What is FPC?
Automotive FPC(Flexible Circuit Board): Flexible printed circuit board is a highly reliable and excellent flexible printed circuit board based on polyimide or polyester film. Referred to as soft board or FPC.
Features: It has the characteristics of high wiring density, light weight and thin thickness. It is mainly used in mobile phones, notebook computers, PDA, digital cameras, LCM and many other products.
Types of FPC
I. Single-Layer FPC
It has a layer of conductive pattern produced by chemical etching, and the layer of conductive pattern on the flexible insulating substrate surface is calendered copper foil. Insulation substrates can be polyimide, polyethylene terephthalate, aromatic amide fiber ester and polyvinyl chloride. Single-layer FPC can be divided into the following four sub-categories:
1. One-sided connection without overburden
The conductor pattern is on the insulating base material, and there is no overlay on the surface of the conductor. Its interconnection is realized by tin welding, fusion welding or pressure welding. It is often used in early telephones.
2. One-sided connection with overlay
Compared with the former, only one layer of overburden is added to the surface of the conductor. When covering, the pad should be exposed. Simply, it can not be covered in the end area. It is the most widely used one-sided soft PCB in automotive and electronic instruments.
3. Double-sided connection without overburden
Connecting disc interface can be connected on both front and back of the conductor. A path hole is opened on the insulating base material at the welding pad. The path hole can be made by punching, etching or other mechanical methods at the required position of the insulating base material.
4. Double sided connection with covering layer
Different from the former, there is a layer of covering layer on the surface, and the covering layer has access holes, which allow both sides to be terminated, and still maintain the covering layer, which is made of two layers of insulation material and one layer of metal conductor.
Ⅱ. Double-sided FPC
Two-sided FPC has an etched conductive pattern on both sides of the insulating base film, which increases the wiring density per unit area. Metallized holes connect the graphics on both sides of the insulating material to form conductive paths to meet the flexibility of the design and use functions. The covering film can protect the single and double-sided conductors and indicate the location of the components. According to the requirements, metallized holes and coatings are optional, and this type of FPC is rarely used.
Ⅲ. Multilayer FPC
Multilayer FPC is to laminate three or more layers of one-sided or two-sided flexible circuits together, through drilling L, electroplating to form metallized holes, forming conductive paths between different layers. In this way, complex welding process is not needed. Multilayer circuits have huge functional differences in higher reliability, better heat conductivity and more convenient assembly performance.
Its advantage is that the base film is light in weight and has excellent electrical properties, such as low dielectric constant. Multilayer soft PCB board made of polyimide film as base material is about 1/3 lighter than rigid epoxy glass cloth multilayer PCB board, but it loses the excellent flexibility of one-sided and two-sided soft PCB. Most of these products do not require flexibility. Multilayer FPC can be further divided into the following types:
1. Finished Flexible Insulation Substrate
2. The finished products of soft insulating substrates are manufactured on soft insulating substrates. The final provisions of the products can be flexible. This kind of multilayer FPC is made of soft insulating materials, such as polyimide film, which is laminated into multilayer plates and loses inherent flexibility after lamination.
FPC Manufacturing Process
So far, almost all FPC manufacturing processes are processed by subtraction method (etching method). Usually, copper clad foil is used as starting material to form an anti-corrosion layer by photolithography, and a circuit conductor is formed by etching off the unwanted copper surface. Because of the problems such as side etching, the etching method has some limitations on the fabrication of micro-circuits.
Based on the fact that the subtraction method is difficult to process or maintain the high qualified rate of micro-circuits, people think that the semi-addition method is an effective method. Various schemes of the semi-addition method have been put forward. An example of micro-circuit fabrication using semi-additive method. In the semi-addition process, polyimide film was used as starting material. First, liquid polyimide resin was cast (coated) on suitable carrier to form polyimide film.
Then, the anti-corrosion layer pattern of the inverse pattern of the circuit is formed on the polyimide matrix film by sputtering method, and then on the polyimide matrix film by photolithography, which is called anti-coating. A conductor circuit is formed by electroplating the blank part. Then the corrosion resistance layer and unnecessary phytoplankton layer are removed to form the first layer of circuit. The photosensitive polyimide resin is coated on the first layer of the circuit, and the photolithography method is used to form holes, protective layer or insulation layer for the second layer of the circuit. Then the phytolith layer is sputtered on the first layer of the circuit, which serves as the base conductive layer of the second layer of the circuit. By repeating the above process, a multi-layer circuit can be formed.
Ultra-micro circuits with pitch of 5um and through hole of 10um can be fabricated by this semi-addition method. The key to fabricating ultrafine circuits by semi-addition method is the properties of photosensitive polyimide resins as insulating layers.
