PCB Tech - Overview of FPC technology in circuit boards

The demand for FPCs in consumer electronic devices around the world is increasing rapidly, and a large number of FPCs are consumed in portable electronic devices such as mobile phones and thin video devices such as flat-panel TVs. The number of points or the total area of the FPC used in a portable phone that also has a digital camera circuit product greatly exceeds that of a rigid PCB. The FPC in the flat panel display (FPD) is arranged in a vertical and horizontal arrangement. With the increase in the size of FPCs, the use of FPCs has rapidly increased.

In the future, FPC will not only increase in number, but will also undergo major changes in quality. From the past centered on single-sided circuits, to the current increase in the proportion of double-sided circuits or multilayer rigid-flex circuits, with the mass production of BOE's Chengdu plant, flexible display screens have officially come to people's eyes. Everyone fantasizes about when the phone can be rolled up and put in the pocket, and when the Pad can be folded. In fact, many technical problems need to be solved in order to roll up the mobile phone, such as making the battery flexible and the circuit board flexible...

Today, I will tell you about the technology of flexible circuit board FPC and take a look at the technology development trend of FPC and the technology trend of FPC materials.

In recent years, the demand for FPCs in consumer electronic devices around the world is rapidly increasing, and a large number of FPCs are consumed in portable electronic devices such as mobile phones and thin video devices such as flat-panel TVs. The number of points or the total area of the FPC used in a portable phone that also has a digital camera circuit product greatly exceeds that of a rigid PCB. The FPC in the flat panel display (FPD) is arranged in a vertical and horizontal arrangement. With the increase in the size of FPCs, the use of FPCs has rapidly increased.

In the future, FPC will not only increase in number, but will also undergo major changes in quality. From the past centered on single-sided circuits, to the current increase in the proportion of double-sided circuits or multilayer rigid-flex circuits, circuit density has continued to increase. For this reason, the manufacturing technology is improved year by year. The traditional subtractive method (etching method) has limitations and requires the development of new manufacturing technologies, and at the same time, the development of higher-performance materials.

The density continues to increase. For this reason, the manufacturing technology is improved year by year. The traditional subtractive method (etching method) has limitations and requires the development of new manufacturing technologies, and at the same time, the development of higher-performance materials.

Basic structure of FPC

The basic structure of a single-sided FPC. In the case of traditional FPC, the copper foil conductor is fixed on a base film such as polyimide interposed with an adhesive such as epoxy resin, and then the circuit formed by etching is covered with a protective film. This structure uses adhesives such as epoxy resin. Due to the high mechanical reliability of this layer composition, it is still one of the commonly used standard structures even now. However, the heat resistance of adhesives such as epoxy resin or acrylic resin is lower than that of polyimide resin matrix film, so it becomes a 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 FPC structure of the binder with low heat resistance. This configuration not only minimizes the thickness of the entire FPC, greatly improves mechanical properties such as bending resistance, but also facilitates the formation of fine circuits or multilayer circuits. Adhesive-free copper clad laminate materials composed only of polyimide layer and conductor layer have been put into practical use, which expands the selection range of materials suitable for various purposes.

There are also FPCs with a double-sided through-hole structure or a multilayer structure in FPC. The basic structure of the double-sided circuit of FPC is roughly the same as that of rigid PCB. The adhesive is used for interlayer bonding. However, the recent high-performance FPC excludes the adhesive and only uses polyimide resin to form the copper clad board. There are many examples. The layer composition of FPC multilayer 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, and reducing the number of layers in the part for bending, or eliminating the adhesion between layers, can increase the degree of freedom of mechanical movement. In order to manufacture multilayer rigid-flex boards, many heating processes are required, so the materials used must have high heat resistance. The use of binder-free copper clad laminates is increasing.

FPC technology trends

With the diversification of uses and compactness, FPCs used in electronic devices require high-density circuits as well as 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 through-hole diameter between conductor layers connecting double-sided circuits or multi-layer circuits is also getting smaller and smaller, and now holes with through-hole diameters below 100um have reached the mass production scale.

Based on the standpoint of manufacturing technology, 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 subtractive method, FPC with a pitch of 150um and a via hole diameter of 15um has been mass-produced. Due to the improvement of materials or processing equipment, a circuit pitch of 30um can be processed even in the subtractive method. In addition, due to the introduction of processes such as CO2 laser or chemical etching, mass production and processing of via holes with a diameter of 50um can be achieved, and most of the high-density FPCs currently mass-produced are processed by 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 increase the yield rate, and new processes or new materials must be introduced. There are various processing methods for the proposed process, 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, the advancement of FPC joining technology requires FPC to have higher reliability performance. With the high density of circuits, the performance of FPCs has put forward diversified and high-performance requirements. These performance requirements depend to a large extent on the circuit processing technology or the materials used.

FPC manufacturing process

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

Based on the processing difficulties of the subtractive method or the difficulty in maintaining high-yield micro-circuits, the semi-additive method is considered to be an effective method, and various semi-additive methods have been proposed. An example of micro-circuit processing using the semi-additive method. The semi-additive PCB process takes polyimide film as the starting material, and first casts (coats) a liquid polyimide resin on a suitable carrier to form a polyimide film. Next, a sputtering method is used to form a seeding layer on the polyimide base film, and then a photolithography method is used to form a resist pattern of the reverse pattern of the circuit on the seeding layer, which is called the PCB anti-plating layer. The blank part is electroplated to form a conductor circuit. Then remove the resist layer and unnecessary seeding layer to form the first layer of circuit. Coating photosensitive polyimide resin on the first layer of circuit, using photolithography to form holes, protective layer or insulating layer for the second layer of circuit layer, and then sputtering on it to form a seeding layer, as the second The base conductive layer of a two-layer circuit. By repeating the above process, a multilayer circuit can be formed.