Screen printing and smart labels

RFID labels (also known as smart labels, electronic labels, etc., hereinafter referred to as smart labels) have been available for many years, but they need to overcome some technical bottlenecks for mass production and universal use. 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 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. 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 bands, namely: LF, HF, UHF and microwave. Its effective reading/writing distance is 0.1~5.0 m, and its data exchange speed, storage capacity, and energy level are 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. The UHF band uses 868 MHz and 950 MHz chips, and the coil section thickness is about 4 μm. 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 125~134 k chips, 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 corrosion-resistant antenna coil printed with UV ink on top of 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 approximately 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 20 μ m. Screen printing here has played its strengths.

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

The thickness of a 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 ink layer thickness is generally more than 20 μm. 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.

Sweden's Stork Print has more than 40 years of experience in rotating screen printing technology. It created a dedicated production line for the antenna coil printing of smart tags, named Rotary Screen Integration (referred to as RSI), see Figure 3. Print widths of 10, 16, 20, and 24 inches (25.4 cm, 40.64 cm, 50.8 cm, and 60.96 cm) are optional. The screen printing cylinder is made of pure nickel material and has a long service life; the mesh hole is hexagonal and the ink is completely transferred. It can continuously print 500,000 m without stopping the machine. The roller can be used up to 15 times. At present, the method of directly printing antenna coils for producing smart tags is still in the research stage, and it seems that only solvent-based inks can be used to meet this printing process. Thus, the drying of the solvent becomes an obstacle. The accuracy of the coil pattern printing is very important, too high drying temperature will affect the printing quality. Because the solvent volatilizes too quickly, a series of crater-like spots will appear on the surface of the ink layer.

Drying of printed circuits requires special care. If the printed circuit does not follow the contours of the design, its conductivity may be affected. The thickness of the coil ink layer and the width of each line have a strict tolerance range. The error of two overlapping overlays must be within 0.1 mm. Lines are too close to each other, and alienation is not enough. These printing errors will cause the antenna to malfunction.

In order to ensure that the ink is fixed in the best shape, the wind speed in the oven must be reduced. The heating temperature is controlled at 140 °C and the heating time is between 30 and 120 s. Extending the length of the drying tunnel can increase the speed of the production line. The printing speed is generally 10 m/min. Due to increasingly stringent environmental protection requirements, solvent recovery is the best measure, not only protecting the environment, but also recycling the solvent can be reused. Therefore, there should be an exhaust passage in the drying tunnel of the production line to allow the gas with volatile solvent to concentrate for condensation.

Ink quality is also an important factor. It is necessary to increase the conductivity and reduce the consumption of ink.

The use of smart labels has become increasingly widespread, and identity-based ID cards, e-passports, supermarket pricing cards, etc. have all been put into practical use. Experts expect that the world market demand in 2005 will increase by 8%. In 2006, it will increase by another 10% to reach an annual output of 250 million. With an average selling price of $1 each, annual sales can reach $2.5 billion.

However, for this emerging business, it is difficult for ordinary printers to enter. One is that a larger initial investment is needed to purchase processing equipment, and the other is to have professional talents who must understand both the quality inspection and software testing. There is also a product standard issue, the current product is not yet standardized, today's equipment can meet the standards after two years? In spite of this, smart labels are still promising for everyone. Through the joint efforts of component manufacturers and equipment manufacturers, production costs have been declining and product quality has gradually improved. The promotion of large-scale use is not far off.

Source: Screen Printing