When designing FPC, the layers need to be bonded together. At this time, FPC glue (Adhesive) needs to be used. Commonly used adhesives for flexible boards include acrylic (Acrylic), modified epoxy (Modified Epoxy), phenolic butyrals (PhenolicButyrals), reinforced adhesives, pressure-sensitive adhesives, etc., while single-layer FPC flexible circuit boards do not need to be glued with glue .

　　In many applications such as device welding, flexible boards need to use stiffeners to obtain external support. The main materials are PI or Polyester film, glass fiber, polymer material, steel sheet, aluminum sheet and so on. PI or Polyester film is a commonly used material for flexible board reinforcement, and the thickness is generally 125um. The hardness of glass fiber (FR4) reinforced board is higher than that of PI or Polyester, and it is relatively difficult to process when used in places where it is required to be harder.

　　Compared with the processing method of PCB pads, there are many methods for FPC pad processing. The common ones are as follows:

　　①Tin-LeadPlating Advantages: You can directly add flat lead-tin to the pad, with good solderability and uniformity. For some processing technologies such as HOTBAR, this method must be used on FPC. Disadvantages: lead is easily oxidized, and the storage time is short; electroplating wires need to be pulled; not environmentally friendly.

　　②Selective Gold Electroplating (SEG) Selective gold electroplating refers to the use of gold electroplating in local areas of the PCB, and another surface treatment method in other areas. Gold electroplating refers to first coating the copper surface of the PCB with a nickel layer, and then electroplating the gold layer. The thickness of the nickel layer is 2.5um-5.0um, and the thickness of the gold layer is generally 0.05um-0.1um. Advantages: The gold coating is thicker and has strong oxidation and wear resistance. "Goldfinger" generally adopts this kind of treatment. Disadvantages: not environmentally friendly, cyanide pollution.

　　③Organic Solderability Protective Layer (OSP) This process refers to covering the exposed PCB copper surface with specific organic substances. Advantages: Can provide a very flat PCB surface, in line with environmental protection requirements. Suitable for PCBs with fine pitch components.

　　④ Electroless nickel gold is also called chemical immersion gold or immersion gold. Generally, the thickness of the non-electrolytic nickel layer used on the copper metal surface of the PCB is 2.5um-5.0um, and the thickness of the immersion gold (99.9% pure gold) layer is 0.05um-0.1um. Replace the gold in the pcb pool). Advantages of this technology: smooth surface, long storage time, easy to solder; suitable for fine-pitch components and thin PCBs. For FPC, it is more suitable for use because of its thin thickness. Disadvantage: Not environmentally friendly.

　　Disadvantages: PCBAs that require conventional wave soldering and selective wave soldering processes are not allowed to use the OSP surface treatment process.

　　⑤Hot air leveling (HASL) This process refers to covering 63/37 lead-tin alloy on the final exposed metal surface of the PCB. The thickness of the hot air leveling lead-tin alloy coating is 1um-25um. The hot-air leveling process is difficult to control the thickness of the coating and the pattern of the pads. It is not recommended for PCBs with fine-pitch components, because the fine-pitch components have high requirements for the flatness of the pads; the hot-air leveling process is for thin FPCs The influence is great, and this kind of surface treatment is not recommended.

　　In the design, FPC often needs to be used in conjunction with the PCB. In the connection between the two, board-to-board connectors, connectors plus gold fingers, HOTBAR, flexible and rigid boards, and manual welding are usually used for connection. For different applications environment, the designer can adopt the corresponding connection method.

　　In practical applications, it is determined whether ESD shielding is required according to the application needs. When the FPC flexibility requirement is not high, it can be realized by solid copper sheet and thick dielectric. When the flexibility requirements are high, copper mesh and conductive silver paste can be used.

　　Due to the flexibility of FPC flexible circuit board, it is easy to break when under stress, so some special means are needed for FPC protection. The commonly used methods are:

　　1. The minimum radius of the inner corner on the flexible contour shape is 1.6mm. The larger the radius, the higher the reliability and the stronger the tear resistance. The corner of the shape can add a trace close to the edge of the board to prevent the FPC from being torn.

　　2. When space allows, a double-sided tape fixing area should be designed at the connection between the gold finger and the connector to prevent the gold finger and the connector from falling off during the bending process.

　　3. In order to achieve better flexibility, the bending area needs to be selected in an area with uniform width, and try to avoid the change of FPC width and uneven wiring density in the bending area.

　　4. The FPC positioning silk screen line should be designed at the connection between the FPC and the connector to prevent the FPC from being skewed and inserted in place during the assembly process. Useful for production inspections.

　　5. Stiffener, also known as reinforcing plate, is mainly used to obtain external support. The materials used are PI, Polyester, glass fiber, polymer material, aluminum sheet, steel sheet, etc. Reasonable design of the position, area and material of the reinforcing plate has a great effect on avoiding FPC tearing.

　　6. The crack or slot on the FPC must be terminated in a circular hole with a diameter of not less than 1.5mm, which is also required in the case that the two adjacent parts of the FPC need to be moved separately.

　　7. In the multi-layer FPC design, the air gap layered design needs to be carried out for the areas that need to be bent frequently during the use of the product, and the thin PI material should be used as much as possible to increase the softness of the FPC and prevent the FPC from being broken during repeated bending.

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