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    2021-10-21 17:42:09
    What is FPC (Flexible Printed Circuit)?


    Ⅰ Overview            
    Ⅱ FPC composing material            
    Ⅲ FPC production process            
    Ⅳ FPC advantages and disadvantages            
    Ⅴ FPC welding operation steps            

    Ⅰ Overview

    FPC, also known as Flexible Printed Circuit, is favored for its lightweight, thin thickness, free bending and folding, and other excellent characteristics. With the rapid development of the electronics industry, the circuit board design is becoming more and more high-precision and high-density. Traditional manual inspection methods can no longer meet production needs. FPC defect automatic detection has become an inevitable trend of industrial development.



    Flexible Printed Circuit (FPC) is a technology developed by the United States for the development of space rocket technology in the 1970s. It is made of polyester film or polyimide as a substrate with high reliability and excellent flexibility. By embedding a circuit design on a flexible thin plastic sheet, a large number of precision components can be stacked in a narrow and limited space to form a flexible circuit. This kind of circuit can be bent at will, folded, lightweight, small size, good heat dissipation, easy installation, and breaks through the traditional interconnection technology. In the structure of the flexible circuit, the materials are insulating film, conductor, and adhesive. Flexible Printed Circuits are the only solution to meet the miniaturization and mobile requirements of electronic products. Flexible printed circuits can greatly reduce the volume and weight of electronic products and are suitable for the development of electronic products in the direction of high density, miniaturization, and high reliability.

    Ⅱ FPC composing material

    1. Insulating film

    The insulating film forms the base layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In a multi-layer design, it is then bonded to the inner layer. They are also used as a protective cover to insulate the circuit from dust and moisture and to reduce stress during flexing. The copper foil forms a conductive layer.

    In some flexible circuits, rigid components made of aluminum or stainless steel are used, which can provide dimensional stability, physical support for the placement of components and wires, and stress relief. The adhesive bonds the rigid component and the flexible circuit together. In addition, there is another material sometimes used in flexible circuits, which is the adhesive layer, which is formed by coating the two sides of the insulating film with an adhesive. The adhesive layer provides environmental protection and electrical insulation functions, and can eliminate a layer of film, and has the ability to bond multiple layers with a small number of layers.

    There are many types of insulating film materials, but the most commonly used are polyimide and polyester materials. Nearly 80% of all flexible circuit manufacturers in the United States use polyimide film materials, and about 20% use polyester film materials. Polyimide materials are non-flammable, geometrically stable, have high tear strength, and have the ability to withstand welding temperatures. Polyester, also known as polyethylene terephthalate (PET), its physical properties are similar to polyimide, it has a lower dielectric constant and absorbs little moisture, but it is not resistant to high temperatures. Polyesters have a melting point of 250°C and a glass transition temperature (Tg) of 80°C, which limits their use in applications that require a lot of end welding. In low-temperature applications, they exhibit rigidity. Nevertheless, they are suitable for use in products such as phones and other products that do not need to be exposed to harsh environments. Polyimide insulating films are usually combined with polyimide or acrylic adhesives, and polyester insulating materials are generally combined with polyester adhesives. 

    2. Conductor

    Copper foil is suitable for use in flexible circuits. It can be electrodeposited (ED) or plated. One side of the electrodeposited copper foil has a glossy surface, while the processed surface on the other side is dull. It is a flexible material that can be made into many thicknesses and widths. The matte side of ED copper foil is often specially treated to improve its bonding ability. In addition to flexibility, forged copper foil also has the characteristics of rigidity and smoothness. It is suitable for applications that require dynamic deflection.

    3. Adhesive

    In addition to bonding the insulating film to the conductive material, the adhesive can also be used as a covering layer, as a protective coating, and as a covering coating. The main difference between the two lies in the application method used. The cover layer is bonded to the cover insulating film to form a circuit with a laminated structure. The screen printing technology used for covering and coating of adhesive. Not all laminate structures contain adhesives, and laminates without adhesives form thinner circuits and greater flexibility. Compared with the laminated structure based on adhesive, it has better thermal conductivity. Due to the thin structure of the adhesive-free flexible circuit and the elimination of the thermal resistance of the adhesive, thereby improving the thermal conductivity. It can be used in a working environment where the flexible circuit based on the adhesive laminated structure cannot be used. 

    Ⅲ FPC production process

    Double panel circuit board

    Cutting → Drilling → PTH → Electroplating → Pre-treatment → Dry Film Pasting → Alignment → Exposure → Development → Graphic Plating → Stripping → Pretreatment → Dry Film Pasting → Alignment Exposure → Development → Etching → Stripping → Surface Treatment → Paste the cover film → Pressing → Curing → Immersion of nickel gold → Printing characters → Shearing → Electrical test → Punching → Final inspection → Packaging → Shipment

    Single panel circuit board

    Cutting → Drilling → Pasting Dry Film → Aligning → Exposure → Developing → Etching → Stripping → Surface Treatment → Covering Film → Pressing → Curing → Surface Treatment → Immersion Nickel Gold → Printing Characters → Cutting → Electrical Measurement → Punching Cutting → final inspection → packaging → shipment

    Ⅳ FPC advantages and disadvantages

    Advantages of Flexible Printed Circuits

    FPC 2.jpg


    Flexible printed circuit boards are printed circuits made of flexible insulating substrates, and have many advantages that rigid printed circuit boards do not have:

    1. It can be bent, wound, and folded freely, can be arbitrarily arranged according to the space layout requirements, and can be moved and expanded in three-dimensional space, so as to achieve the integration of component assembly and wire connection.

    2. The use of FPC can greatly reduce the volume and weight of electronic products, which is suitable for the development of electronic products in the direction of high density, miniaturization, and high reliability. Therefore, FPC has been widely used in aerospace, military, mobile communications, laptop computers, computer peripherals, PDAs, digital cameras, and other fields or products.

    3. FPC also has the advantages of good heat dissipation and solderability, easy assembly and low overall cost, etc. The design of soft and hard combinations also makes up for the slight deficiency of the flexible substrate in the component carrying capacity to a certain extent.

    Disadvantages of Flexible Printed Circuits

    1. High one-time initial cost: Since the flexible PCB is designed and manufactured for special applications, the initial circuit design, wiring, and photographic masters require higher costs. Unless there is a special need to apply a flexible PCB, it is usually best not to use it in a small amount of application.

    2. It is difficult to change and repair the FPC: once the flexible PCB is made, it must be changed from the base map or the programmed light drawing program, so it is not easy to change. The surface is covered with a protective film, which must be removed before repair and restored after repair, which is a relatively difficult task.

    3. Size is restricted: Flexible printed boards are usually manufactured by a batch method when it is not yet common. Therefore, due to the size of production equipment, they cannot be made very long and wide.

    4. Easily damaged: Improper operation by the installation and connection personnel can easily cause damage to the circuit, and its soldering and rework need to be operated by trained personnel.

    Ⅴ FPC welding operation steps

    1. Before soldering, apply flux to the pad and process it with a soldering iron to avoid poor tin plating or oxidation of the pad, resulting in poor soldering. The chip generally does not need to be processed.

    2. Use tweezers to carefully place the PQFP chip on the PCB board, not to damage the pins. Make it aligned with the pad and ensure that the chip is placed in the correct direction. Adjust the temperature of the soldering iron to more than 300 degrees Celsius, dip a small amount of solder on the tip of the soldering iron, press down the aligned chip with a tool, and add a small amount of solder to the two diagonal pins. Hold down the chip and solder the pins on the two diagonal positions to make the chip fixed and unable to move. After soldering the opposite corners, recheck the alignment of the chip position. If necessary, adjust or remove and realign the position on the PCB board.

    3. When starting to solder all the pins, add solder to the tip of the soldering iron, and apply flux to all the pins to keep the pins moist. Touch the end of each pin of the chip with the tip of a soldering iron until you see the solder flowing into the pin. When soldering, keep the tip of the soldering iron parallel to the soldered pin to prevent overlap due to excessive soldering.

    4. After soldering all the pins, wet all the pins with flux to clean the solder. Suck off the excess solder where needed to eliminate any short circuits and overlaps. Finally, use tweezers to check whether there is any false soldering. After the inspection is completed, remove the solder from the circuit board, and soak the hard brush with alcohol and wipe it carefully along the pin direction until the solder disappears.

    5.SMD resistance-capacitance components are relatively easy to solder. You can put tin on a solder joint first, then put one end of the component, clamp the component with tweezers, and then see if it is placed correctly after soldering one end. If it has been aligned, then solder the other end.

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