Progress in RFID label printing technology
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Always limited by factors such as high cost, RFID Labels have not been widely available in the domestic market.
At present, the progress of RFID tag technology mainly focuses on the manufacturing materials of antennas and chips. In terms of printing technology, it has become an important research direction of printed electronics technology.
Compared with other technologies of RFID tag production, the birth of printing technology makes RFID tags have incomparable advantages in terms of low cost, high efficiency and environmental protection, which provides great possibilities for further use in the civilian field.
Progress in manufacturing materials
Antenna material
Mainly used in copper and aluminum, the mainstream RFID antennas are made by coil winding and etching processes. The products have great advantages in reliability and service life, so they occupy most of the market.
However, with the rise of nanotechnology, nano inks (made of conductive materials) that can directly print RFID antennas present challenges to existing materials. According to IDtechEX (well-known market research company), global conductive inks accounted for US$2.86 billion in 2012. It is expected that the conductive ink market will reach US$3.36 billion in 2018. By then, the market for nano-silver conductive inks and nano-copper conductive inks will reach 735 million. Dollar.
However, the nano-conductive ink printed RFID antenna technology still faces problems such as ink adhesion, abrasion resistance, printing uniformity, conductivity and oxidation resistance.
(1) Nano silver conductive ink
Nano silver conductive ink has excellent conductivity and oxidation resistance, and its high technical maturity is the preferred material for printing RFID antennas.
The research group of South China University of Technology used self-made nano silver particles with an average particle size of 62.79 nm as the conductive filler, polyurethane/acrylic resin as the binder, ethyl acetate and ethanol as the solvent, and added appropriate amount of oleic acid dispersant. Mixed dispersion, formulated into conductive ink. The ink resistivity can be on the order of 10-4 Ω·m, and the number of rubbing resistance is more than 6,000 times, which can resist multiple tearing of the sealing tape.
Huazhong University of Science and Technology related research group used sodium hypophosphite as reducing agent, sodium hexametaphosphate as dispersing agent, PVP as protective agent, reacted with silver nitrate solution to obtain purple-red silver sol, and added pH adjuster to adjust the pH of silver sol. Adjusted to 3, filtered, passivated, washed, etc., vacuum dried at 60 ° C to obtain nano silver powder with an average particle size of about 30 nm, and then added to the FA-406 ink to form a conductive ink, after curing, The conductive ink has good flexibility and heat and humidity resistance, and has high conductivity and uniform printing.
(2) nano copper conductive ink
Although the nano-copper conductive ink is slightly inferior in conductivity than the nano-silver conductive ink, the price has a great competitive advantage and has become a research hotspot in recent years. However, the chemical nature of copper is active, which makes it difficult to avoid oxidation of nano-copper particles in synthesis and application.
The related research group of Huazhong University of Science and Technology used the method of laser heating liquid metal to vaporize it to prepare nano-copper powder with an average particle diameter of 88 nm. After being coated with organic polymer, the particles were directly oxidized by contact with air. After the obtained copper powder is added to the solvent, the resin and the auxiliary agent for high-speed dispersion, the adhesion on the PI film can be as high as 5B, the resistivity is 10-7 Ω·m, and the UHF RFID antenna is fabricated by the squeegee-mask process. After the chip is packaged, the recognition distance can reach 3 meters, as shown in Figure 1 and Figure 2.
figure 1
figure 2
Guangdong Dongshuo Technology Co., Ltd. successfully prepared spherical nano-copper with a diameter of 30-50 nm in the organic solvent ethylene glycol, and no other oxides existed. This indicates that ethylene glycol can prevent the oxidation of nano-copper. The polyethylene glycol dispersant can increase the dispersion stability and uniformity of the nano copper particles.
2. Chip material
In the field of chip manufacturing, the benchmark for measuring the manufacturing process is the width of the connection between the transistor and the wire in the chip, that is, the line width and the finer the width, the more precise the manufacturing process. Due to the limitation of the size of the atomic component, the line width of the semiconductor chip has its physical limit, and the chip volume cannot be infinitely small. Therefore, it is difficult to increase the frequency of the on and off power to a certain extent.
The use of carbon nanotubes (CNTs) instead of silicon transistors is the best way to break through this bottleneck. Compared with other semiconductor materials, CNTs are not only small in size, excellent in electrical properties, but also stable in physical and chemical properties. Moreover, transistors constructed by CNTs have the advantages of less heat generation and higher operating frequency. At the same time, CNTs are easy to be solutionized, separated and purified. The latter CNTs inks can print high performance thin film transistor devices.
Inevitably, CNTs also have the disadvantage that the prepared CNTs contain both metallic and semiconducting CNTs. Therefore, in the field of chip manufacturing, it needs to be separated to obtain semiconductor-like CNTs in order to play a corresponding role.
At present, the separation of single-walled CNTs is generally based on chemical methods, which are relatively mature, such as electrophoretics and chromatographic methods, but often involve various chemical agents, such as the addition of surfactants to produce doping effects. Therefore, separation requires multiple steps of physical and chemical processes to complete.
In recent years, a new method of separating CNTs has emerged, the Coulomb explosion method. The Coulomb explosion method uses the principle of electrostatic repulsion to bring a bundle of single-walled CNTs with the same kind of charge. When the repulsive force between the charges is greater than the van der Waals force between the CNTs, a Coulomb explosion occurs. This explosion separates a bundle of carbon nanotubes into a unique, novel radial pattern called the "nanotree." The Ramanspectra and other tests have shown that the separated CNTs have a smaller diameter, less than 3 nm, and even a single CNTs with complete structure.
Other new technologies have emerged, such as the identification of metallic and semiconducting single-walled CNTs using DNA from different sequences. Although DNA identification is sensitive, it is costly and time consuming. Shortening this length can separate the single-walled CNTs with different properties by two-phase extraction. The whole process takes only 4 minutes, which is dozens of times faster than the traditional method, and the cost is greatly reduced, which is the separation of single-walled CNTs. Really meaningful applications offer the possibilities.
Progress in printing technology
1. Antenna printing process
At present, RFID antennas are often used in screen printing, but the screen printing production efficiency is low, the printing precision is not high, the ink layer is thick, and the cost is high, which has gradually failed to meet the requirements of fine-grained antenna printing. Many experts and scholars at home and abroad have passed theoretical analysis and experimental verification that RFID tag antennas can be passed by conductive inks. It is printed by inkjet printing and has the advantages of thin printing ink layer, low printing cost, good conductivity and easy control of antenna pattern.
The Institute of Industrial Informatics, Institute of Industrial Automation, Shenyang Institute of Automation, Chinese Academy of Sciences, printed a chipless RFID tag antenna using digital inkjet printing technology. The tag antenna was printed directly using nano silver conductive ink instead of the traditional one. Complex etching process to achieve the performance requirements of high frequency RFID tags 13.56MHz. The research team is further improving the nozzle drive system, motion control system, and photoelectric detection system of the printing equipment, and fully overcomes the key technologies such as the use of the printed resonant circuit to replace the codec function of the RFID tag chip.
Shanghai Heyu Technology Development Co., Ltd. uses its self-developed high-speed roll-to-roll automatic production equipment to produce RFID tag antennas by printing, which greatly improves production efficiency. At the same time, its unique all-copper copper addition technology is compatible with the SMT (SurfaceMountedTechnology) placement process. The electrical properties of copper itself make the product superior to aluminum etched and silver-printed antennas, and product quality consistency is guaranteed in a roll-to-roll precision printing process.
2. Chip printing process
Unlike traditional inorganic RFID tags, new organic RFID tags are all passed Printing technology prints metal and organic inks on the same substrate to obtain antennas and chips. The working principle, structure, function and spectrum division of organic RFID tags are not much different from inorganic RFID. The main difference between the two is whether the transistors in the chip are made by printing process.
When an RFID chip is fabricated using a silicon transistor, industrial waste such as heavy metals and toxic gases is generated, and a large amount of water is consumed, which takes a long time to produce. The transistors of the organic RFID chip can be directly printed on a common plastic substrate by using a high-speed multicolor printer. Mass production of organic RFID tags through roll-to-roll printing technology can significantly reduce costs. According to related reports, the cost of all-organic RFID tags will be reduced to 0.01-0.02 USD per piece. According to research, the RFID market using organic circuits in 2015 will reach $11.6 billion.
Based on the original work, the Institute of Nanotechnology and Nano-Bionics of the Chinese Academy of Sciences has developed a variety of methods for the selective and selective separation of semiconductor carbon nanotubes through chemical methods and polymer selective coating methods, and constructed by modern printing technology. A simple logic circuit such as a carbon nanotube thin film transistor device (having a mobility and a switching ratio of 40 cm 2 /Vs and 107, respectively), a fully printed flexible thin film transistor device, and an inverter. This has laid a solid foundation for the application of carbon nanotube printed thin film transistor devices in the fields of LED, OLED, RFID and the like.
Manufacturers and consumers are concerned about the performance and price of RFID tags. RFID tags manufactured by printing methods are only improved in materials and processes compared with traditional RFID tags. In terms of frequency, reading distance, and usage environment, There has been no substantial improvement and, therefore, it is not entirely an innovative product. However, due to the impact of environmental protection policies, the manufacture of RFID tags by printing is still the trend of the times.
It is believed that with the unremitting efforts of researchers, the manufacturing materials and processes used in RFID tags will gradually break through. By then, a true low-cost, environmentally-friendly RFID tag will enter people's lives.
The above content is selected from the 7th issue of “ Printing Technology †of Keyin Media in 2014. For more journal content, please pay attention to the journal channel .
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