ABSTRACT: the characteristics and present situation of rolled copper foil are introduced, and the development trend of rolled copper foil is briefly analyzed. Key words: rolled copper foil; Present Situation; Development Trend, classification number: TG339 document identification number: A

Copper foil is an essential raw material for making printed circuit board, CCL and lithium-ion battery. Copper foil for industrial use can be divided into calendered copper foil and electrolytic copper foil according to its manufacturing process. The electrolytic copper foil is made by electrolysis of copper on the principle of electrochemistry. The inner structure of the produced foil is vertical needle-like crystal structure, and its production cost is relatively low. The calendered copper foil is made by the principle of plastic processing and the repeated rolling-annealing process of copper ingots. The inner structure of the calendered copper foil is flake crystalline structure, and the calendered copper foil products have good ductility. At present, electrolytic copper foil is mainly used in the production of rigid circuit board, while calendered copper foil is mainly used in flexible and High Frequency Circuit Board.

1. Characteristics of calendered copper foil calendered copper foil compared with electrolytic copper foil has the following characteristics. 1.1 The production process is complex and the production cost is high

The technological process of calendering copper foil (melting-casting-heating-hot rolling-milling-cold Rough rolling-annealing-pickling-cold finishing rolling-annealing-shearing-foil rolling-surface treatment-shearing) is much more complicated than that of electrolytic copper foil (dissolving copper-electrolytic foil-surface treatment-shearing) , there are many factors that affect the quality and yield of rolled copper foil, so it is difficult and costly to produce rolled copper foil.

1.2 The thickness and width of copper foil are limited. The thickness and width of rolled copper foil are limited by the processing method, especially the rolling technology. At present, the limit thickness and width of commercial rolled copper foil are above 6 m and below 800 mm. 1.3 high purity, high flexibility and low Surface roughness due to the different manufacturing methods of calendered copper foil and electrolytic copper foil, the purity of calendered copper foil can reach 99.9% , higher than that of electrolytic copper foil 99.8% . Compared with the vertical needle-like crystalline state of electrolytic copper foil, the bending times of the ordinary rolled copper foil are more than 2 times that of the standard electrolytic copper foil Because of the difference between the surface roughness of electrolytic copper foil and that of smooth surface, the flexure resistance of standard electrolytic copper foil is different from that of calendered copper foil. The smoothness of the surface is determined by the mode of production of the rolled copper foil. Generally speaking, the Surface roughness of rolled copper foil can reach 1 m, which is one fifth of the Surface roughness of standard electrolytic copper foil. Due to the skin effect of charge, the electric performance of the rolled copper foil with small roughness is much better than that of the electrolytic copper foil with large roughness in the process of high frequency charge transmission. The main properties of calendered copper foil and electrolytic copper foil are compared in Table 1. Table 2 is the measured value of the main characteristics of electrolytic copper foil and rolled copper foil.

1.4 means of improving performance and developing new products flexible electrolytic copper foil is produced by copper electrolysis. If the electrolytic copper foil is to be improved or new products developed, this can only be achieved by adjusting the conditions of the copper foil electrolysis process. The calendered copper foil is made on the basis of the traditional strip billet by repeated rolling-annealing process, which can be coordinated by means of microalloying of the strip billet material, controlling the processing rate of the copper foil, adjusting the annealing schedule, etc. , the calendered copper foil with high strength, high flexibility, high elongation and high softening temperature was obtained.

2. Current status and development trend of rolled copper foil at present, more than 90% of global rolled copper foil production capacity is in the hands of Japanese manufacturers, See Table 3; and more than 80% of Japanese manufacturers production capacity is concentrated in Japan, see table 4. Because the production capacity of rolled copper foil is too concentrated and the production cost is high, its output and price are uncertain. In order to reduce the cost and avoid the risk of raw material supply, most industries such as printed circuit boards and lithium batteries have to use electrolytic copper foil instead of rolled copper foil, the development of special electrolytic copper foils such as high ductility electrolytic copper foils and low profile electrolytic copper foils has been promoted. However, due to the limitation of its manufacturing method, its flexibility, Surface roughness and other properties can not be compared with calendered copper foil.

Because the cost of rolling copper foil is higher, at present, calendered copper foil is mainly used in flexible circuit board industry, High Performance Lithium Battery Industry and PCB industry which need high frequency signal transmission, rolling Copper Foil demand changes and development is closely related to the development of these industries. Flexible Printed Circuit Board is a kind of printed circuit with high reliability and flexibility, which is made of polyester film or polyimide by etching and forming lines on copper foil. The Circuit Board has the advantages of high wiring density, thin thickness, light weight, less wiring space limitation, folding, high flexibility, and can be moved and telescoped at will in the 3D, so as to achieve the integration of components assembly and wire connection, its characteristics and electronic products to light, thin, small development in line with. With the development of electronic products in the direction of light, thin and small, electronic equipment is required to be more and more complex, more and more parts are needed, and the requirement for interconnecting and mounting density of printed circuit parts is higher and higher, the contact distance of circuit components on the printed circuit board is reduced, which requires the application of high-density wiring and micro-hole technology to achieve the goal. The thinner the copper foil is, the easier it is to avoid the short circuit caused by the "side corrosion" and to realize the drilling of micro holes, thus ensuring the high reliability of PCB. Compared with other high-performance batteries, lithium-ion batteries have many advantages, such as high specific energy, no memory effect, long cycle life, no pollution, small volume and light weight, etc. . Due to the advantages of low Surface roughness and good ductility after heat treatment, rolled copper foil has been widely used in high performance lithium battery industry, especially in coiled batteries. In high frequency signal transmission, because of "skin effect" , the signals on the Printed Circuit Board will accumulate on the surface of the conductor, so the copper foil is an important part of the conductive layer of the printed circuit board, the effect of the Surface roughness on the transmission loss is significant (at high frequencies, the higher the Surface roughness, the greater the transmission loss) . Low Profile (RZ: 2.7 ~ 3.3 m) or smooth copper foil (RZ: ≤1.7 m) should be used to reduce the transmission loss of high frequency signals. In order to adapt to the rapid development of information network technology, electronic products are required to process information more and more quickly and efficiently, the low profile of copper foil surface is the key to improve the speed of signal transmission, reduce the signal loss and attenuation in high speed signal transmission. In recent years, in order to meet the development needs of electronic products, the development of new calendered copper foil materials has made some progress. For example: by changing the calendering processing conditions to produce ultra-low rough surface calendering copper foil, its Surface roughness up to 0.4 m, more suitable for high-frequency signal transmission; The copper foil with cubic recrystallized structure was fabricated by combining different calendering processing conditions with annealing. The flexure resistance of the foil was more than 4 times of that of the common calendered copper. The flexure reliability of the flexible PCB was higher In order to improve the mechanical strength and elasticity of the rolled copper foil, the micro-alloying treatment was carried out to meet the manufacturing process and requirements of different flexible copper clad laminates.

3. Conclusion

 (1) high purity, high flexibility, low Surface roughness, improving product performance or flexible means to develop new products are the main characteristics of copper foil rolling. 

(2) the complexity of production process, high production cost and high concentration of production capacity are the main factors restricting the development of copper foil rolling, and reducing production cost and breaking industrial monopoly are the key means to enhance the market competitiveness of copper foil rolling.

 (3) the portability of electronic products, the development and wide application of high frequency signal transmission, the requirements of high density interconnect electronic installation technology, and the rapid development of the lithium battery industry, it is decided that the calendered copper foil will be a strong competitor to electrolytic copper foil in the future. 

Source: Chinanews.com, by Zhao Jingsong