Summary of Printed RFID Antenna Problems

With the maturity of RFID (Radio Frequency Identification) technology and the gradual reduction of the price of RFID tags, 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 RFID Antenna Requirements

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 system of RFID tags is mainly composed of three parts, namely tags, readers and antennas. Among them, the manufacturing and printing of antennas have a more and more "close" relationship-due to the high cost and slow speed of the copper wire winding process of the traditional manufacturing technology, and the disadvantages of low precision, polluting the environment, and poor waterproof and folding resistance in the metal foil etching process Therefore, it is a method commonly used in the industry in recent years to directly print RFID tag antennas by printing.

In fact, flexographic printing, gravure printing, inkjet printing, and screen printing can all complete the printing of RFID tag antennas, but from 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 reach 20 μm or more, which is naturally not too difficult for screen printing with an ink layer thickness of 300 μm, but for other printing methods, it is necessary to rely on repeated printing. In order to achieve the desired thickness, this will inevitably put forward higher requirements for printing accuracy. Therefore, the author believes 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, since conductive ink is used in the printing process of RFID tag antennas, it is different from traditional screen printing in some aspects. Special attention should be paid to the following issues.

1. Determination of antenna structure

The antenna mainly plays the role of receiving and sending signals in the whole 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 the 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 as circular or rectangular rings—and with different materials for the substrate—both flexible and rigid.

The RFID tag antenna of the long-distance application system of more than 1 meter needs to adopt a microstrip patch or dipole antenna structure, which mainly works in the ultra-high frequency and microwave frequency bands, and the typical working distance is 1 to 10 meters.

2. Determination of the printing method

Screen printing methods are generally divided into two types: contact type and non-contact type. In the process of contact printing, the substrate is in direct contact with the screen, and the squeegee moves on the screen for printing. Its advantage is that the screen will not be tilted and deformed. 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, it tilts the screen and contacts the substrate to print out graphics. Since the screen can rebound immediately after printing, the printed pattern will not be blurred. When the RFID tag antenna is printed by contact, due to the performance of the conductive ink, it is very easy to smudge, which will have a negative impact on fine printing. Therefore, in order to obtain good printing quality, in actual operation, non-contact printing is often used as the printing method for RFID tag antennas.

3. Selection of conductive ink

The conductivity of conductive ink will be affected by many factors such as the type of conductive material, particle size, shape, filling amount, dispersion state, type of binder, and curing time. The combination of different variables will also have different effects on the conductivity. In view of the extremely high conductivity requirements of the RFID tag antenna, silver-based conductive ink is the first choice. Silver powder for ink is mainly divided into two types: micron-scale and nano-scale, and the commonly used micron-scale silver powder includes two types: flake and spherical. In order to make the silver powder have better contact between the binders, flake silver powder is generally used as the main filler, and nano-silver powder is assisted.

During the printing process, ink resistance may increase due to incomplete drying and thin printing thickness. In addition, if the ink is not stirred thoroughly before printing, due to the high specificity of silver, it is easy to deposit to the bottom, which will lead to 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. These should be paid enough attention to.

Issues requiring special attention

After determining the basic factors such as the printing method and antenna structure, the printing process was not all smooth sailing. In the process of printing RFID tag antennas by screen printing, there will be some unavoidable problems. Here are some examples 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: the partial conductivity is good, the overall conductivity is poor or there is no obvious conductivity, and intermittent lines will be found when observed with a magnifying glass, 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 number is 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 200~300 mesh; Insufficient printing force of the squeegee or uneven force will also lead to uneven ink leakage, the strength of the silk screen squeegee 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 evenly transferred to the substrate, if it is too low, it will cause paste.

2. Electrostatic discharge

Electrostatic discharge, referred to as ESD (ElectroStatic Discharge), is a huge hidden danger in the electronics manufacturing industry and seriously affects the development of the industry. The friction between any two phases in solid, liquid and gas will generate static electricity. During printing, the speed, pressure, ink volume, screen distance, and substrate peeling speed of the squeegee will all generate static electricity, and the operation of the machine itself will also generate static electricity. After the static electricity is generated, it will absorb dust, make the surface of the material dirty or block the screen, resulting in printing defects; static electricity can also cause wire drawing or flying hairs, which will have a greater impact on fine film lines; excessive electrostatic voltage may Break down the air, and then generate sparks, causing a fire.

Electrostatic hazards are so great. In view of its invisibility, randomness, potentiality, and complexity, etc., prevention of ESD phenomena should be given priority, and the following two measures can be used for protection.

① Release method. Through effective grounding, the generated static electricity will be discharged directly to the ground, thereby eliminating static electricity.

② Neutralization method. Neutralize static electricity on label substrates and machines by discharging static electricity of different polarities.

3. Migration of silver powder

In daily work, such a phenomenon often occurs: the performance of the product is good during the factory inspection, and all parameters are completely qualified, but after using it for a period of time, the user finds that the resistance of some products increases, and even short-circuit self-connection occurs. . The reason is that the migration of silver is at work. The problem of silver migration is also the biggest crux that affects the expansion of the application range of silver paste inks. Of course, there is no silver paste without silver migration at all, but we can suppress the migration of silver to a certain extent by properly treating the silver powder. Since the silver powder has a catalytic effect on the gel-removing property of the slurry, ultra-fine flake silver powder with a particle size of 0.1-0.2 μm and an average surface area of 2 m2/g can be used. The Ag-Pd conductive paste prepared by air spray method has relatively stable conductivity even under 200°C and humid conditions, and there is little short circuit phenomenon caused by silver migration.