We present the possibilities and challenges of passive UHF RFID tag antennas manufactured by inkjet printing silver nanoparticle ink on versatile paper-based substrates. The most efficient manufacturing parameters, such as the pattern resolution, were determined and the optimal number of printed layers was evaluated for each substrate material. Next, inkjet-printed passive UHF RFID tags were fabricated on each substrate with the optimized parameters and number of layers. According to our measurements, the tags on different paper substrates showed peak read ranges of 4–6.5 meters and the tags on different cardboard substrates exhibited peak read ranges of 2–6 meters. Based on their wireless performance, these inkjet-printed paper-based passive UHF RFID tags are sufficient for many future wireless applications and comparable to tags fabricated on more traditional substrates, such as polyimide.
The development of the Internet of Things has created a need for cost-effective wireless electronics on environmentally friendly substrates. Great potential lies especially in inkjet printing and inkjet-printed antennas [
RFID technology is a wireless identification technology to automatically identify and track physical objects or people by using radiofrequency waves. When using RFID tags for identification, multiple devices can be read simultaneously and a line-of-sight is not necessary [
The goal of this paper is to study the possibility of inkjet printing on various paper and cardboard substrates and to optimize the printing parameters in order to effectively fabricate passive UHF RFID tags on these substrates. The ready RFID tags are evaluated for their wireless performance and compared to tags fabricated on a more traditional polyimide substrate.
The used ink was Harima NPS-JL silver nanoparticle ink [
Specifications of the utilized ink [
Ink | Ag NPS-JL NanoPaste® |
---|---|
Solid content (wt%) | 52–57 |
Particle size (nm) | 5–12 |
Average particle size (nm) | 7 |
Resistivity ( |
4–6 |
Viscosity (mPa·s) | 11.5 (measured at 20°C and 60 rpm) |
Recommended thermal curing | 120–150°C for 60 minutes |
The substrate material plays a big role in additive RFID tag manufacturing. Different substrate materials need different printing parameters, because they have different surface properties and morphologies. In [
Substrate properties.
Substrate | Thickness | Speciation |
---|---|---|
Paper A | 100 |
Uncoated paper |
Paper B | 80 |
Coated, calendered |
Paper C | 80 |
Double coated: film + blade |
Cardboard A | 500 |
Double coated: film + blade |
Cardboard B | 500 |
Base board |
There are several parameters that can affect the printing quality, such as the jetting pulse shape, the jetting frequency, the jetting voltage, and the temperature of the ink cartridge, as well as the pattern resolution. The used jetting pulse shape is shown in Figure
Printing resolution of every substrate.
Substrate | Average drop size ( |
Angle (degree) | Resolution (dpi) |
---|---|---|---|
Paper A | 110 | 11.4 | 508 |
Paper B | 90 | 9.1 | 635 |
Paper C | 95 | 10.2 | 564 |
Cardboard A | 90 | 9.1 | 635 |
Cardboard B | 105 | 11.4 | 508 |
Printing parameters.
Cartridge temperature (°C) | 40 |
Platen temperature (°C) | 50 |
Jetting voltage (V) | 28 |
Jetting frequency (kHz) | 23 |
Sintering time (minutes) | 60 |
Sintering temperature (°C) | 150 |
The used jetting pulse shape.
Droplet size test on all substrates: (a) droplets on Paper A, (b) droplets on Paper B, (c) droplets on Paper C, (d) droplets on Cardboard A, and (e) droplets on Cardboard B.
After finding the optimized printing parameters, simple lines with dimensions of 5 mm × 30 mm were printed on each substrate to study the optimal number of layers. In theory, when the antenna design and the substrate are the same, antennas with better conductivity (lower DC resistance) should have higher read ranges. Based on the datasheet of the ink, the sintering was done at 150°C for 60 minutes to maximize the conductivity of the printed layer [
Resistances of printed lines on all substrates.
Substrate | Total layer(s) | Resistance (Ω) | Description |
---|---|---|---|
Paper A | 2 | ∞ | Not conductive, absorbed |
4 | ∞ | Not conductive, absorbed | |
8 | ∞ | Not conductive, absorbed | |
|
|||
Paper B | 1 | 1.8 | Good conductivity, totally metallic |
2 | 0.8 | Good conductivity, totally metallic | |
4 | 0.3 | Good conductivity, totally metallic | |
|
|||
Paper C | 2 | 9.7 | Good conductivity, mostly metallic |
4 | 2.8 | Good conductivity, totally metallic | |
6 | 2.1 | Good conductivity, totally metallic | |
|
|||
Cardboard A | 1 | ∞ | The edge of line is metallic, mostly black |
2 | ∞ | The edge of line is metallic, mostly black | |
4 | 79 | The edge of line is metallic, mostly black | |
6 | 14 | The edge of line is metallic, mostly black | |
8 | 1.3 | The edge of line is metallic, partially black | |
1-S-1 | 5 | Totally silver, good conductivity | |
2-S-2 | 0.8 | Totally silver, good conductivity | |
4-S-4 | 0.3 | Totally silver, good conductivity | |
|
|||
Cardboard B | 2 | ∞ | Not conductive |
4 | 2.5 M | Badly conductive | |
8 | 161 | Badly conductive |
Inkjet-printed line patterns for fabrication optimization: (a) four-layer lines on Paper B, (b) four-layer lines on Paper A, (c) four-layer lines on Paper C, (d) four-layer lines on Cardboard A, and (e) four-layer lines on Cardboard B.
Microscopic images of the inkjet-printed layers: (a) surface of a one-layer line on Cardboard A, (b) surface of an eight-layer line on Cardboard A, (c) surface of a four-layer line on Cardboard B, (d) surface of a four-layer line on Paper A, (e) surface of a one-layer line on Paper B, and (f) surface of a two-layer line on Paper C.
One-layer lines were firstly printed on Cardboard A, and they showed no conductivity. But from Figure
The conductivity of the lines on Cardboard B was not good. The resistances of the four-layer lines and the eight-layer lines were found to be 2.5 MΩ and 161 Ω, respectively. Figure
On Paper A, the inkjet-printed lines showed no conductivity even with eight printed layers. The surface of the four-layer line is mostly black and no coherent metallic trace formed after sintering, which is shown in Figure
On Paper B, the one-layer lines obtained good conductivity after sintering and the resistance was measured to be around 1.8 Ω. As shown in Figure
The two-layer lines on Paper C show tolerable conductivity as the average resistance is around 9.8 Ω, although the surface of the printed line is partially black, as shown in Figure
After finding the optimized printing parameters and the optimal number of layers, passive UHF RFID tag antennas were fabricated on Paper B, Paper C, Cardboard A, and Cardboard B. The tag antenna structure applied in this study is shown with a manufactured tag in Figure
(a) A ready inkjet-printed UHF RFID tag on Paper B. (b) The utilized tag antenna geometry.
The used tag IC was NXP UCODE G2iL series RFID IC [
The wireless performance of the tags was evaluated with read range measurements using an RFID measurement unit. The tags were tested wirelessly using Voyantic Tagformance measurement system [
As expected based on the resistance measurements, Paper B was found to be the most suitable substrate. As can be seen from Figure
The measured read ranges of all tags: (a) the read ranges of tags on Paper B, (b) the read ranges of tags on Paper C, (c) the read ranges of tags on Cardboard A, and (d) the read ranges of tags on Cardboard B.
Figure
Judging from the data in Figure
Thus, depositing multiple layers directly does not always correspond to a higher read range and the optimized number of layers for antennas on each substrate material needs to be studied separately. In case of the tags printed on Cardboard A and Paper C, which were both double coated materials, the double coating most probably causes the substrate not to absorb as much ink as the other substrates. Thus, twelve printed layers are too much on these coated substrate materials, causing the read ranges to decrease compared to tags with eight or six printed layers. On paper-based porous substrates, the substrate will absorb some of the deposited ink. When depositing too much ink on the substrate, most probably the conductive layers are not as uniformly connected. In addition, the ink can spread and destroy the shape of the printed pattern. Thus, the read range will decrease when too many layers are printed.
The resistances of the printed lines on Cardboard B were very high. Also the performance of the tags on this substrate is unsatisfying. The read ranges of the inkjet-printed tags on Cardboard B are shown in Figure
Generally, the dielectric substrate affects the tag performance through its electrical properties, such as loss tangent and relative permittivity. Porous substrates like paper and cardboard also have an indirect effect on the tag through the ink film morphology [
In this paper, the possibility of inkjet printing on versatile paper and cardboard substrates using silver nanoparticle ink was studied. The printing parameters were optimized for each substrate material in order to fabricate passive UHF RFID tags on these substrates. It was discovered that, in addition to the printing parameters, also the number of printed layers needs to be studied separately for each substrate material. The wireless performance of the fabricated tags was evaluated and the read ranges of the tags were found to be comparable to tags inkjet-printed on a polyimide substrate. In the future, the use of copper nanoparticle ink on these paper and cardboard substrates will be studied for potential cost reduction.
The authors declare that there are no competing interests regarding the publication of this paper, and the mentioned received funding in Acknowledgments did not lead to any competing interests regarding its publication.
This research work was supported by the Academy of Finland and TEKES.