When designing FPC, it is necessary to bond each layer together. At this time, FPC glue (Adhesive) is needed. Commonly used adhesives for flexible boards include Acrylic, Modified Epoxy, Phenolic Butyrals, Reinforced Adhesives, Pressure Sensitive Adhesives, etc., while single-layer FPC flexible circuit boards do not need to be glued. . In many applications such as device welding, the flexible board needs to use a stiffener to obtain external support. The main materials used 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 the reinforcement of flexible boards, and the thickness is generally 125um. The hardness of the glass fiber (FR4) reinforcement board is higher than that of PI or Polyester, and it is relatively difficult to process when it is used in places where it is required to be harder. Compared with the processing method of PCB pads, there are also many methods for FPC pad processing. The common ones are as follows:
①Tin-Lead Plating: The advantage of tin-lead plating is that you can directly add flat lead and tin to the pad, with good solderability and uniformity. For some processing techniques such as HOTBAR, this method must be adopted on FPC. Disadvantages: lead is easy to oxidize, and the storage time is short; it needs to pull the electroplated wire; it is not environmentally friendly.
②Selective gold electroplating (SEG): Selective gold electroplating refers to the use of electroplated gold in local areas of the PCB and another surface treatment method for other areas. Gold electroplating refers to coating the copper surface of the PCB with a nickel layer first, 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: thick gold plating, strong oxidation resistance and wear resistance. “Golden Finger” generally adopts this processing method. Disadvantages: not environmentally friendly, cyanide pollution.
③Organic Solderability Protective Layer (OSP): This process refers to covering the exposed PCB copper surface with specific organic matter. Advantages: It can provide a very flat PCB surface, which meets environmental protection requirements. Suitable for PCBs with fine pitch components.
④ Chemical nickel gold (also known as chemical immersion gold or immersion gold): Generally, the thickness of the electroless 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). The technical advantages: flat surface, longer storage time, easy to solder; suitable for fine-pitch components and thinner PCBs. For FPC, it is more suitable for use because of its thinner thickness. Disadvantages: not environmentally friendly. Disadvantages: PCBA using conventional wave soldering and selective wave soldering processes is required, and OSP surface treatment processes are not allowed.
⑤Heat Air Leveling (HASL): This process refers to covering the 63/37 lead-tin alloy on the final exposed metal surface of the PCB. The hot-air leveling thickness of the lead-tin alloy coating is required to be 1um-25um. The hot air leveling process is difficult to control the thickness of the coating and the land pattern. It is not recommended to be used on PCBs with fine-pitch components because the fine-pitch components have high requirements for the flatness of the land; the hot air leveling process is for thin FPCs The impact is great, this kind of surface treatment is not recommended.
In the design, FPC often needs to be used in conjunction with PCB. In the connection between the two, board-to-board connectors, connectors with gold fingers, HOTBAR, soft and hard boards, and manual soldering are usually used for connection. For different applications Environment, the designer can adopt the corresponding connection method. In actual applications, determine whether ESD shielding is required according to the application needs. When the FPC flexibility requirements are not high, it can be implemented with solid copper and thick media. When the flexibility requirements are high, it can be realized by using copper skin grid and conductive silver paste. Due to the flexibility of the FPC flexible circuit board, it is easy to break when under stress. Therefore, some special means are required for FPC protection. Commonly used methods are:
1. The minimum radius of the inner corner on the flexible contour is 1.6mm. The larger the radius, the higher the reliability and the stronger the tear resistance. At the corner of the shape, a trace near the edge of the board can be added to prevent the FPC from being torn.
2. When space permits, a double-sided tape fixing area should be designed at the junction of the golden finger and the connector to prevent the golden 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 a uniform width area, and try to avoid the FPC width change 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 skewing and improper insertion during the assembly process. Conducive to production inspections.
5. Stiffener, also known as stiffener, is mainly used to obtain external support. The materials used are PI, Polyester, glass fiber, polymer materials, aluminum sheets, steel sheets, 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 end in a round hole with a diameter of not less than 1.5mm. This is also required when the two adjacent parts of the FPC need to be moved separately.
7. In the multi-layer FPC design, the area that needs to be bent frequently during the use of the product needs to be designed with air gap layers. Try to use thin PI materials to increase the softness of the FPC and prevent the FPC from breaking during repeated bending.
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