Detailed analysis of printed RFID antennas and summary of issues that need attention
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With the maturity of RFID (Radio Frequency Idenficaon) technology and the gradual decrease in the price of RFID tags, RFID tags are likely to replace traditional one-dimensional bar codes and two-dimensional codes. If the two-dimensional code is an extension of the one-dimensional code label, then the birth of RFID may be called a revolution in the label industry.
Screen printing hits the demand for RFID antennas
RFID is a non-contact automatic identification technology that automatically recognizes target objects and obtains relevant data through radio frequency signals. It can work in a variety of harsh environments without manual intervention. The RFID tag system is mainly composed of three parts, namely the tag, the reader and the antenna. Among them, the manufacturing of antennas and printing have a more and more “close” relationship-due to the high cost and slow speed of the copper wire winding process of traditional manufacturing technology, and the metal foil etching process has the disadvantages of low precision, environmental pollution, and poor waterproof and folding resistance. Therefore, the direct printing of RFID tag antennas by printing is a method commonly used in the industry in recent years.
In fact, flexographic printing, gravure printing, inkjet printing and screen printing can all complete the printing of RFID tag antennas. However, considering many aspects, it seems that screen printing is superior to other printing processes, especially the ink layer. The factor of thickness gives screen printing an absolute advantage. In the actual printing process, the thickness of the ink layer is generally required to be more than 20μm. This is naturally not too difficult for screen printing where the thickness of the ink layer can reach 300μm, but for other printing methods, it needs to rely on repeated printing. In order to achieve the desired thickness, this will inevitably put forward higher requirements for printing accuracy. Therefore, I believe that screen printing is the most suitable printing process for printing RFID tag antennas.
Non-traditional rules of non-traditional silk screen printing
Although screen printing is the most suitable printing process for printing RFID tag antennas, conductive ink is used in the printing process of RFID tag antennas, so it is different from traditional screen printing in some aspects. Need to pay special attention to the following issues.
1. Determining the antenna structure
The antenna mainly plays the role of receiving and sending signals in the entire working process of the RFID tag, including 4 working frequency bands of low frequency, high frequency, ultra high frequency and microwave. According to different frequency bands, RFID tag antennas can be divided into three basic forms: coil type, microstrip patch type and dipole type.
The RFID tag antenna of a short-range application system less than 1 meter generally adopts a coil-type antenna structure with simple process and low cost, and its working frequency band is mainly located in low frequency and high frequency. Coil antennas can be constructed in different ways-either a circular ring or a rectangular ring; the substrate can be made of different materials-either a flexible substrate or a rigid substrate.
The RFID tag antenna of a long-distance application system of more than 1 meter needs to adopt a microstrip patch or dipole-type antenna structure, which mainly works in ultra-high frequency and microwave frequency bands, and the typical working distance is 1 to 10 meters.
2. Determination of printing method
Screen printing methods are generally divided into contact type and non-contact type. In the contact printing process, the substrate is in direct contact with the screen, and the squeegee moves on the screen for printing. The advantage is that the screen will not be tilted or deformed. In the non-contact printing process, there is a fixed distance between the screen and the substrate. When the squeegee pushes the slurry through the screen, the screen is tilted and contacts the substrate to print graphics. Since the screen can rebound immediately after printing, the printed pattern will not be blurred. When contact printing RFID tag antennas are used, due to the properties of conductive ink, smearing is extremely easy to occur, which has a bad influence on fine printing. Therefore, in order to obtain a good printing quality, in actual operation, non-contact printing is often used as the printing method of the RFID tag antenna.
3. Selection of conductive ink
The conductivity of conductive inks will be affected by many factors such as the type of conductive material, particle size, shape, filling amount, dispersion state, type of adhesive, and curing time. The combination of different variables will also have different effects on the electrical conductivity. In view of the extremely high requirements for conductivity of RFID tag antennas, silver-based conductive ink is the first choice. Silver powder for ink is mainly divided into two types: micron-level and nano-level, and commonly used micron-level silver powder includes flake and spherical two types. In order to make the silver powder have better contact between the connecting materials, generally choose the flake silver powder as the main filler, and the nano-silver powder is auxiliary.
During the printing process, the ink resistance may increase due to incomplete drying and thin printing thickness. In addition, if the pre-printing ink is not thoroughly stirred, due to the high proportion of silver, it is easy to deposit to the bottom, which will cause problems such as low silver content in the upper layer of the ink, increased resistance, high silver content in the lower layer, and reduced adhesion. All these should arouse enough attention.
Issues that require special attention
After determining the printing method, antenna structure and other basic factors, the printing process is not smooth. In the process of printing RFID tag antennas by screen printing, there will be some unavoidable problems. Special examples are given for readers to learn from.
1. Uneven ink leakage
In the process of printing RFID tag antennas by screen printing, this situation is often encountered: local conductivity is good, overall conductivity is poor or no obvious conductivity, when you observe with a magnifying glass, you will find intermittent lines, that is, the substrate. There is no ink on the surface, which is what we often call uneven ink leakage. There are many reasons for this phenomenon. For example, if the screen mesh is selected too high, it will lead to poor ink permeability, while the mesh number is too low, which will result in a decrease in line accuracy and affect the quality of fine prints. Therefore, the screen mesh is generally selected. The number is 200~300 mesh; insufficient squeegee imprinting force or uneven force will also cause uneven ink leakage, so the strength of the screen printing squeegee should be adjusted; ink viscosity is also one of the reasons for uneven ink leakage, and the viscosity is too high , Ink penetration is low and cannot be transferred evenly to the substrate. If it is too low, it will cause paste.
2. Electrostatic discharge
Electrostatic discharge is abbreviated as ESD (Electro Stac Discharge), which is a huge hidden danger in the electronics manufacturing industry and seriously affects the development of the industry. The friction of any two of solid, liquid and gas will generate static electricity. During printing, the speed of the squeegee, the pressure, the amount of ink, the net distance, and the peeling speed of the substrate will generate static electricity, and the operation of the machine itself will also generate static electricity. After static electricity is generated, it will attract dust, make the surface of the material dirty or block the screen, causing printing defects; static electricity can also cause wire drawing or flying, which will have a greater impact on fine film circuits; excessively high electrostatic voltage may be possible Break through the air, and then generate sparks, causing a fire.
The electrostatic hazard is so great. In view of its invisibility, randomness, potentiality and complexity, the prevention of ESD phenomenon should be the mainstay, and the following two measures can be used to protect it.
①Bleeding method. Through effective grounding, the generated static electricity is directly discharged to the ground, thereby eliminating static electricity.
② Neutralization method. By releasing static electricity of different polarities, the static electricity on the label substrate and the machine can be neutralized.
3. Silver powder migration
In daily work, there will often be such a phenomenon: the product performs well during factory inspection, and all parameters are fully qualified, but after a period of use, users find that some products have increased resistance or even short-circuit and self-passing. . The reason is that the migration of silver is at work. The problem of silver migration is also the biggest sticking point affecting the expansion of the application range of silver paste inks. Of course, there is no silver paste that does not cause silver migration at all, but we can suppress the migration of silver to a certain extent by properly treating the silver powder. Since silver powder has a catalytic effect on the debinding of the slurry, ultra-fine flake silver powder with a particle size of 0.1~0.2μm and an average surface area of 2m2/g can be used. The Ag-Pd conductive paste prepared by the air-spraying method has relatively stable conductivity even under 200°C and humid conditions, and short-circuit phenomena caused by silver migration are rare.
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