In passive Radio Frequency Identification (RFID), transponders or tags are used to label objects to be identified. In this study passive tag antennas were produced using etching, screen-printing, and gravure printing methods. The threshold and backscattered signal strengths of the tags were measured to determine the effect of different manufacturing methods on the tags' performance. Conductivity, skin depth, thickness, and the quality of the conducting layer have a major effect on tag performance. Each manufacturing method sets its own boundary conditions on the processibility of the high quality conduction layer and such conditions need to be considered in tag design. Tag design also affects the manufacturing parameters used in the different techniques. The results of the study show that each of the studied fabrication methods can be used to manufacture reliable RFID tags.
The use of passive UHF RFID systems is increasing rapidly in numerous applications. Globally, the allowed frequencies for UHF RFID range from 840 MHz to 955 MHz, depending on the local regulations [
Antennas are integrated into a variety of applications such as product packaging and clothing. Currently copper is the most commonly used conductor in tag antennas and the etching is the most widely used manufacturing technique to produce the conductive patterns. However, the cost of antennas is a crucial factor in the mass production of antennas and there is an increasing need to develop new manufacturing techniques to enable the manufacturing of RFID tags on complicated curved surfaces at economically competitive cost. This can be achieved by applying new economical manufacturing methods to produce the antenna structures. Printing techniques may provide a new and fast way to do this. In printed electronics silver particles are often used to form the conductive layer, and therefore it is important to optimize the amount of silver and the thickness of the conductive layer. At the same time it is also important that the ink layer is thick enough to achieve low ohmic losses. However, in RFID systems thin conducting layers are preferred to maintain low manufacturing costs.
The penetration depth and ohmic losses of the conductive layer set the boundary condition for the manufacturing methods and manufacturing parameters such as the amount of silver and the thickness of the layer. One factor controlling the thickness of the conductive layer is the skin effect or penetration depth. At high frequencies, like UHF, the current density is packed in the region near the surface of a good conductor. This is called the skin effect. Skin depth or penetration depth is defined as the depth below the surface of the conductor at which the amplitude of the incident electric field decays to 0.37 percent of the amplitude at the surface of the conductor.
For good conductors, the penetration depth has the well-known approximate expression
RFID tags can be made in various ways. In this study, etching, screen, and gravure printing techniques were compared and used to produce RFID antennas. Etching, which is the traditional way of making the conductors, was used as a reference technique for the printing techniques. It is a well-known method and is widely used to produce conductive patterns in the electronics industry. However, printing techniques have several advantages over the etching technique. Printing is a fast and environmentally friendly process; less hazardous chemicals are needed and printing can be used to produce 3D conductive patterns in an economically competitive way.
In etching process, a thin copper or aluminum foil on a substrate material is etched into the form of the designed antenna pattern. In an industrial process the IC-chip can be connected to the antenna, for example, by reflow soldering. In case the antenna is etched on a special substrate, which does not tolerate reflow process or the antenna pattern is printed, conductive adhesives can be used. Usually the antenna structure is also shielded with a thin layer of plastic to improve its endurance. In etching process, the substrate material must tolerate the chemicals used in the etching process, and this restricts the choice of substrate material. Polyethelene Terephthalate (PET) is commonly used as an antenna substrate material. The thickness of the foils is typically 18
In the electronics industry there is increased use of a variety of printing techniques to produce electrical conductors. The main components used in a printed RFID antenna are conductive ink and a substrate. Conductive inks consist of a polymer matrix, conductive fillers, and solvents. Silver particles are commonly used as the conductive filler material. Paper, plastic, and fabric can be used as the substrate material. The printing techniques offer a new way to transfer electronics circuits onto several materials, such as paper, plastic, and fabric. Different printing technologies require different ink characteristics [
Screen printing is commonly used in electronic manufacturing. The conductive ink is pressed through a stencil onto the substrate with a squeegee. The stencil consists of the frame and a fabric mesh of threads. The advantage of this technique is that it enables very thin printing and also very thick films (from 0.02
Gravure printing is a technique, which uses an engraved cylinder to transfer the figure to the substrate. The engraved figure on the cylinder consists of gravure cells which hold the ink while transferring the figure [
Input impedance at the antenna terminals depends on the geometry and material of the antenna and all other proximate materials. By definition, the radiation resistance of an antenna is the resistance, which relates the radiated power to the current at the antenna terminals. Similarly antenna loss resistance relates the terminal current to the difference of the accepted power and radiated power. The total resistance at the antenna input is the sum of these two resistances and its input reactance is related to the reactive power in the near field zone [
Tag antennas used in the present study are rectangular short dipole tags equipped with a triangular matching loop. This matching technique is a slightly modified embedded T-matching, which is discussed in [
Figure
Dimensions of different antenna designs.
Dimension [mm] | Screen- and gravure-printed | Etched |
---|---|---|
8 | 9 | |
97 | 100 | |
17 | 15.6 | |
0.5 | 0.5 | |
2 | 2 | |
5 | 6 |
Shape and dimensions of the printed tag design.
From the simulated antenna impedance we calculated the Power Reflection Coefficient (PRC) [
In our study we have used impedance
In practice the nonuniform distribution of the ink becomes apparent when the printed layer is very thin, as in the gravure-printed tag. This effect is particularly evident in narrow traces, where the current density is high, such as in the matching loop of the manufactured tag (see Figure
Simulated surface current density in the antenna structure under a plane wave incidence with different phases of incident wave.
The effect of the non-ideal print quality in this part the antenna was modelled afterwards by reducing the nominal trace width by 30% to account for the effect of the ragged edge of the actual printed conductor. This makes it possible to predict the frequency shift in the measurement results (see Figures
The RFID tags shown in Figure
Screen-printed tags were printed with screen printable polymeric silver ink. The characteristics of the screen printing ink are presented in Table
Characteristics of screen printable ink.
Manufacturer’s description | Curing conditions ( | Viscosity (P) | Conductivity (MS/m) |
---|---|---|---|
Single component silver ink consisting of polyester resin and silver particles. | 120, 20 | 200–300 | 1.25 |
Gravure-printed tags were printed using polymeric rotogravure ink. The characteristics of the gravure printable ink are presented in Table
Characteristics of gravure printable ink.
Manufacturer’s description | Curing conditions ( | Viscosity (P) | Conductivity (MS/m) |
---|---|---|---|
Silver pigment in a thermoplastic resin for flexographic or rotogravure printing techniques. | 120, 20 | 40 | 4 |
Information on the printed samples is presented in Table
The printed samples.
Sample | Symbol | Printing equipment | Number of ink layers |
---|---|---|---|
Sample 1 | Screen | Screen (mesh/width of thread: 124/27) | 2 |
Sample 2 | Gravure | Gravure cylinder (Depth of cells: 60 | 1 |
Copper tags were produced by etching. Copper of 20
The thickness of the conductive layer was measured before antenna measurements using software connected to an optical microscope. The thickness values are an average value of 20 different measurement points in the middle of the cross-section, which is marked Figure
Location of the cross-section for the thickness measurement.
The copper layer is more uniform than the printed layers and has a thickness of 20
Two different quantities, threshold power and backscattered signal power, were measured to study the performance of the fabricated tags. All the measurements were made with Voyantic Tagformance measurement unit [
Threshold power is the minimum sufficient power to activate the IC-chip. Measurement of threshold power was performed by increasing the transmitted power until the tag can respond to the reader’s query command. The effect of path loss and tag antenna gain can be approximated by using a calibration tag, with known properties. Calibration tag is provided by the manufacturer of the measurement device, as part of the measurement system. Decisive factors for the threshold power are the quality of the conjugate match between the antenna and the IC-chip, and the sensitivity of the IC-chip.
Backscattered signal power is a critical factor for the performance of an RFID tag. Assuming that the tag has sufficient power to activate the IC-chip, backscattered signal power together with the reader’s sensitivity are the key factors for the tag-reader read range and reliability of the reception.
In addition to the above-described performance parameters an intuitive way to evaluate the performance of an RFID tag is the detection range. It is the maximum distance at which a valid response from the tag can be received. In a passive RFID system the detection range is typically limited by the forward link operation and more specifically the IC-chip’s sensitivity. Detection range also depends on the radiation patterns and polarization of the reader antenna and the tag and the propagation channel. The detection range patterns in two orthogonal planes were measured for the fabricated tags. In this measurement the polarizations of the reader and tag antenna were matched and measurement was done with 2 W transmitted ERP-power, which is the maximum allowed transmitted power in the European RFID band.
When printing is used to produce prototype tag antennas there are thickness variations in the ink layer. The thickness measurement results of the printed samples are presented in Table
Thickness measurement statistics for the printed tags.
Sample | Average [ | Min [ | Max [ | Standard deviation [ |
---|---|---|---|---|
Screen | 21.5 | 16.8 | 26.0 | 2.5 |
Gravure | 3.8 | 1.6 | 5.4 | 0.8 |
Scanning electron micrograph of a cross-section of a screen-printed sample.
Simulated antenna impedances.
Calculated PRC’s according to simulated antenna impedances with
Simulated antenna impedances for different antennas are shown in Figure
PRC’s of different antenna models according to the simulated antenna impedances.
Measurement results for threshold power and backscattered signal power are presented in Figures
Measured threshold power versus frequency.
Measured backscattered signal power versus transmitted power at 866 MHz.
Simulation results in Figure
In reality accurate knowledge of the chip impedance is important for realization of impedance matching at the intended frequency. To account for uncertainties in the strap impedance, due to parasitics of the chip package and attachment to the antenna,
Simulation results in Figure
The measurement results for backscattered signal power in Figure
Since the manufactured tags are very similar to each other, differences in the measured power levels can be attributed to differences in conductivity and conductor thickness. The commercial Alien Squiggle tag was measured here to provide a frame of reference for the measured values. However, since it is smaller in size, no direct comparison can reasonably be made of its performance against the results from the other tags.
Measured detection range patterns for the fabricated tags are presented in Figure
Measured detection ranges (m) at 866 MHz in two orthogonal planes, using 2 W ERP-power.
Narrow conductive line in antenna, printed with gravure printing.
The penetration depth of gravure ink is 8.6
The penetration depth of screen printable ink is 15
The penetration depth in copper is 2.2
The performance of printed silver ink dipole antennas and copper dipole antennas are investigated, for example, in [
The results we obtained are in accordance with above mentioned results in the references. Printed tags were tuned at the desired frequency and we managed to manufacture a printed tag with almost the same geometry as the copper tag; only a minor change was done to optimize the antenna impedance.
Possible restrictions of printed tags are the maximum ink layer thickness and resolution achieved with different printing methods. Still it is possible to increase the conductivity of the ink, for example, by decreasing the particle size and adding solid content of the ink, but this increases the tag price. Also the printing method has to be selected according the tag model and materials. So it is most probably possible to achieve the same performance as the etched tags, but this increases the costs.
Antenna designing is always a compromise between, among other things, antenna performance, physical dimen-sions of the tag, price, materials (substrate material, conductor material, materials nearby during the use of the tag), and manufacturing requirements (process limitations).
RFID tags can be manufactured using each of the methods investigated in this paper. The performance of the fabricated screen-printed tag is comparable to the copper etched tag with the same conductor thickness. Observed minor difference in their performance is due to the lower conductivity of the screen-printing ink. Decreasing conductor thickness increases losses and thereby decreases efficiency and results to weaker backscatter from the tag.
For a thin conductor layer, the nonuniform distribution in the conducting material in a narrow trace which was included in the matching network of the antenna detuned the antenna significantly impairing its performance. Thus the print quality needs special attention when UHF RFID tags are printed using very thin conducting layers or narrow traces.
Obtained results support the use of printing process as an alternative to etching in manufacturing of RFID tags. Especially printing is an advantageous manufacturing method when the substrate does not tolerate the etching chemicals or tags need to be integrated directly on the product packages.
The financial support of the Finnish Cultural Foundation, Satakunta Regional Fund, and the Ulla Tuominen Foundation is gratefully acknowledged.