Raw material analysis of flexible circuit board (PCB)

- May 26, 2017-

The flexible circuit (FPC) is also called soft circuit, is the last century the United States in 70s and for the development of space rocket technology development technology, or a polyester film made from polyimide substrate for a printed circuit with high reliability, excellent flexibility, through the thin plastic sheet can be bent, embedded in circuit design. In the narrow and limited space in the heap block large precision component, thus forming a flexible circuit can be bent. This circuit can be bent, folded light weight, small volume, good heat dissipation, easy installation, break through the traditional interconnection technology.

In a flexible circuit structure, the material consists of an insulating film, conductor, and adhesive.

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 multilayer design, it is bonded to the inner layer. They are also used as protective covers to separate the circuit from dust and moisture, and to reduce stress during flexing. Copper foil forms a conductive layer.

In the flexible circuit, using a rigid member formed by aluminum or stainless steel, they can provide stability of size, provides a physical support for the placement of components and wires, and the release of stress. The adhesive bonds the rigid member to the flexible circuit. There is also a kind of material, sometimes used in flexible circuit, it is adhesive layer, it is coated on the two sides of the insulating film coated with adhesive formed. The adhesive layer provides environmental protection and electronic insulation, and eliminates the ability to remove a layer of film and multilayer with fewer adhesive layers.

There are many kinds of insulating film materials, but the most commonly used are poly imide and polyester. 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 is non flammable, stable geometrical dimension and high tear strength, and has the ability to withstand temperature, also known as polyester, Polyethylene double stupid two formate (Polyethylene terephthalate: PET), its physical properties similar to polyimide, with low dielectric constant, the absorption of moisture very small, but not high temperature.

The melting point of polyester is 250 DEG C and the glass transition temperature (Tg) is 80 DEG C, which limits their use in applications where large amounts of end welding are required. In low temperature applications, they exhibit rigidity. Still, they are suitable for use in products such as telephones and other products that do not need to be exposed to harsh environments.

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


Copper foil is suitable for use in flexible circuits, which can be electrodeposited (Electrodeposited: ED), or plated. The electrodeposited copper foil has a shiny surface on one side and a dull matte finish on the other. It is a flexible material that can be made into a variety of thickness and width, ED copper foil matte side, often after special treatment to improve its adhesion ability. Wrought copper foil, besides its flexibility, is also hard and smooth, and is suitable for use in situations requiring dynamic flexing.


In addition to bonding the insulating film to the conductive material, the adhesive can also be used as a covering, as a protective coating, as well as as an overlay coating. The main difference between the two lies in the way in which the coating is applied to cover the insulating film to form a laminated structure. The screen printing technique used in the coating of adhesives.

Not all laminated structures contain adhesive, and no adhesive laminates form thinner circuits and greater flexibility. It has better thermal conductivity in comparison with laminated structures based on adhesive. The structure characteristics of thin adhesive free flexible circuit, as well as the adhesive resistance is eliminated, thereby improving the thermal conductivity, it can be used in flexible circuit to use adhesive laminated structure based on the working environment.

Previous:How to choose SMT processing plant Supplier? Next:Three major factors causing welding defects of circuit board