Application trend of FPC technology

With jingdong Fangchengdu factory mass production, flexible display formally came to people's eyes. People fantasize about when they can roll up a phone and put it in their pocket, or fold a Pad. In fact, there are a lot of technical challenges that need to be solved to roll up the phone, such as making the battery flexible, making the circuit board flexible...

Today, we are together with the flexible circuit board FPC technology, FPC technology development trends and FPC material technology trends.

In recent years, the demand for FPC in civil electronic equipment is increasing rapidly all over the world, especially in portable electronic equipment such as mobile phone and thin image equipment such as flat screen TV. The number of FPCS, or total area, used in portable phones with digital camera circuits far exceeds that of rigid PCBS. The FPC in the flat panel display (FPD) is configured in horizontal and horizontal arrays. The use of FPC has increased rapidly as FPC has become larger.

In the future, FPC will not only increase in quantity, but also change in quality. From the past to single-sided circuit as the center, to improve the proportion of double-sided circuit or multi-layer rigid flexible circuit, circuit density continues to increase. For this reason, manufacturing techniques are improved year after year. The limitations of the traditional reduction method (etching) require the development of new manufacturing techniques, as well as materials with higher performance.

The basic structure of FPC

Basic composition of single - sided FPC. In the traditional FPC case, copper foil conductor is fixed on the substrate film such as polyimide, which is inserted into epoxy resin and other binders, and then a protective film is covered on the circuit produced by etching. This structure, which uses adhesives such as epoxy resin, is still one of the commonly used standard structures due to its high mechanical reliability. However, the heat resistance of epoxy resin or acrylic resin and other binders is lower than that of polyimide resin matrix film, so it becomes the Bottle Neck to determine the upper limit of the temperature of the whole FPC.

In this case, it is necessary to exclude the FPC composition of a binder with low heat resistance. This structure can not only minimize the thickness of the entire FPC, greatly improve mechanical properties such as bending resistance, but also facilitate the formation of micro circuit or multilayer circuit. The use of unbonded coppers, consisting of only a polyimide layer and a conductor layer, has expanded the choice of materials for a variety of applications.

In FPC, there are also double-sided through-hole structure or multi-layer structure OF FPC. The basic structure of the FPC's double-sided circuit is roughly the same as that of hard PCBS, with the use of adhesives for interlayer bonding. However, there are many recent examples of high performance FPCS that have removed the adhesives and used only polyimide resin to form copper-clad foils. The layers of FPC Multilayer circuits are much more complex than printed PCB, they are called Multilayer Rigid? Flex) or MultilayerFlex. Increasing the number of layers reduces flexibility, and reducing the number of layers or eliminating the bond between layers in the part used for bending can improve the degree of freedom of mechanical movement. In order to manufacture multilayer rigid flexible plates, many heating processes are required, so the materials used must have high heat resistance. Now the use of unbonded copper clad foil is increasing.

FPC technology trends

With the diversification and miniaturization of applications, FPC used in electronic equipment requires high density circuits as well as high performance in a qualitative sense. Recent FPc circuit density changes. The subtraction method (etching) can be used to form single-sided circuits with conductor pitch of less than 30um, and double-sided circuits with conductor pitch of less than 50um have also been practical. The conduction aperture between the conductor layers of the double-sided circuit or multi-layer circuit is also getting smaller and smaller. Now the conduction aperture of holes below 100um has reached the mass production scale.

The possible manufacturing range of high-density circuits based on the manufacturing standpoint. According to circuit pitch and on-hole aperture, high-density circuits can be roughly divided into three types :(1) traditional FPC; (2) High density FPC; (3) Ultra-high density FPC.

In the traditional reduction method, FPC with a pitch of 150um and a through-hole aperture of 15um has been mass-produced. Due to the improvement of materials or processing devices, even in the reduction method can be machined line pitch of 30um. In addition, due to the introduction of CO2 laser or chemical etching, it is possible to achieve 50um aperture through hole mass production processing, most of the current mass production of high-density FPC using these technologies.

However, if the pitch is less than 25um and the through-hole diameter is less than 50um, it is difficult to improve the pass rate even if the traditional technology is improved, and new process or new material must be introduced. There are various processing methods proposed now, but the use of electroforming (sputtering) technology is the most suitable method, not only the basic process is different, and the use of materials and auxiliary materials are also different.

On the other hand, the progress of FPC joint technology requires FPC to have higher reliability performance. As the circuit becomes more dense, the performance of FPC requires diversification and high performance. These performance requirements are largely dependent on the circuit processing technology or materials used.

FPC manufacturing process

So far the FPC manufacturing process is almost all by the reduction method (etching) processing. Usually with copper-clad foil plate as the starting material, the photoresist layer is formed by lithography, and the copper surface is etched away to form the circuit conductor. Etching has the limitation of micro circuit processing due to problems such as side erosion.

Based on the subtraction method is difficult to process or maintain a high pass rate of the micro circuit, people think that the half-addition method is effective, people put forward a variety of half-addition method. Micro circuit processing example using half addition method. The semi-addition process takes polyimide film as the starting material, and first casts (covers) the liquid polyimide resin on the appropriate carrier to form polyimide film. Then, a phytic layer is formed on the polyimide matrix film by sputtering method, and then the reverse pattern of the circuit is formed on the phytic layer by photolithography, which is called the coating resistance. Electroplating the blank part to form a conductor circuit. The corrosion resistant layer and unnecessary phytic layer are then removed to form the first circuit. The photosensitive polyimide resin is coated on the first circuit and photolithography is used to form holes, a protective layer or an insulating layer for the second circuit layer, and then sputtered on it to form a phytic layer as the base conductive layer for the second circuit layer. By repeating the above process, a multilayer circuit can be formed.

This method can be used to manufacture ultra-fine circuits with a pitch of 5um and a hole of 10um. The key to making ultrafine circuit by semi-addition method is the performance of photosensitive polyimide resin used as insulation layer.

The basic building materials of FPC

The basic material of FPC is the matrix film or the heat-resistant resin that constitutes the matrix film, followed by the copper clad foil plate and protective layer material that constitutes the conductor.

FPC substrate film materials from the initial polyimide film to the heat resistant film can be welded. The first generation of polyimide film has some problems such as high hygroscopicity and high thermal expansion coefficient, so people use the second generation of polyimide material for high density circuit.

To date, several heat-resistant membranes have been developed for FPC that can replace the first generation of polyimide membranes. However, the position of polyimide resin as the main material of FPC is not expected to change over the next 10 years. In addition, with the high performance of FPC, the material morphology of polyimide resin will change, so it is necessary to develop polyimide resin with new functions.

The copper-clad plate

Many FPC manufacturers often purchase in the form of copper-clad foil, and then process FPC products from copper-clad foil as raw materials. FPC coppers or Cover Lay films using first-generation polyimide films are composed of adhesives such as epoxy resin or acrylic resin. The binder used here has a lower heat resistance than polyimide, thus limiting the heat resistance or other physical properties of FPC.

In order to avoid the disadvantages of using traditional adhesive copper-clad foil boards, the high performance FPC, including high density circuits, uses a non-adhesive dosage form copper-clad foil board. There are many manufacturing methods available to date, but the following three are now available:

1) Casting process

The casting process is based on copper foil. The surface activated copper foil is directly coated with liquid polyimide resin after heat treatment to form a film. The polyimide resin used here must have excellent adhesion to copper foil and excellent dimensional stability, but there is no polyimide resin that can meet these two requirements. The surface of activated copper foil is coated with a thin layer of polyimide resin with good adhesion (bonding layer), and then coated with a certain thickness of polyimide resin with good dimensional stability (core layer) on the bonding layer. Due to the difference in the physical properties of these polyimide resins to heat, large pits will appear in the matrix film if copper foil is etched. In order to prevent this phenomenon, a bonding layer is coated on the core layer to obtain a good symmetry of the matrix layer.

In order to fabricate double-sided copper-clad foil, the adhesive layer is made of hot-melt polyimide resin and the copper foil is laminated on the adhesive layer by hot-pressing method.

2) Sputtering/plating process

The starting material of sputtering/plating process is heat resistant film with good dimensional stability. The initial step is to form a phytic layer on the surface of the activated polyimide film by sputtering process. The implanted crystal layer can ensure the bonding strength of the conductor base layer and also perform the task of electroplating the conductor layer. Usually nickel or nickel alloy is used. To ensure electrical conductivity, a thin layer of copper is sputtered on the nickel or nickel alloy layer, and then the copper is thickened to a specified thickness by electroplating.

3) Hot pressing

Hot pressing method is in good dimensional stability of heat resistant polyimide film surface coated with thermoplastic resin (thermoplastic adhesive resin), and then in the heat soluble resin high temperature, laminated copper foil, here the use of composite polyimide film.

This composite polyimide film is marketed by a specialized manufacturer, and its manufacturing process is relatively simple. During the manufacture of copper clad foil, the composite film and copper foil are folded together and hot pressed under high temperature. Equipment investment is relatively small, suitable for a small number of varieties of production. Double-sided copper-clad foil is also easier to manufacture.

Another important material element that constitutes the FPC is the Cover Lay, and various protection materials have been proposed. The first practical protective layer is on the same heat resistant film as the substrate, coated and copper-clad foil with the same binder. This construction is characterized by good symmetry and still occupies a major part of the market, often referred to as "Film Cover Lay". However, this kind of film protective layer is difficult to realize the automation of processing engineering, making the whole manufacturing cost rise, and because it is difficult to carry out micro window processing, so it can not meet the needs of high-density SMT, which has become the mainstream in recent years.

In order to meet the requirements of high density installation, photosensitive protective layers have been adopted in recent years. Photosensitive resin is coated on the copper foil circuit, and then photolithography is used to open Windows on the necessary parts. The forms of photosensitive resin materials are liquid and dry film. At present, epoxy resin or acrylic resin as the matrix of protective layer materials have been used, but their physical properties, especially mechanical properties are far less than polyimide as the matrix of protective film. In order to improve this situation, it is necessary to use polyimide resin or epoxy resin or acrylic resin as the matrix of the physical properties of the protective layer material, or in the processing technology and other aspects of improvement. The photosensitive polyimide resin used here promises to be used as an interlayer insulation material in multilayer circuit formation engineering.

conclusion

The need for FPC increased rapidly, circuit density continued to increase, and manufacturing technology improved and improved year by year. The base material, protective layer and interlayer insulation of the rapidly growing FPC will remain centered on polyimide resin in the future.

With the high performance and high density of FPC, it is required not only to develop polyacrymatte resin films with higher performance, but also to develop more diversified product forms.