Digital Magazine

Rotary Screen Printings Contribution to RFID

Part 1 | Part 2

RFID production certainly has come a long way in the last few years—but a number of technological jumps are necessary before RFID tags can be used at product level. For progress to be made, it is paramount for all in the value chain to work together and show how they can contribute.

Rotary screen technology certainly brings benefits to the RFID label value chain. The unique benefit of screen is that you can bring a deep layer of ink or varnish onto your substrate in one pass—up to 300 µm in fact. That’s many more times than flexo, offset, or gravure, for example. This is useful in three of the main production processes for RFID labels.

So, for beginners, what is RFID (radio frequency identification)? In short, it’s a technology to determine the identity of and to exchange data with individuals (in the form of cards and tickets) and objects, with labels and tags. The main uses are anti-theft, track and trace, and access control.

RFID, of course, is a system, not just a label. You need a reading and rewriting hard and software and the infrastructure to go with it along the value chain. RFID labels differ in the following ways:

  • The frequency at which they work. There are four bands: LF, MF, UHF and Microwave.
  • The read and write range, from 10 cm to 5 m.
  • Data Speed
  • Energy Levels—or active and passive transponders.
  • Memory Capacity—1 to 64 KBit memory.

The standards are based on four main frequencies at which the antennae work.

The LF (low frequency) 125 Khz is for close range, up to 0.5 m. The HF (high Frequency) 13.56 MHz frequency functions at a distance of up to 1 m. UHF frequencies, which range from 868–950 MHz, are for distances up to 5 m and beyond. There is also a microwave frequency of between 2.45 and 5.8 GHz. This has a range of about a meter.

The organizations ISO and EPC Global are working to set international standards for RFID labels. There will be, among others, standards for each of the main frequencies.

The frequency determines the number of coils in the antennae, the thickness of the tracks, and distance between them. HF, whose frequency is 13.56 MHz, usually require between four and six loops. If covered with silver ink the thickness is approximately 20 µm.

HF tag antennae feature a coil formation. They are passive tags—with inductive or capacitive coupling. They would be used for smart cards, item level tracking like library books, and baggage handling outside the US. So their success all depends on the adoption of smart cards.

The long distance UHF cards eventually may drop in price because of the IC design improvements. Applications are pallet tracking and baggage handling. There are differing standards between regions. Japan is 950 MHz, Europe 868 MHz and US 915 MHz. The ink volume requirement is significantly less: about 5 µ

Resistivity is important!
As we have seen, the distance at which the labels function depends a lot on the frequency at which they work. The more conductive the material is, the lower its resistivity will be. And the resistivity determines the frequency and thus the functionality of the label. Resistivity, specifically of an RFID antenna, is determined by:

  • The conductivity and thus type of metal used;
  • The shape of the antenna;
  • The thickness of the antenna;
  • The temperature and air pressure at which it is subjected in the drying chamber and the length of drying time;
  • The temperature and length of time at which it is cured;
  • The pressure applied to the antenna when the RFID construction is laminated, making it part of the main label.

Resistivity varies according to the chemical properties of each metal. Printed silver has to be between 7 and 20 m?, copper between 30 and 120 m?, at 25 µm thickness in both cases.

There are two RFID antenna formations—inductor coil and dipole.

The inductor coil is used for low and high frequency RFID; LF antennae are made of wound copper and have between 200 and 250 coil loops. HF ones have just four to six and can be made out of wound copper wire, etched copper or aluminium, plated copper, or silver ink.

The other type is a square-bracket shaped ‘dipole formation’ for UHF and microwave applications. These can be made out of all processes mentioned above, apart from wound copper wire.

Creating the Antenna—Options
Let us look at all the RFID antenna application technologies:

Wound Copper
Copper is a relatively poor conductor, so its functionality is limited to the low frequencies. But it is more effective at catching the magnetic field lines over short distances, which etched metals fail to do. Its high production costs prohibit its uptake in the high frequency band.

Etching is restricted to filmic substrates since paper does not survive the process. First, the substrate is coated with wither copper or aluminium. The etch-resistant UV-based ink, 30 µm thick, is printed over the intended antenna path or area. This forms a negative image of the RFID antenna. After drying, the web goes through another printing station—to print the conductive strap underneath. The web is treated with an acidic etching solution to remove the unmasked coat. The mask is then removed in an alkaline bath. Production speeds run at about 5 m/min—this speed is usually determined by the time required in the etching bath.

Here, a copper-plate attracting layer, containing a catalyst like silver or palladium, is first printed, for example, with screen or other methods like flexo, and is then fortified with copper by the plating process. In total, ink deposits need to be about 10 µm thick. Although you need less material, the process requires two steps, thus making it slow.

Direct Laydown of Silver Ink
The other technique is the direct laydown of silver conductive ink on paper and filmic substrates. Ink laydown can vary between 5 and 25 µm depending on the required frequency.

Rotary screen printing can be used to make the antennae in all processes except for the wound copper method. It plays an important role because of the speed and accuracy of the printing process, together with the drying and curing system.

Eventually, rotary screen printing must play an important role in these areas of production if the printing industry is going to realistically meet the market’s requirements for these applications.

Drying evaporates the solvents; but in the case of silver ink and the plating processes, a curing operation is needed, too, in order to soften the particles and boost the conductivity. An etch resistant, UV-ink would require curing under a UV A lamp, too.

How Rotary Screen Printing Works
Rotary screen printing enables a clean transfer of the printed image onto the substrate, continuously, in a single operation that repeats with every revolution of the screen. As already mentioned, the maximum deposit is 300 µm, and no other print process comes anywhere near even the 25 µm deposit required to print either the etch-resist or the silver ink. It has a distinct advantage over flatbed, which requires a back-and-forth movement. The doctor blade has to pass over the screen to fill the mesh openings with ink, and then the squeegee pass needs to bring the stencil and substrate into contact to allow ink transfer.

You can control the thickness of the ink layer and its width on the substrate with great precision, whether you are printing the etch-resistant part or the conductive ink itself. This is essential because these factors determine the resistivity of the ink. Equally, care has to be taken when drying and curing the ink.

At the moment, direct laydown is under development. It still looks like solvent-based ink will remain the only suitable means of producing RFID. The use of solvent inks means that drying is a vital part of the screen printing operation. Accuracy is paramount. A drying temperature that is too high also can affect the print quality. If the solvents are evaporated too quickly, one runs the risk of boiling the ink and creating craters along the ink surface.

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