Screen printing and smart labels

Radio frequency identification tags (RFID labels, also known as smart tags, electronic tags, etc., hereinafter referred to as smart tags) have been available for many years, but to mass production and popularization, there are still some technical bottlenecks to overcome. In this regard, there is an urgent need for all parties to coordinate and overcome difficulties. For example, in every smart tag, a metal antenna is indispensable. What can screen printing do to produce antenna coils for smart tags?

1. What is a smart label

Smart tags are cards or tags attached to items that record detailed item information. see picture 1. Each smart tag has 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, etc. A smart tag is not just a simple tag. It is a system that includes hardware and software for reading and writing. Smart tags use 4 different frequency bands due to different operating frequencies, namely: LF, HF, UHF and microwave. Its effective reading / writing 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 cross-section and the distance between each turn. See Figure 2. The HF band uses a 13.56MHz chip, usually it requires 6 coil turns and a thickness of about 20μm. The UHF band uses 868MHz and 950MHz chips, and the coil cross-section thickness is about 4μm. Can this requirement be met by screen printing?

Second, the manufacturing method of the antenna coil

There are currently four methods for making smart tag antenna coils.

1. Copper wire winding method. The chip used for low frequency 125 ~ 134k is suitable for capturing magnetic lines of force in a short distance. The disadvantage is high cost and slow production speed.

2. Chemical etching method. Limited to smart labels based on plastic film, paper-based labels cannot withstand chemical corrosion treatment. Here, a plastic film roll covered with copper foil or aluminum foil is required. Print the circuit diagram (mask) of 30μm thick anti-corrosion antenna coil on the metal foil with UV ink. After drying, the film web passes through another printing unit to print the connection line with conductive ink on the back of the coil. Then remove the copper foil without mask protection through the corrosion bath, and wash off the mask with an alkali bath. The speed of the rotary screen printing unit depends on the corrosion speed, and the general starting speed is 5m / min.

3. Electroplating method. First, a coil circuit that attracts copper is printed on the plastic film base. The ink contains the catalyst substance silver or metal palladium. The printing method is screen printing or flexo printing. Then immersed in the plating tank, the thickness of the copper plating layer is about 10μm. This method saves material, but the processing speed is too slow.

4. Direct printing. Directly printing paper or plastic film rolls with silver paste conductive ink, the thickness of the ink layer is about 20μm. Here screen printing has played its specialty.

As can be seen from the above, three of the four production methods are related to screen printing.

The thickness of one printing ink layer in screen printing can be up to 300μm, which is several times that of flexo, offset and gravure printing. This is very effective for printing antenna coils with conductive ink, or printed circuit masks for corroding copper foil. In actual printed circuit production, the thickness of the ink layer is generally above 20 μm. Different types of smart tags have different requirements for the thickness of the ink layer of the coil and the spacing of the wires.

3. Rotary screen printing process

Screen printing is divided into platform type and rotary type. During the flat screen printing, the scraper moves back and forth, first inking the screen, and then scraping 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 the production efficiency is low. Rotary screen printing is continuous ink supply, continuous printing, faster printing speed, and better ink transfer ability.

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