As RFID (Radio Frequency Identification) technology matures and the price of RFID tags gradually decreases, RFID tags are likely to replace traditional one-dimensional barcodes 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 in the middle of RFID antenna demand
RFID is a non-contact automatic identification technology that automatically identifies target objects and obtains relevant data through radio frequency signals. It can work in various harsh environments without manual intervention. The RFID tag system is mainly composed of three parts, namely the tag, reader and antenna. Among them, the manufacture and printing of antennas have an increasingly "close" relationship-due to the high cost and slow speed of the traditional manufacturing technology copper wire winding process, and the metal foil etching process has the disadvantages of low accuracy, pollution of the environment, and poor waterproof and folding resistance Therefore, 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 complete the printing of RFID tag antennas, but from many aspects, it seems that screen printing is superior to several other printing processes, especially the ink layer The thickness factor makes screen printing an absolute advantage. In the actual printing process, the thickness of the ink layer is generally required to be more than 20μm, which is naturally not too difficult for screen printing with an ink layer thickness of 300μm, but for other printing methods, you need to rely on repeated printing In order to achieve the desired thickness, this will inevitably place higher requirements on 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 screen printing
Although screen printing is the most suitable printing process for printing RFID tag antennas, the conductive ink is used in the RFID tag antenna printing process, so it is different from the traditional screen printing in some aspects. The following issues need special attention.
1. Determination of antenna structure
The antenna mainly plays the role of receiving and sending signals in the entire working process of the RFID tag, including four working frequency bands of low frequency, high frequency, ultra high frequency and microwave. According to the different frequency bands, the antennas of RFID tags can be divided into three basic forms: coil type, microstrip patch type and dipole type.
The RFID tag antenna of the close-range application system of 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 at low frequency and high frequency. Coil antennas can be constructed in different ways-either circular or rectangular; substrates can be made of different materials-either flexible or rigid.
The RFID tag antenna of the long-distance application system of more than 1 meter needs to use a microstrip patch or dipole type antenna structure, which mainly works in the ultra-high frequency and microwave frequency bands, and the typical working distance is 1-10 meters.
2. Determination of printing method
Screen printing methods are generally divided into two types of contact and non-contact. In the contact printing process, the substrate directly contacts the screen, and the squeegee moves on the screen for printing. The advantage is that the screen does not tilt and deform. In the non-contact printing process, there is a fixed distance between the screen and the substrate. When the squeegee pushes the slurry to flow through the screen, the screen is tilted and printed with the substrate. Since the screen can rebound immediately after printing, the printed pattern will not be blurred. When contact-printed RFID tag antennas are used, due to the performance of the conductive ink, it is extremely easy to smear, which has an adverse effect on fine printing. Therefore, in order to obtain good printing quality, in actual operation, mostly use non-contact printing as the RFID tag antenna printing method.
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 electrical conductivity requirements of RFID tag antennas, the silver conductive ink is the first choice. Ink silver powder is mainly divided into two types of micron level and nano level, and the commonly used micron silver powder includes two kinds of flake and spherical. In order to make the silver powder have better contact between the connecting materials, the flake silver powder is generally used as the main filler, and the nano silver powder is used as an auxiliary.
During the printing process, you may encounter an increase in ink resistance due to incomplete drying and thin printing thickness. In addition, if the pre-press ink mixing is not thorough enough, due to the large proportion of silver, it is easy to deposit to the bottom, so it 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 attract enough attention.
Issues requiring special attention
After determining the basic factors such as printing method and antenna structure, the printing process was not smooth. In the process of using screen printing to print RFID tag antennas, there will be some inevitable problems. Special examples are provided for readers' reference.
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 there is no obvious conductivity. When you observe with a magnifying glass, you will find intermittent lines, that is, substrates There is no ink on the surface, which is what we often say is 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, and if the mesh number is too low, it will lead to a decrease in line accuracy and affect the quality of fine prints. The number is between 200 and 300 meshes; insufficient squeegee pressing force or uneven force will also cause uneven ink leakage, and the screen printing squeegee strength should be adjusted; ink viscosity problem is also one of the reasons for uneven ink leakage, the viscosity is too high The ink penetration is low and cannot be transferred to the substrate evenly. Too low will cause paste.
2. Electrostatic discharge
Electrostatic discharge (ESD) is a huge hidden danger in the electronics manufacturing industry, which seriously affects the development of the industry. The friction of any two phases in solid, liquid and gas will generate static electricity. During printing, the speed of the doctor blade, pressure, ink volume, screen pitch, and 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 will also cause wire drawing or flying hairs, which will have a greater impact on fine thin film circuits; excessively high electrostatic voltage may be Breaking through the air, which in turn generates sparks and causes fire.
Static electricity is so harmful. In view of its invisibility, randomness, potentiality and complexity, the prevention of ESD should be the mainstay, which can be protected by the following two measures.
â‘ Bleeding method. Through effective grounding, the generated static electricity is directly discharged to the ground, thereby eliminating static electricity.
â‘¡ Neutralization method. By discharging static electricity of different polarities, neutralize the static electricity on the label substrate and the machine.
3. Silver powder migration
In daily work, such a phenomenon often occurs: the product performs well during factory inspection, and all parameters are fully qualified, but after using for a period of time, users find that some products have increased resistance, and even short circuit self-opening phenomenon . The reason is that the migration of silver is causing trouble. The problem of silver migration is also the biggest problem affecting the expansion of the application range of silver paste inks. Of course, at present, there is no silver paste where silver migration does not occur at all, but we can suppress the migration of silver to a certain extent through proper treatment of silver powder. Since silver powder has a catalyst effect on the debinding of the slurry, ultrafine flake silver powder with a particle size of 0.1 to 0.2 μm and an average surface area of ​​2 m2 / g can be used. The Ag-Pd conductive paste prepared by the air-flow spray method has a relatively stable conductive performance even at 200 ° C and humid conditions, and there is rarely a short circuit phenomenon caused by silver migration.
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