The aim of this paper is to examine the potential of inkjet printing technology for the fabrication of Near Field Communication (NFC) coil antennas. As inkjet printing technology enables deposition of a different number of layers, an accurate adjustment of the printed conductive tracks thickness is possible. As a consequence, input resistance and
Near Field Communication (NFC) is a recent wireless technology working on HF band that enables the exchange of data in a short range. It is an ideal solution where reliable exchange of data is paramount to quality user operations such as mobile payments. It is estimated that this system will be used in more than 1.2 billion smartphones by 2015 [
In this context, there are a number of approaches to implement an NFC antenna layout onto a selected substrate. The earliest and most cumbersome method of antenna fabrication has been subtractive etching on a metal-plated laminate (e.g., FR4). Subtractive etching has matured to the point where it has become a low-tech process requiring inexpensive off-the-shelf materials and equipment. However, for larger production runs, etching creates significant amounts of metal salt and chemical waste products, incurring increased costs due to regulatory fees.
Contrary to the traditional etching technique, inkjet printing technology is a direct-write technology by which the designed pattern is transferred directly to the substrate [
It has been estimated that there are about 1500 worldwide research units working on various aspects of flexible electronics [
Up to date, inks made of silver conductive nanoparticles have been used to produce printed electronic circuits on flexible surfaces. Metallic nanoparticle inks typically consist of silver or gold nanoparticles with diameters ranging from just 2 nm to over 50 nm, encapsulated by a protective shell and dispersed in a liquid solvent [
In the fabrication process, several parameters are involved, such as, size of ink nanoparticles, substrate, surface tension of the particle and of the substrate, time and temperature of the ink sintering, among others [
The usage of inkjet technology for NFC implementation on flexible terminals stills finds another motivation for the antennas involved. The
As long as a decreasing of
Therefore, an exploration of the potentials and limitations of inkjet printing technology for NFC flexible terminals should be addressed in this context since it provides a low cost, environmentally friendly manufacturing process on flexible materials with no damping resistance involved.
This paper provides a rigorous approach, involving analytical and computational models, for rectangular planar spiral coils at 13,56 MHz with subsequent experimental verification on flexible substrates. It is divided into four parts. The second section deals with the methodology, where a general method for the design, fabrication, simulation, and measurement of inkjet printed loop antennas for NFC systems on a flexible substrate is explained. The third part includes the implementation and obtained results. Lastly, the fourth part states the conclusions.
In this section, all the necessary equations to analytically calculate the total inductance,
Rectangular planar spiral inductor.
The total inductance of an N-turn planar spiral inductor coil (Figure
The self inductances of a rectangular cross-section conductor can be obtained using the following equation [
The mutual inductance is the inductance that results from the magnetic fields produced by adjacent conductors. The mutual inductance is positive when the directions of the current along the conductors are in the same direction, and negative when the directions of currents are in opposite directions. The mutual inductance between two parallel conductors of equal length is a function of the length of the conductors and of the geometric mean distance
The geometric mean distance
Two conductor segments for mutual inductance calculation.
The thickness of the conductive track deposited by inkjet printing technology can be adjusted by the number of layers micron by micron. Provided that skin effect at 13,56 MHz is approximately 47
The spiral inductor
Frequency domain solver implemented by CST MICROWAVE STUDIO simulation software has been selected to make the numerical computation of the design. Adaptative hexahedral mesh is used and local mesh refinement strategy is adopted within the coil conductive track.
The material used as substrate is Dupont Kapton polyimide film 127
SunTronic Jettable Silver U5714 ink by Sunjet has been used as conductive ink. It is a solvent-based silver nanoparticle inkjet ink and 20% of its weight is due to silver particles.
The printer that has been used is the Dimatix Materials Printer DMP-2831 (Figure
Cartridge and Printer Dimatix 2831 [
DMC-11610 cartridge which has 16 nozzles, 254
Once the prototypes have been printed, a sintering process is required to improve the conductivity of the ink. The manufacturer recommends a range of temperature and time to sinter the silver ink and this depends on the amount of solid material present in the ink. In general, the lower the sintering time or temperature, the higher the electric resistance since a larger gap between the nanoparticles occurs. This process (Figure
Sintering process [
Once the methodology to print is defined, some traces are printed into a substrate to study the corresponding thickness depending on the number of layers.
A KLA-Tencor P-16+ Profiler has been used to measure the overall thickness of the printed traces and estimate the one of each layer by average. In Figure
Obtained profiles for different number of printed layers.
A thickness of 0.8
Once the thickness of the printed layers is found, conductivity is characterized.
Although conductivity is independent of the number of printed layers, the conductivity study has been done in samples with different number of layers, which correspond to different electrical resistance and thickness pairs for comparison purposes. The electrical resistance of the structure has been measured using a Keithley 6517B Electrometer/High Resistance Meter.
The conductivity can be calculated using (
Obtained conductivity values with 30 min@200°C sintering process.
Material: silver | Conductivity | Conductivity with respect to pure silver (%) |
---|---|---|
Pure | 100 | |
One layer | 12.6 | |
Two layers | 14.3 | |
Three layers | 12.5 | |
Six layers | 13.5 |
The obtained results by this method are in accordance with the ones found in bibliography [
Once the antenna has been printed, its impedance can be measured by a standard VNA according to the setup shown in Figure
Measurement set up.
The reflection coefficient will be used as figure of merit to estimate the correlation between the results obtained by analytical formulas, computed numerical models, and measurements.
The amplitude of the reflection coefficient between two different complex impedances can be calculated as [
An analytical study of the effect of the thickness for a 5 turn rectangular (80 mm × 50 mm) planar spiral inductor with
Nominal dimensions of ID-1 card type.
Width (mm) | Height (mm) | Thickness (mm) |
---|---|---|
85.6 | 53.98 | 0.76 |
The analytically obtained inductance for the presented antenna is 2.82
Calculation of
A
In practice, it is not feasible to fabricate coil antennas with such a high thicknesses due to the ink spilling over the track when too many layers are mounted on it. Two different prototypes have been implemented; one with 4 layers and another one with 6 layers (Figure
Obtained results of the two prototypes: analytical (A), simulation (S), and measured (M).
A | S | M | ||
---|---|---|---|---|
4 layers | 46,58 | 51,13 | 50,25 | |
2,82 | 2,79 | 2,9 | ||
5,16 | 4,64 | 4,91 | ||
6 layers | 31 | 35,118 | 32,35 | |
2,82 | 2,79 | 2,947 | ||
7,75 | 6,77 | 7,76 |
Printed prototype (6 layers).
The correlation between the three outcomes (analytical, simulated, and measured) is studied as explained in the methodology validation section. The obtained values are summarized in Table
Results validation.
Comparison | |||
---|---|---|---|
4 layers | A | S | −25.16 |
A | M | −21.94 | |
S | M | −20.35 | |
6 layers | A | S | −22.78 |
A | M | −15.39 | |
S | M | −14.08 |
All the correlations are below −10 dB; therefore, it can be concluded that the obtained results are reliable. In order to explore inkjet printing technology limits once the procedure is validated, a prototype with 10 layers has been fabricated. This represents a reasonable trade off between delivered performance in terms of
The potential of using inkjet printing technology to fabricate NFC antennas has been investigated. A general methodology for the design, simulation, fabrication, and measurement process has been explained.
The ink conductivity is shown to be around
Inkjet printing technology is therefore a promising technique for the fabrication of NFC coil antennas. Moreover, due to its low cost manufacturing process and substrate flexibility, it is a perfect candidate for the implementation of environmentally friendly NFC terminals into flexible substrates.
Iñaki Ortego’s scholarship has been granted by the Department of Education of the Basque Government. Joseba García’s contract is partially supported by the Spanish Ministry of Education within the framework of the Torres Quevedo Program, and cofinanced by the European Social Fund. The authors would like to thank the collaboration agreement between Tecnun and CST that has made possible the achievements of this piece of research.