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What are the characteristics of flexible printed circuit board materials?

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What are the characteristics of flexible printed circuit board materials?
FPCB, also known as flexible printed circuit board, is also referred to as “soft board”. Compared with hard, non-flexible PCB or HDI, FPCB forms a sharp contrast of soft and hard material characteristics. It has become equivalent in the design of electronic products today. Common soft and hard interoperable mixed use flexibility, and this article will focus on the “soft” characteristics of “soft boards”, discuss from the perspective of materials, processes, and key components, and explain the use restrictions of soft boards.

FPCB material characteristics
The product characteristics of FPCB, in addition to the soft material, actually have a light texture, extremely thin and light structure. The material can be flexed many times without breaking the insulating material of a hard PCB. The flexible plastic substrate of the board and the wire layout method make the flexible board unable to cope with the excessively high conduction current and voltage. Therefore, it is almost impossible to see the flexible board design in the application of high-power electronic circuits. For consumer electronics products, 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 in product design, 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 digital camera electronic zoom lens, or the soft board material of the optical drive read head electronic circuit, are all due to the situation where the electronic components or functional modules must be moved and the hard circuit board materials are not compatible. , An example of implementing a soft circuit design.
It was mostly used in aerospace and military applications in the early days, now in consumer electronics applications
In the 1960s, the use of soft boards was quite common. At that time, the unit price of finished soft boards was high. Although they were light, bendable, and thin, the unit cost was still high. At that time, they were only used for high-tech, aerospace, and military purposes. For more. In the late 1990s, FPCs began to be widely used in consumer electronic products. Around 2000, the United States and Japan were the main producers of FPCs. The main reason is that FPC materials are controlled by major suppliers in the United States and Japan. Due to the limitations, the cost of flexible circuit boards remains high.
PI is also known as “polyimide”. Among PI, its heat resistance and molecular structure can be divided into different structures such as fully aromatic PI and semi-aromatic PI. Fully aromatic PI is a linear type. The materials are Infusible and infusible and thermoplastic substances, the properties of infusible materials cannot be injection molded during production, but the material can be compressed and sintered, and the other can be produced by injection molding.

Semi-aromatic PI, Polyetherimide belongs to this kind of material. 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 board 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 circuit boards. Fully aromatic materials, organic 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.
bismale type PI has excellent mechanical material properties, extremely low temperature changes, 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 amount of change of the material is small. In addition, bismale 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 degree 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. To
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, and the insulating base material, rolled copper foil, and adhesive form the 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 and 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 cost is not considered, and the production quality is directly structured on the flexible board, not only the design volume is relatively reduced, the overall product volume can also be greatly reduced due to the characteristics of the board.
FPCB’s substrate structure 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 matched 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 design on the outer layer, the flexible circuit board can also achieve high-efficiency EMI problem improvement design .
On the key circuit boards, the top layer structure of 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 fault. RA (Rolled Annealed Copper) has a higher production cost, but its flexibility is better. Therefore, most of the flexible circuit boards used in the high deflection state are made of RA materials.
As for the forming of FPCB, 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 categories. 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 production 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 circuit to be etched is processed in advance. After the exposure and development processing is completed, the solvent etching operation is carried out. 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 FPCB base layer and the surface of the etched copper foil, and then the covering layer is attached.
After completing the above operations, the FPCB has been roughly 80% completed. At this time, we need to deal with the connection points of the FPCB, such as increasing the opening of the guide welding process, and then proceed to the appearance 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, then the secondary processing is performed at this time, or it is designed with a reinforcement board.
FPCB has a variety of uses, and the production difficulty is not high, but FPCB itself cannot make too complicated and compact circuits, because the circuit that is too thin will cause the cross-sectional area of ​​the copper foil to be too small. If the FPCB is flexed, it is easy The internal circuit breaks, so the circuit that is too complicated will mostly use the core HDI high-density multilayer board to handle the related circuit requirements. Only a large number of data transmission interfaces or data I/O transmission connections of different functional carrier boards will be used. Use FPCB for board connection.

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