With the mass production of the BOE Chengdu plant, flexible display screens have officially come to people's eyes. Everyone has fantasies about when the phone you bought can be rolled up and put in your pocket, and when you can fold the Pad. In fact, in order to roll up the mobile phone, many technical problems need to be solved, such as making the battery flexible and the circuit board flexible ...

Today, I will discuss the technology development trend of Flexible PCB (FPC) and the technology trend of Flexible circuit board materials with everyone.

In recent years, the demand for Flexible circuit board in civilian electronic devices around the world is increasing rapidly, and a large number of Flexible PCBs are consumed in portable electronic devices such as cellular phones and thin imaging devices such as flat-screen TVs. The FPC used in mobile phones with digital imaging circuit products has more points or total area than rigid PCBs. FPCs in flat panel displays (FPD) are arranged in a horizontal and vertical arrangement. With the increase in the size of Flexible PCB, the use of Flexible PCB has increased rapidly.

The future FPC is not only an increase in quantity, but also a qualitative change. From the past, single-sided circuits have been the center, to the current increase in the ratio of double-sided circuits or multilayer rigid-flex circuits, the circuit density has continuously increased. To this end, the manufacturing technology is improved every year. The traditional subtractive method (etching method) has limitations, which requires the development of new manufacturing techniques, and at the same time the development of higher performance materials.

The basic structure of Flexible circuit board

The basic structure of a single-sided Flexible PCB. In the case of traditional Flexible PCB, the copper foil conductor is fixed on a base film such as polyimide interposed with an adhesive such as epoxy resin, and then a protective film is covered on the circuit processed by etching. This structure uses adhesives such as epoxy resin. Due to the high mechanical reliability of this layer, it is still one of the commonly used standard structures. However, the heat resistance of the adhesive such as epoxy resin or acrylic resin is lower than the heat resistance of the polyimide resin base film, so it becomes the bottleneck that determines the upper limit of the use temperature of the entire FPC (Bottle Neck).

In this case, it is necessary to exclude the Flexible PCB configuration of the binder with low heat resistance. This configuration can not only minimize the thickness of the entire Flexible PCB, greatly improve mechanical properties such as bending resistance, but also facilitate the formation of fine circuits or multilayer circuits. The adhesive-free copper-clad foil material composed of only the polyimide layer and the conductor layer has been put into practical use, and it has expanded the selection of materials suitable for various uses.

There are also FPCs with double-sided through-hole structures or multilayer structures in FPCs. The basic structure of the FPC double-sided circuit is almost the same as that of the rigid PCB, and the adhesive is used for interlayer bonding. However, the adhesive is excluded from the recent high-performance FPC, and only the polyimide resin is used to form the copper clad There are many examples. The layer structure of FPC's multi-layer circuits is much more complicated than that of printed PCBs. They are called Multilayer Rigid? Flex or Multilayer Flex. Increasing the number of layers will reduce the flexibility, reducing the number of layers in the part for bending purposes, or excluding the adhesion between the layers, can increase the degree of freedom of mechanical activity. In order to manufacture a multi-layer rigid-flex board, many heating processes are required, so the materials used must have high heat resistance. The use of copper-clad plates without adhesives is increasing.

FPC technology trends

With the diversification and miniaturization of applications, FPCs used in electronic equipment require high-density circuits and high performance in a qualitative sense. Recent changes in FPc circuit density. The subtractive method (etching method) can be used to form a single-sided circuit with a conductor pitch of 30um or less, and a double-sided circuit with a conductor pitch of 50um or less has also been put into practical use. The conduction aperture between the conductor layers connecting the double-sided circuit or the multilayer circuit is also getting smaller and smaller. Now the holes with the aperture diameter of less than 100um have reached mass production scale.

Based on the standpoint of manufacturing mother technique, the possible manufacturing range of high-density circuits. According to the circuit pitch and via hole diameter, high-density circuits are roughly divided into three types: (1) traditional FPC; (2) high-density FPC; (3) ultra-high-density FPC.

In the traditional subtraction method, FPC with a pitch of 150um and a via hole diameter of 15um has been mass-produced. Due to improvements in materials or processing equipment, it is possible to process 30um line pitch even in the subtractive method. In addition, due to the introduction of processes such as CO2 laser or chemical etching, mass production processing of 50um diameter vias can be achieved. Most of the high-density FPCs currently in mass production are processed using these technologies.

However, if the pitch is less than 25um and the via hole diameter is less than 50um, even if the traditional technology is improved, it is difficult to improve the yield, and a new process or new material must be introduced. There are various processing methods currently proposed, but the semi-additive method using electroforming (sputtering) technology is the most suitable method. Not only the basic process is different, but the materials and auxiliary materials used are also different.

On the other hand, advances in FPC bonding technology require FPC to have higher reliability performance. With the increase in the density of circuits, the performance of FPC has raised the requirements for diversification and high performance. These performance requirements are largely dependent on circuit processing technology or materials used.

FPC manufacturing process

Almost all the FPC manufacturing processes so far have been processed by the subtractive method (etching method). Generally, a copper-clad plate is used as a starting material, a resist layer is formed by photolithography, and an unnecessary part of the copper surface is etched to form a circuit conductor. Due to problems such as undercutting, the etching method has limitations in the processing of fine circuits.

The processing based on the subtractive method is difficult or it is difficult to maintain a fine circuit with a high pass rate. It is considered that the semi-additive method is an effective method, and various schemes of the semi-additive method are proposed. An example of microcircuit processing using the semi-additive method. The semi-additive process uses a polyimide film as the starting material. First, a liquid polyimide resin is cast (coated) on an appropriate carrier to form a polyimide film. Next, a sputter method is used to form a seed layer on the polyimide base film, and then a photoresist pattern is formed on the seed layer by a photolithography method, which is called a plating resist layer. Conducting circuit is formed by plating on the blank part. Then remove the resist layer and unnecessary seed crystal layer to form the first layer of circuit. A photosensitive polyimide resin is coated on the first layer of circuit, a hole is formed by photolithography, a protective layer or an insulating layer for the second circuit layer, and then a sputtered layer is formed on it as The base conductive layer of the two-layer circuit. By repeating the above process, a multilayer circuit can be formed.

Using this semi-additive method, ultrafine circuits with a pitch of 5um and a via hole of 10um can be processed. The key to making ultrafine circuits using the semi-additive method is the performance of the photosensitive polyimide resin used as an insulating layer.

The basic constituent materials of FPC

The basic constituent material of the FPC is the base film or the heat-resistant resin constituting the base film, followed by the copper-clad foil constituting the conductor and the protective layer material.

The base film material of FPC ranges from the initial polyimide film to a heat-resistant film that can withstand soldering. The first-generation polyimide film has the problems of high hygroscopicity and large coefficient of thermal expansion, so people use the second-generation polyimide material for high-density circuits.

So far, people have developed several heat-resistant films that can replace the first-generation polyimide films for FPC. However, in the next 10 years, it is believed that the position of the polyimide resin as the main material of the FPC will not change. In addition, with the higher performance of FPC, the material form of polyimide resin will change, and it is necessary to develop polyimide resin with new functions.

Copper clad laminate

Many FPC manufacturers often purchase copper clad laminates and then process them into FPC products using copper clad laminates as starting materials. The copper foil clad laminate for FPC or protective film (Cover Lay Film) using the first-generation polyimide film is composed of an adhesive such as epoxy resin or acrylic resin. The heat resistance of the binder used here is lower than that of polyimide, so the heat resistance or other physical properties of FPC are limited.

In order to avoid the shortcomings of copper-clad laminates using traditional adhesives, high-performance FPCs including high-density circuits use adhesive-free copper-clad laminates without adhesives. There have been many manufacturing methods so far, but the following three methods are now available for practical use:

1) Casting process

The casting process is based on copper foil. A liquid polyimide resin is directly coated on the surface-activated copper foil, and is heat-treated to form a film. The polyimide resin used here must have excellent adhesion to copper foil and excellent dimensional stability. However, to date, there has been no polyimide resin that can meet the requirements of both aspects. First, a thin layer of polyimide resin (adhesive layer) with good adhesion is coated on the surface of the activated copper foil, and then a certain thickness of polyimide resin with good dimensional stability is coated on the adhesive layer (Core layer). Due to the difference in physical properties of these polyimide resins against heat, if the copper foil is etched, large pits will appear in the base film. In order to prevent this phenomenon, the core layer is coated with an adhesive layer in order to obtain good symmetry of the base layer.

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

2) Sputtering / plating process

The starting material for the sputtering / plating process is a heat-resistant film with good dimensional stability. The initial step is to form a seed layer on the surface of the activated polyimide film using a sputtering process. This planted crystal layer can ensure the bonding strength to the conductor base layer, and at the same time bear the task of the conductor layer for electroplating. Usually nickel or nickel alloy is used. In order to ensure conductivity, a thin layer of copper is sputtered on the nickel or nickel alloy layer, and then the copper thickened to a specified thickness is electroplated.

3) Hot pressing method

The hot pressing method is to coat a thermoplastic resin (thermoplastic adhesive resin) on the surface of a heat-resistant polyimide film with good dimensional stability, and then laminate the copper foil on the hot-melt resin at high temperature, here A composite polyimide film was used.

This composite polyimide film is commercially available from a specialized manufacturer. The manufacturing process is relatively simple. When manufacturing a copper clad laminate, the composite film and the copper foil are laminated together and hot pressed at high temperature. The equipment investment is relatively small, which is suitable for a small amount of multi-variety production. The manufacture of double-sided copper clad laminates is also relatively easy.

Another important material element constituting the FPC is the protective layer (Cover Lay), and various protective materials are now proposed. The first practical protective layer was coated on the same heat-resistant film as the substrate, using the same adhesive as the copper clad laminate. The characteristic of this structure is good symmetry, and it still occupies a major part of the market, usually called "Film Cover Lay". However, it is difficult to automate the processing engineering of this kind of film protection layer, which increases the overall manufacturing cost, and because it is difficult to perform fine windowing, it cannot meet the needs of high-density SMT, which has become mainstream in recent years.

In order to meet the requirements of high-density mounting, in recent years, a photosensitive protective layer has been used. Apply photosensitive resin on the copper foil circuit, and then use the photolithography process to open the window in the necessary part. The forms of photosensitive resin materials are liquid and dry film. At present, the protective layer material based on epoxy resin or acrylic resin has been put into practical use, but their physical properties, especially mechanical properties, are far inferior to the film protective layer based on polyimide. In order to improve this situation, it is necessary to use polyimide resin or to perform physical properties of the protective layer material based on epoxy resin or acrylic resin, or to improve the processing technology. The photosensitive polyimide resin used here is expected to be used as an interlayer insulating material in a multilayer circuit formation process.

Conclusion

The demand for FPC has increased rapidly, the circuit density has continued to increase, and manufacturing technology has also been improved and advanced year by year. The rapidly growing base material, protective layer and interlayer insulating material of FPC will continue to focus on polyimide resin in the future.

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