What is the structure of FPC

- Apr 29, 2020-

What is the structure of FPC

     In the structure of a flexible circuit, the materials that are composed are an insulating film, an adhesive, and a conductor.

Insulating film

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

    In some flexible circuits, rigid members formed of aluminum or stainless steel are used, which can provide dimensional stability, provide physical support for the placement of components and wires, and relieve stress. The adhesive bonds the rigid member and the flexible circuit together. Another material is sometimes used in flexible circuits. It is an adhesive layer, which is formed by coating adhesive on both sides of an insulating film. Adhesive layers provide environmental protection and electrical insulation, and can eliminate a thin film, and the ability to have multiple layers with fewer adhesive layers.


     There are many types of insulating film materials, but the most commonly used are polyimide and polyester materials. At present, nearly 80% of all flexible circuit manufacturers in the United States use polyimide film materials, and about 20% use polyester film materials. Polyimide material is non-flammable, stable in geometric dimensions, has high tear strength, and has the ability to withstand welding temperature. Polyester, also known as Polyethylene Terephthalate: PET), which has physical properties similar to polyimide, has a low dielectric constant, absorbs very little moisture, but 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 large amount of end welding. In low temperature applications, they exhibit rigidity. Nevertheless, they are suitable for use in products such as telephones 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. The advantages of combining with materials with the same characteristics can be dimensionally stable after dry welding or after multiple lamination cycles. Other important characteristics in adhesives are lower dielectric constant, higher insulation resistance, high glass transition temperature (Tg) and low moisture absorption.


     In addition to the adhesive used to bond the insulating film to the conductive material, it can also be used as a cover layer, as a protective coating, and cover coating. The main difference between the two lies in the application method used. The cover layer is adhered to cover the insulating film to form a circuit with a laminated structure. Screen printing technology used for adhesive coating.

    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 characteristics of the adhesive-free flexible circuit and the elimination of the thermal resistance of the adhesive, which improves the thermal conductivity, it can be used in working environments where flexible circuits based on the adhesive laminate structure cannot be used .


     Copper foil is suitable for use in flexible circuits. It can be electrodeposited (Electrodeposited for short: ED) or plated. Electrodeposited copper foil has a shiny surface on one side and a dull matte surface on the other side. It is a flexible material that can be made into many thicknesses and widths. The matte side of ED copper foil is often treated with special treatment to improve its bonding ability. In addition to its flexibility, forged copper foil also has the characteristics of hard smoothness. It is suitable for applications where dynamic deflection is required.

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