Flexible Circuit Board Composition Material

1. Insulating film

Flexible circuit board

The insulating film forms the base layer of the circuit, and the adhesive bonds the copper foil to the insulating layer. In multi-layer designs, it is then bonded to the inner layer. They are also used as a protective covering to insulate circuits 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 that is sometimes used in flexible circuits is an adhesive layer, which is formed by coating an adhesive on both sides of an insulating film. Adhesive plies provide environmental protection and electrical insulation, as well as the ability to eliminate a single film, as well as the ability to bond multiple layers with fewer 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, geometrically stable, has high tear strength, and has the ability to withstand welding temperatures. Polyester, also known as polyethylene bisphthalate (Polyethyleneterephthalate for short: PET) ), its physical properties are similar to polyimide, it 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 requiring extensive end welding. In low temperature applications, they exhibit rigidity. Nonetheless, they are suitable for use in products such as telephones and other products that do not need to be exposed to harsh environments. Polyimide insulating films are usually combined with polyimide or acrylic adhesives, and polyester insulating materials are generally combined with polyester adhesives. Combined with the advantages of materials with the same properties, dimensional stability can be achieved after dry welding, or after multiple lamination cycles. Other important properties 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, which can be electrodeposited (Electrodeposited for short: ED) or plated. The electrodeposited copper foil has a glossy surface on one side, while the processed surface on the other side is dull. It is a flexible material that can be made in many thicknesses and widths, and the matte side of ED copper foil is often specially treated to improve its adhesion. In addition to flexibility, wrought copper foil also has the characteristics of hardness and smoothness, which is suitable for applications requiring dynamic deflection.

3. Adhesive

In addition to being used to bond insulating films to conductive materials, adhesives can also be used as overlays, as protective coatings, and as overlay coatings. The main difference between the two is the application used, the cover layer bonding the cover insulating film to form the circuit of the laminated construction. The screen printing technique used for the overlay coating of the adhesive. Not all laminate structures contain adhesives, and laminates without adhesives result in thinner circuits and greater flexibility. It has better thermal conductivity than adhesive-based laminate constructions. Due to the thin structure of the adhesiveless flex circuit and the improved thermal conductivity due to the elimination of the thermal resistance of the adhesive, it can be used in working environments where the flex circuit based on the adhesive laminate structure cannot be used. .