Screen Printing: The Best Choice for Printing RFID Antennas

Screen printing median RFID antenna needs

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 a variety of harsh environments without human intervention. The system of RFID tags mainly consists of three parts: tags, readers and antennas. Among them, the manufacturing and printing of antennas have become increasingly "close" to each other - due to the relatively high cost and slow speed of the traditional manufacturing technology of copper wire winding process, the metal foil etching process has the disadvantages of low accuracy, environmental pollution, and poor waterproof and folding resistance. Therefore, the use of printing methods to directly print RFID tag antennas is a commonly used method in the industry in recent years.

In fact, flexo printing, gravure printing, inkjet printing, and screen printing can all complete the printing of RFID tag antennas. However, from many aspects, it seems that screen printing is superior to other printing processes, especially ink layers. 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 20 μm, which naturally does not have much difficulty for screen printing where the ink layer thickness can reach 300 μm, but for other printing methods, it needs to rely on repeated printing. In order to achieve the desired thickness, this inevitably imposes higher requirements on printing accuracy. Therefore, I believe that screen printing is the most suitable printing process for printing RFID tag antennas.

Non-traditional screen printing of non-traditional rules

Although screen printing is the most suitable printing process for printing RFID tag antennas, since the RFID tag antenna is printed with a conductive ink, it differs from traditional screen printing in some respects in the printing process. You need to pay special attention to the following issues.

1. Determination of antenna structure

The antenna mainly plays the role of receiving and transmitting signals during the entire working process of the RFID tag, including low-frequency, high-frequency, ultra-high-frequency and microwave four working frequency bands. According to the different frequency bands, the antenna of the RFID tag can be divided into three basic forms: a coil type, a microstrip type, and a dipole type.

The RFID tag antenna of a close-range application system less than 1 meter generally adopts a coil-type antenna structure with a simple process and low cost, and its operating frequency band is mainly located at low frequency and high frequency. Coil antennas can be constructed in different ways—either circular or rectangular, and the substrate can be made of different materials—either flexible or rigid.

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

2. Determination of printing methods

Screen printing is generally divided into contact and non-contact two. In the contact printing process, the substrate is in direct contact with the screen, and the squeegee moves on the screen to perform printing, which has the advantage 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 in contact with the substrate. Since the screen can rebound immediately after printing, the printing pattern will not be blurred. When contact-type printed RFID tag antennas are used, due to the performance of conductive inks, dirt pick-up is extremely likely to occur, which has an adverse effect on fine printing. Therefore, in order to obtain good printing quality, non-contact printing is often used as an RFID tag antenna printing method in actual operations.

3. Selection of conductive ink

The conductive properties of the conductive ink will be influenced 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 requirements on the conductivity of RFID tag antenna, the first choice is silver conductive ink. Silver powders for inks are mainly classified into micrometer and nanometer grades, while commonly used micron silver powders include sheets and spheres. In order to make the silver powder have better contact between the connecting materials, flake silver powder is generally used as the main filler and nano silver powder is auxiliary.

In the printing process, the increase in ink resistance due to incomplete drying and thin printing thickness may be encountered. In addition, if the ink before printing is not sufficiently stirred, the silver is likely to be deposited on the bottom because of its large specific gravity, which may 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. These should be given enough attention.

Problems that require special attention

After determining the basic factors such as printing mode and antenna structure, the printing process is not always smooth. The use of screen printing printing RFID tag antenna, there will be some unavoidable problems, special examples of tips, for the reader to learn from.

Ink leakage is not uniform

In the process of printing RFID tag antennae by screen printing, this situation is often encountered: local electrical conductivity is good, overall electrical conductivity is poor or there is no obvious electrical conductivity, and observation with a magnifying glass reveals intermittent lines, that is, substrates. There is no ink on the surface, which is what we often call ink leakage. There are many reasons for this phenomenon. For example, if the number of screen meshes is too high, the ink permeability will be poor. If the number of meshes is too low, the line accuracy will be degraded and the quality of fine prints will be affected. Therefore, the screen is generally selected. The number is 200~300 mesh; the insufficient imprinting force of the squeegee or the uneven force can also cause the ink leakage to be uneven, and the intensity of the screen printing squeegee should be adjusted; the viscosity of the ink is also one of the reasons that cause ink leakage unevenness, and the viscosity is too high. The ink has low penetration and cannot be uniformly transferred to the substrate. Too low will result in a paste plate.

2. Electrostatic discharge

Electrostatic Discharge (ESD), which is referred to as Electro Static Discharge (ESD), is a huge hidden danger in the electronics manufacturing industry and seriously affects the development of the industry. Friction between any two solids, liquids, and gases creates static electricity. During printing, static electricity is generated by the speed, pressure, amount of ink, net distance, and peeling speed of the substrate of the squeegee, and the operation of the machine itself generates static electricity. After static electricity is generated, dust will be absorbed, causing the surface of the material to be dirty or the screen will be blocked, causing printing defects; static electricity will also cause wire drawing or flying, which will have a greater impact on the fine film circuit; excessive electrostatic voltage may be Break through the air, creating sparks and causing fire.

The electrostatic hazard is so great that given its invisibility, randomness, potentiality and complexity, the ESD phenomenon should be mainly prevention, and it can be protected by the following two measures.

1 discharge method. By effectively grounding, the generated static electricity is directly discharged to the earth to eliminate static electricity.

2 Neutralization method. Unleash static electricity from the label substrate and the machine by releasing static electricity of different polarity.

3. Silver migration

In the daily work, there is often such a phenomenon: the product performs well in the factory inspection, and the parameters are completely qualified. However, after a period of time, the user finds that some products have increased resistance, and even short-circuit self-pass phenomenon. . The reason is that the migration of silver is working. The issue of silver migration is also one of the biggest issues affecting the expansion of silver ink applications. Of course, there is no silver paste that does not undergo silver migration at present, but we can suppress silver migration to a certain extent by appropriate treatment of silver powder. Since the silver powder has a catalyst effect on the degumming property of the slurry, an ultra-fine flake silver powder having a particle size of 0.1 to 0.2 μm and an average surface area of ​​2 m 2 /g can be used. The Ag-Pd conductive paste prepared by the air-spraying method has a relatively stable electrical conductivity even at a temperature of 200° C. and moisture, and short-circuiting due to silver migration rarely occurs.