Screen printing and smart labels
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Radio frequency identification tags (RFIDLabel, also known as smart tags, electronic tags, etc., hereinafter referred to as smart tags) have been available for many years, but to be mass-produced and widely used, there are still some technical bottlenecks to overcome. In this regard, it is urgent for all parties to cooperate and overcome difficulties. For example, in every smart tag, a metal antenna is indispensable. What can screen printing do for antenna coils that produce smart labels?
First, what is a smart label
A smart tag is a card or tag that is attached to an item with detailed item information. Each smart tag carries a chip, and the data in the chip exchanges information with the outside world through the antenna. Mainly used for logistics management, anti-theft, access control, automatic payment, precious animal tracking and so on. A smart tag is more than just a simple tag. It is a system that includes both reading and writing hardware and software. Smart tags use four bands, LF, HF, UHF, and microwave, depending on the frequency of operation. Its effective read/write distance is 0.1~5.0m, and its data exchange speed, storage capacity and energy level are all related to the frequency of antenna operation. The operating frequency determines the number of turns of the antenna coil, the thickness of the wire section, and the distance between each turn. The HF band uses a 13.56 MHz chip, which typically requires a coil number of 6 and a thickness of about 20 μm. The UHF band uses 868MHz and 950MHz chips with a coil cross-section thickness of approximately 4μm. Can such a requirement for screen printing be done?
Second, the antenna coil manufacturing method
At present, there are four methods for manufacturing smart tag antenna coils.
1. Copper wire winding method. It is used for low frequency 125~134k chips, suitable for capturing magnetic lines in short distances. The disadvantage is high cost and slow production.
2. Chemical corrosion method. Limited to smart labels based on plastic film, paper-based labels cannot withstand chemical etching. Here, a plastic film web having a copper foil or an aluminum foil coated on its surface is required. A wiring pattern (mask) of a 30 μm thick anti-corrosion antenna coil was printed on the metal foil with UV ink. After drying, the film web is printed with a conductive ink on the back side of the coil through another printing unit. The copper foil without the mask protection is then removed through the etching bath, and the mask is washed away with an alkali bath. The speed of the rotary screen printing unit depends on the corrosion speed, and the general starting speed is 5 m/min.
3. Electroplating method. First, a coil circuit for attracting copper is printed on the plastic substrate, and the ink contains silver or metal palladium as a catalyst material, and the printing method is screen printing or flexo printing. Then, it was immersed in a plating bath, and the thickness of the copper plating layer was about 10 μm. This method saves material, but the processing speed is too slow.
4. Direct printing. The paper or plastic film web is directly printed with a silver paste conductive ink, and the thickness of the ink layer is about 20 μm. Here, screen printing has played its specialties.
As can be seen from the above, three of the four manufacturing methods are related to screen printing.
The thickness of a printing ink layer in screen printing can be up to 300μm, which is several times that of flexo, offset and gravure. This is very effective for printing antenna coils with conductive inks or for printing circuit masks for etching copper foil. In the actual production of printed circuits, the thickness of the ink layer is generally required to be 20 μm or more. Different types of smart labels have different requirements for the thickness of the ink layer of the coil and the spacing of the wires.
Third, the rotary screen printing process
Screen printing has a platform type and a rotary type. In the case of flat screen printing, the scraper moves back and forth. First, the screen is inked, and then the printing is performed again. At this time, the film is in contact with the substrate, and the ink is squeezed to transfer it. This movement is intermittent and production is less efficient. Rotary screen printing is continuous ink supply, continuous printing, faster printing speed and better ink transfer ability.