Brief introduction to smart labels and screen printing processes

Radio frequency identification tags (RFID Labels, also called smart tags, electronic tags, etc., hereinafter referred to as smart tags) have been available for many years, but it is necessary to overcome some technical bottlenecks in mass production and popularization. In this regard, it is imperative that all parties cooperate to overcome the 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 tag

A smart tag is a card or tag attached to an item with detailed item information. see picture 1. 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. A smart tag is not just a simple tag. It is a system that includes reading and writing hardware and software. Due to the different frequencies of work, smart tags use four types of bands: 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 work. The operating frequency determines the number of turns of the antenna coil, the thickness of the conductor cross-section, and the distance between each turn. See Figure 2. The HF band uses a 13.56 MHz chip, which typically requires 6 turns and a thickness of about 20 μm. UHF band uses 868MHz and 950MHz chip, coil section thickness is about 4μm. Can such requirements require screen printing?

Second, antenna coil manufacturing method

At present, there are four methods for producing smart tag antenna coils.

1. Copper wire winding method. For low-frequency 125 ~ 134k chip, suitable for capturing magnetic lines within a short distance. The disadvantages are high costs and slow production speeds.

2. Chemical etching method. Confined to smart labels based on plastic film, paper-based labels cannot be chemically treated. There is a need for plastic film webs with copper or aluminum foils on their surfaces. A circuit diagram (mask) of a 30 μm thick anti-corrosion antenna coil was printed with UV ink over the metal foil. After drying, the film web passes through another printing unit to print the connecting line with conductive ink on the back of the coil. Then the copper foil without mask protection was removed through the etched tank, and the mask was washed off with an alkali bath. The speed of the rotary screen printing unit depends on the etching speed. The general starting speed is 5 m/min.

3. Plating method. First, a coil circuit for attracting copper is printed on the plastic substrate. The ink contains catalyst material silver or palladium, and the printing method is screen printing or flexo printing. Then immersed in a plating bath, the thickness of the copper plating layer is about 10 μm. This method saves material, but the processing speed is too slow.

4. Direct printing. Direct printing of paper or plastic film with silver paste conductive ink, ink layer thickness of about 20μm. Screen printing here has played its strengths.

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

The thickness of one printing ink layer for screen printing can reach up to 300 μm, which is several times that of flexo, offset and gravure. This is very effective for printing antenna coils with conductive ink, or printing circuit masks for etching copper foils. In actual printed circuit production, the thickness of the ink layer is generally 20 μm or more. The different types of smart labels have different requirements for the ink layer thickness and the wire spacing of the coil.

Third, rotary screen printing process

Screen printing has a platform type and rotation type. When the squeegee moves back and forth during the printing of the flatbed screen, the screen is first inked and then the squeegee is printed again. At this time, the stencil is in contact with the substrate, and the squeezing ink is transferred. This kind of exercise is intermittent and the production efficiency is low. Rotary screen printing is continuous ink supply, continuous printing, faster printing speed, better ink transfer capability.