PCB Tech - Brief description of the structure and materials of FPC

In the structure of the flexible circuit board, the materials are composed of the edge film, the adhesive and the conductor.

Edge film

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

In some FPCs, rigid members 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 adhesive on both sides of the edge film. The adhesive layer provides environmental protection and electronic integrity 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 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, geometrically stable, has high tear strength, and has the ability to withstand welding temperature. Polyester, also known as polyethylene terephthalate (Polyethylene terephthalate for short: PET), whose physical properties are similar to polyimide, has a lower dielectric constant, 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 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 phones and other products that do not need to be exposed to harsh environments.

Polyimide edge film is usually combined with polyimide or acrylic adhesive, and polyester edge material is generally combined with polyester adhesive. The advantages of combining with materials with the same characteristics can have dimensional stability after dry welding is completed, or after multiple lamination cycles. Other important characteristics in the adhesive are lower dielectric constant, higher end resistance, high glass transition temperature (Tg) and low moisture absorption.

glue

In addition to the adhesive used to bond the edge film to the conductive material, it 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 cover the edge film to form a laminated circuit. 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 characteristics of the adhesive-free FPC and the elimination of the thermal resistance of the adhesive, thereby improving the thermal conductivity, it can be used in the working environment where the flexible circuit based on the adhesive laminated structure cannot be used.

conductor

Copper foil is suitable for use in flexible circuits. It can be electrodeposited (Electrodeposited for short: ED) or plated. The surface of the copper foil deposited by electrodeposition is glossy on one side, while the processed surface on the other side is dull and 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.