Material characteristic of FPC (flexible printed circuit board)
FPCB, also known as Flexible Printed Circuit Board (FPCB), also known as "Flexible Board". Compared with PCB or HDI, which can not be flexibly used, FPCB forms a distinctive material characteristic of soft and hard. In the design of electronic products nowadays, it has become quite common flexibility of mixed use of soft and hard. This paper will focus on the "soft" of "Flexible Board". The characteristics are discussed from the point of view of material, process and key components, and the limitations of the use of the flexible board are also explained.
Material characteristic of FPC (flexible printed circuit board)
In addition to the softness of the material, the product characteristics of FPCB have a lightweight structure with a very thin/very light configuration.
Flexible PCB material can be deflected many times without fracture of rigid PCB insulating material.
The flexible PCB board plastic substrates and wire layouts of the flexible board make it impossible for the flexible circuit board to cope with the high conduction current and voltage. Therefore, the flexible board design can hardly be seen in the application of high-power electronic circuits. On the contrary, the flexible board is used in consumer electronic products with small current and power, and the usage of the flexible board is quite large.
Because the cost of flexible circuit boards is still controlled by the key material PI, the unit cost is high, so in product design, flexible printed circuit boards are not usually used as the main carrier board, but are used locally in key design requiring "soft" characteristics, such as the application of digital camera electronic zoom lens or the application of flexible printed circuit boards. It is the soft board material of the electronic circuit of the CD-ROM reader head. It is an example of the design of the soft board circuit under the condition that the electronic components or functional modules must move and the hard board material cannot match.
Early flexible PCB was mostly used for aerospace and military purposes, and now it is brilliant in consumer electronics applications.
In the 1960s, the use of flexible board was quite common. At that time, the unit price of the finished product was high. Although it had the characteristics of light weight, flexibility and thin, the unit cost remained high. At that time, it was only used for high-tech, aerospace and military purposes.
In the late 1990s, flexible boards began to be widely used in consumer electronics products, while around 2000, most of the countries producing flexible boards were in the United States and Japan, mainly under the control of the main suppliers in the United States and Japan, coupled with material constraints, the cost of flexible boards remained high.
PI is also known as "polyimide". In PI, it can be divided into different structures such as full aromatic PI, semi-aromatic PI and so on, depending on its heat resistance and molecular structure.
Full aromatic PI belongs to the straight chain type. The material has the material of melting and thermoplasticity. The properties of the melting material can not be injected into the production, but the material can be compressed and sintered, while the other can be injected into the production.
Semi-aromatic PI, in Polyetherimide, belongs to this kind of material.
Polyetherimide is generally thermoplastic and can be produced by injection moulding. As for thermosetting PI, different characteristics of raw materials can be used for lamination, compression, or progressive die forming of impregnated materials.
FPCB board material with high heat resistance and stability performance
In the final forming products of chemical materials, PI can be used as gaskets, gaskets and sealing materials.
Bimales can be used in flexible PCB multi-layer circuit substrates, all-aromatic materials, organic polymer materials in use is the highest heat-resistant materials, heat-resistant temperature can reach 250-360 degrees C! As for bismaletype PI used as flexible circuit board, its heat resistance is slightly lower than that of full aromatic PI, generally around 200 C.
BimalePI has excellent mechanical properties, and can maintain a high stability under high temperature, with minimal creep deformation and low thermal expansion rate due to very low temperature variation. In addition, bismale PI exhibits excellent resistance to antibiotics. If impregnated with 5% hydrochloric acid at 99 C, the retention rate of tensile strength of the material can be maintained to a certain extent. In addition, the friction and wear characteristics of bismale PI are also excellent. It can also be used in wear-prone applications with a certain degree of wear resistance.
In addition to the main material properties, the structure of FPCB substrate is also a key factor.
FPCB is a covering film (upper layer) used as insulation and protective material. The whole FPCB is composed of insulating base material, calendered copper foil and additives. FPCB has insulation properties. Generally, two kinds of materials, polyester (PET) and polyimide (PI), are commonly used. PET or PI have their own advantages and disadvantages.
FPCB Material and Procedure Improve Flexibility of Terminal
FPCB has a lot of uses, but basically nothing more than lead circuit, printed circuit, connector and multi-functional integrated system. According to the function, it can be divided into space-based design, shape change, folding and flexing design, and FPCB design can be used to prevent electrostatic interference of electronic equipment. The use of flexible circuit boards, regardless of cost, allows products to be directly structured on the flexible board, not only the design volume is relatively reduced, but also the overall product volume can be greatly reduced due to the characteristics of the board.
The structure of FPCB board is quite simple, mainly from the upper protective layer and the middle conductor layer. In mass production, soft particle board can be matched with positioning holes for production process alignment and post-processing. As for the use of FPCB, the shape of sheet metal can be changed according to space needs or used in folded form. As long as the multi-layer structure adopts anti-EMI and static barrier design form in the outer layer, the flexible circuit board can also improve the design of EMI problem efficiently.
On the key circuit board, the top structure of FPCB is copper, including 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. The production cost of RA (Rolled Annealed Copper) is high, but its flexibility is better. Therefore, most of the flexible circuit boards used in high flexural state are made of RA material.
For FPCB to be formed, different coatings, calendered copper and substrates need to be bonded through the following agents.
Adhesives commonly used are Acrylic and Mo Epoxy.
The thermal resistance of epoxy resin is lower than acrylic resin, which is mainly used for household goods. Although acrylic resin has the advantages of high thermal resistance and high subsequent strength, its insulation electrical property is poor. In FPCB fabrication structure, the thickness of the following agent accounts for 20-40 um (micron) of the overall thickness.
For high deflection applications, reinforcement and integration design can be used to improve material performance.
In FPCB process, copper foil and base plate will be made first, after truncation, perforation and electroplating will be adopted.
Roughly after the hole position of FPCB is pre-completed, the photoresistive material is coated, and the exposure development procedure of FPCB is carried out immediately after the coating is completed, the lines prepared for etching are pre-processed, and the solvent etching operation is carried out immediately after the exposure development process is completed. At this time, etching to a certain extent leads to the formation of the conducting line, which is carried out on the surface. The solvent is cleaned and removed. In order to uniformly coat the FPCB base and the etched copper foil surface with the follow-up agent, the additional work of coating is carried out.
At this time, we have to deal with the connection points of FPCB, such as increasing the conduction welding of openings, and then processing the shape of FPCB. For example, after cutting a specific shape by laser, if FPCB is a hard and soft composite plate, or needs to be welded with functional modules. At this time, the secondary processing or the processing design of the reinforcement plate are carried out.
The use of FPCB is quite diverse, and the difficulty of making FPCB is not high. FPCB itself can not make too complicated and compact circuits, because too thin circuits will be too small because of the copper foil cross-section area. If FPCB is deflected, it is easy to make the internal lines break.
Therefore, most of the complex circuits will use the core HDI high density multilayer board to deal with the related circuit requirements. Only a large number of data transmission interfaces, or data I/O transmission connections of different functional boards, will FPCB be used for plate connection.
