1. The insulating film
The insulating film forms the basic layer of the circuit, and the copper foil is bonded to the insulating layer by the adhesive. In a multi-layer design, it is then bonded to the inner layer.
Fig. Flexible circuit boards
The insulating film is also used as protective cover to insulate the circuit from dust and moisture and to reduce stress during bending. Copper foil forms a conductive layer.
In some flexible circuit board materials, rigid components made of aluminum or stainless steel are used, which can provide dimensional stability, provide physical support for the placement of components and wires, and release stress. The adhesive is used to bond rigid components to flexible circuits. In addition, there is another material sometimes used in flex printed circuit board materials. It is a bonding layer, which is formed by coating two sides of insulating film with adhesive. The adhesive layer provides environmental protection and electronic insulation, and it can eliminate one layer of film and has the ability to bond multiple layers with fewer layers.
There are many kinds of insulating film materials, but polyimide and polyester are the most commonly used materials. Nearly 80% of all flexible circuit manufacturers in the United States use polyimide film materials, and about 20% use polyester film materials. Polyimide materials are non-inflammable, geometrically stable, have high tensile strength, and are capable of withstanding welding temperature. Polyester, also known as Polyethylene benzoate, has similar physical properties to polyimide, with low dielectric constant, little moisture absorption, but not resistant to high temperature. The melting point of polyester is 250℃, and the glass transition temperature is 80℃. This limits their use in applications requiring a lot of terminal weld. They are rigid in the applications of lower temperature. Nevertheless, they are suitable for using in products such as telephones and other products that do not require exposure to harsh environments. Polyimide insulation film is usually combined with polyimide or acrylic adhesive, while polyester insulation material is usually combined with polyester adhesive. With the advantages of combining with materials that have the same features, it can have dimensional stability after dry welding or after multiple lamination cycles. The other important properties of adhesives are low dielectric constant, high insulation resistance, high glass transition temperature and low moisture absorption.