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Flexible circuit board (FPC for short) is a flexible printed circuit board with high reliability and excellent reliability, which is made of polyimide woven glass fabric copper-clad laminates or polyester film as the base material. It has the characteristics of high wiring density, light weight, thin thickness and good bendability. Also known as flexible circuit boards, it is favored for its excellent characteristics such as light weight, thin thickness, and free bending and folding. With the rapid development of the electronics industry, the design of circuit boards is becoming more and more high-precision and high-density. Traditional manual inspection methods have been unable to meet production needs. FPC defect automatic inspection has become an inevitable trend in industrial development.

Flexible 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 the circuit design on the thin and flexible plastic sheet, a large number of precision components are stacked in a narrow and limited space to form a flexible circuit. This kind of circuit can be bent freely, folded light weight, small size, good heat dissipation, easy installation, and breaks through the traditional interconnection technology. In the structure of a flexible circuit, the materials composed are insulating film, conductor and adhesive.

The material of flexible circuit board

1. 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 coverings 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. 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, has stable geometric dimensions, has high tear strength, and has the ability to withstand welding temperatures. Polyester, also known as polyethylene ethylene terephthalate ), whose physical properties are 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 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 combined 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 and low moisture absorption.

2. Conductor

Copper foil is suitable for use in flexible circuits. It can be 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, wrought copper foil also has the characteristics of hard smoothness. It is suitable for applications where dynamic deflection is required.

3. Adhesive

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 is the application method they 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 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 .

Flexible circuit board characteristics

1. Short: short assembly hours

All circuits are configured. Eliminate the need to connect extra cables

2. Small: smaller than PCB

Can effectively reduce the volume of the product. Increase the convenience of carrying

3. Light: lighter than PCB (rigid board)

Can reduce the weight of the final product

4. Thin: thinner than PCB

Can improve the softness. Strengthen the assembly of three-dimensional space in a limited space

Advantages and disadvantages of flexible circuit boards

The advantages of multi-layer circuit boards: high assembly density, small size, light weight, because high-density assembly, reduced wiring between components (including parts), thereby increasing reliability; can increase the wiring layer, and then increase design flexibility; It can also form the impedance of the circuit, can form a certain high-speed transmission circuit, can set the circuit, the electromagnetic shielding layer, and can also install the metal core layer to meet the functions and needs of special thermal insulation; easy installation and high reliability.

Disadvantages of multi-layer PCB board (unqualified): high cost and long cycle; high reliability inspection method is required. Multilayer printed circuit is the product of electronic technology, multi-function, high speed, small volume and large capacity. With the development of electronic technology, especially the wide application of large-scale and ultra-large-scale integrated circuits, fine lines appear in the direction of high-density, high-precision and high-number change of multilayer printed circuits.

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