal creep deformation and low thermal expansion rate! In the temperature range of -200~+250°C, the change in the material is small. In addition, the bismale type PI has excellent chemical resistance. If immersed in 5% hydrochloric acid at 99°C, the material's tensile strength retention rate Can still maintain a certain level of performance. In addition, the bismale-type PI has excellent friction and wear characteristics, and it can also have a certain degree of wear resistance when used in applications that are prone to wear.

In addition to the main material characteristics, the structural composition of the FPCB substrate is also a key factor. The FPCB is a cover film (upper layer) as an insulating and protective material, with an insulating base material, a rolled copper foil,and an adhesive to form an overall FPCB. The substrate material of FPCB has insulating properties. Generally, two major materials, polyester (PET) and polyimide (PI), are commonly used.PET or PI each has its own advantages/disadvantages.

FPCB production materials and procedures improve the flexibility of the terminal

FPCB has many uses in products, but basically it is nothing more than wiring, printed circuits, connectors, and multifunctional integrated systems. According to the function, it can be divided into space design, change its shape, adopt folding, flexural design and assembly, and FPCB design can be used to prevent the electrostatic interference problem of electronic equipment. With the use of flexible circuit boards, if the product quality is directly structured on the flexible board regardless of the cost, not only the design volume is relatively reduced, but the overall product volume can also be greatly reduced due to the characteristics of the board.

The substrate structure of the FPCB is quite simple, mainly composed of the upper protective layer and the middle wire layer. When mass production is carried out, the soft spot circuit board can be used with positioning holes for production process alignment and post-processing.As for the use of FPCB,the shape of the board can be changed according to the space needs, or it can be used in a folded form.As long as the multi-layer structure adopts anti-EMI and static resistance isolation design on the outer layer,the flexible circuit board can also achieve high-efficiency EMI problems to improve the design. 

On the critical circuit of the circuit board,the uppermost structure of the FPCB is copper,which includes RA (Rolled Annealed Copper),ED (Electro Deposited), etc. The manufacturing cost of ED copper is quite low,but The material will be more prone to fracture or faults.The production cost of RA (Rolled Annealed Copper) is relatively high,but its flexibility is better. Therefore, most of the flexible circuit boards used in the state of high deflection are RA materials.

As for FPCB to be formed,it is necessary to bond different layers of covering layer,calendered copper,and base material through an adhesive.Adhesives generally used include Acrylic and Mo Epoxy. There are two main types.Epoxy resin has lower heat resistance than acrylic and is mainly used for household goods.Acrylic has the advantages of high heat resistance and high bonding strength,but its insulation and electrical properties Inferior,and in the FPCB manufacturing structure,the thickness of the adhesive accounts for 20-40μm (micrometers) of the overall thickness.

For highly flexural applications, reinforcement and integrated design can be used to improve material performance

In the FPCB manufacturing process,the copper foil and the substrate are made first, and then the cutting process is performed,and then the perforation and electroplating operations are carried out.After the holes of the FPCB are completed in advance,the photoresist material coating process is started,and the coating process is completed.In the FPCB exposure and development process,the etched circuits are processed in advance.After the exposure and development processing is completed,solvent etching is performed.At this time,after etching to a certain degree to form the conductive circuit,the surface is cleaned to remove the solvent.The adhesive is evenly coated on the surface of the FPCB base layer and the etched copper foil, and then the cover layer is attached.

After completing the above operations,the FPCB has been roughly 80% completed.At this time, we still have to deal with the connection points of the FPCB,such as increasing the opening of the guide welding process,etc.,and then perform the appearance processing of the FPCB,such as using laser cutting After a specific appearance,if the FPCB is a soft and hard composite board or needs to be welded with the functional module,the secondary processing is carried out at this time, or it is designed with a reinforcement board.

FPCB is versatile and not difficult to produce.It's just that FPCB themselves can't produce circuits that are too complex or too compact,because a circuit that's too thin results in too small a cross-sectional area of copper foil. If the FPCB is flexed,it can easily cause the internal circuits to break, so most of the overly complex circuits will use the core HDI high-density multilayer board to deal with the relevant circuit requirements.Only a large number of data transfer interfaces or data I/O connections to different functional carrier boards would require the use of FPCBs for board connections.