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The structure and material of FPC board

Author: Date:5/7/2018 6:33:52 AM
FPC Structure and Material Knowledge


In the structure of the flexible circuit, the constituent materials are insulating films, adhesives, and conductors.

Insulation 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 bonded to the inner layer. They are also used as protective coverings to isolate the circuit from dust and moisture and to reduce stress during flexing. 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, physical support for the placement of components and wires, and stress relief. The adhesive bonds the rigid member and the flexible circuit together. Another material is also sometimes used in flexible circuits. It is an adhesive layer formed by coating an adhesive on both sides of an insulating film. The adhesive plies provide environmental protection and electrical insulation, and can eliminate one layer of film and the ability to have multiple layers with fewer layers of adhesive.

There are many types of insulating film materials, but the most commonly used are polyimine and polyester materials. Nearly 80% of all flexible circuit manufacturers in the United States currently use polyimide film materials, and about 20% use polyester film materials. Polyimide material has non-flammability, geometrical stability, high tear strength, and ability to withstand welding temperatures, polyester, also known as Polyethylene terephthalate (abbreviation: PET), which has physical properties similar to those of polyimide, has a low dielectric constant and absorbs 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 requiring extensive end welding. In low temperature applications, they exhibit rigidity. However, they are suitable for use in products such as telephones and other devices that do not require exposure 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, after dry welding, or after multiple lamination cycles, can be dimensionally stable. Other important properties in the adhesive are lower dielectric constant, higher insulation resistance, high glass transition temperature (Tg) and low moisture absorption.

glue

In addition to bonding an insulating film to a conductive material, an adhesive can also be used as a cover layer, as a protective coating, and as an overlay coating. The main difference between the two lies in the application method used. The cover layer is covered with an insulating film to form a circuit of a laminated structure. The screen printing technology used for overlay coating of adhesives.

Not all laminate structures contain adhesives, and laminates without adhesives create thinner circuits and greater flexibility. It has better thermal conductivity than adhesive-based laminates. Due to the thin structure characteristic of the adhesive-free flexible circuit and the elimination of heat resistance of the adhesive, the thermal conductivity is improved, and it can be used in a work environment in which the flexible circuit based on the adhesive laminated structure cannot be used. .

Conductor

The copper foil is suitable for use in a flexible circuit, which can be electrodeposited (ED) or plated. The electrodeposited copper foil has a glossy surface on one side and a matte and dull surface on the other side. It is a flexible material that can be made into a wide variety of thicknesses and widths. The matte side of ED copper foil is often treated to improve its adhesion. In addition to its flexibility, wrought copper foils are also hard and smooth. They are suitable for applications where dynamic flexing is required.

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