1. Insulating film
Flexible circuit board
The copper foil, acting as a conductive layer, is bonded to the insulating film which serves as the base layer of the circuit. The adhesive is crucial in securely attaching the copper foil to the insulating layer. In the case of a multilayer design, the insulating film is further bonded to the inner layer. Apart from its functional role, the insulating film also acts as a protective covering, ensuring the circuit remains isolated from dust and moisture. Additionally, it aids in minimizing stress during flexure, contributing to the circuit's durability and longevity.
Aluminum or stainless steel rigid components are utilized in certain flexible circuits to offer dimensional stability, physical support for component and wire placement, and stress relief. These rigid members are bonded to the flexible circuit using adhesive. Furthermore, a bonding layer, consisting of adhesive coated on both sides of an insulating film, is sometimes employed in these flexible circuits. This adhesive layer sheet serves to provide environmental protection and electronic insulation, reducing the number of film layers needed and enabling bonding of multiple layers.
Insulating films are a crucial component in many industries, with two popular materials being polyimide and polyester. Polyimide film is the preferred choice for roughly 80% of flexible circuit manufacturers in the US due to its non-flammable properties, high tensile strength, stable dimensions, and ability to withstand welding temperatures. On the other hand, polyester film, also known as polyethylene phthalate, shares many physical properties with polyimide but has a lower dielectric constant and is unable to resist high temperatures. Its melting point caps at 250°C, and it has a glass transition temperature of 80°C, restricting its use in welding-intense applications, particularly those that require rigidity in cryogenic applications. Regardless, polyester remains viable in less demanding contexts - think phones and other devices that don't see harsh conditions - and is usually paired with polyester adhesive. When selecting adhesives, it's critical to consider factors like low dielectric constant, high insulation resistance, high glass transition temperature, and low moisture absorption, ensuring dimensional stability after dry welding or after multiple laminating cycles.
2. Conductor
Flexible circuits often utilize copper foil, which can be obtained through either electrodeposition or plating methods. The resulting copper foil exhibits a glossy surface on one side and a dull, lusterless appearance on the other. Its flexibility allows for the production of various thicknesses and widths to suit specific requirements. To enhance its adhesion, the matte side of electrodeposited copper foil is often subjected to special treatment. Alternatively, forged copper foil possesses both a hard and smooth texture, making it ideal for applications where dynamic bending is necessary.
3. Adhesive
Apart from bonding the insulating film with the conductive material, the adhesive can serve as a protective coating and covering layer as well. The primary distinction lies in the method of application. The covering layer is applied to cover the insulating film, creating a laminated circuit structure. This is achieved using screen printing technology for the adhesive coating. It's important to note that not all laminated structures contain adhesives. Layers without adhesives result in thinner circuits and enhanced flexibility. Furthermore, they exhibit superior thermal conductivity compared to adhesive-based laminated structures. The thin composition of the adhesive-free flexible circuit and the absence of thermal resistance from adhesives contribute to the improved thermal conductivity. This makes it suitable for environments where flexible circuits based on adhesive laminated structures cannot be used effectively.