3d-printed rfid tags for a specific application
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A NOVEL METHOD FOR 3D PRINTING HIGH CONDUCTIVITY ALLOYS FOR UHF APPLICATIONS
Undergraduate Paper Authors: Craig Bishop, Ian Armstrong, Rolando Navarrete
Advisors: Dr. Michael Marcellin, Dr. Hao Xin Electrical and Computer Engineering Department, University of Arizona
ABSTRACT Traditional approaches to constructing 3D structural electronics with conductive and dielectric materials include ink-‐jet printed, silver-‐bearing ink and fine copper wire meshes. One approach combines stereo-‐lithographic 3D-‐printed photo-‐polymers with direct-‐printed silver-‐bearing conductive inks. Results have shown 3D conductive structures with conductivities in the range 2x106 to 1x107 S/m using annealing temperatures ranging from 110°C to 150°C for 10 to 15 minutes. However, the stereo-‐lithographic approach suffers from the high cost of the printer and structural deformation during annealing. This paper presents a new method for 3d printing high conductivity metal alloys using consumer-‐grade 3D printer. The design and construction of the necessary modification will be presented in addition to the new 3D design process. The method yields metal structures with expected conductivities exceeding 2.6x106 S/m. The process is performed without an annealing step, so the polymeric structural material is not exposed to high temperatures for any prolonged time. A UHF ISM band antenna is constructed for an RFID application using this method, the antenna performance is measured, and the results are compared simulations in Ansys HFSS. This new method can reduce total cost, and several low melting-‐point alloys could raise the conductivity. KEY WORDS RFID, 3D-‐Printing, UHF, Security INTRODUCTION Additive manufacturing has great potential for low-‐volume prototyping and even volume manufacturing in the future. Current commercially available printers extrude a variety of plastics (ABS, PLA, polyester resin), photo-‐cured polymers, and laser annealed granules or powders. In addition, the laser annealing process is capable of printing some metals including steel and titanium. However, only in the last few years has research focused on
integrating conductive metals and dielectric polymers in a single additive process. Doing so promises to change how rapid prototyping of antennas, non-‐planar circuit boards, and even semiconductor packaging is performed. This paper explores a novel method for integrating high-‐conductivity metal with a standard 3D-‐printed ABS plastic without an annealing step. Previous research in the field has achieved full integration of conductors and polymers using stereolithography and direct print technologies (Lopes et al., 2012). By combining a line-‐scan sterolithography system and a direct-‐print system, a seamless process for printing conductors and polymers was achieved. The stereolithographic system laser cures a photosensitive liquid polymer, and silver-‐based conductive inks are deposited using a direct-‐print system with 25 micron lines and spaces. The candidate inks used required curing temperatures of 110°C to 138°C. Medina et al. developed a 3D printed accelerator sensor system using ink dispensing combined with stereolithography. Additionally Medina et al. developed the precursor to the system above for arbitrary-‐form structural electronics printing. For printed RFID tags alone, much research has focused on using traditional and widely available inkjet print heads to dispense silver bearing inks (Pranonsatit, 2012), operating the tags in the 902-‐928 MHz band. For this research project, the high cost of stereolithography 3D-‐printers and the associated materials necessitated an alternative and lower cost method. SYSTEM DESIGN The scope of the project described in this paper extended beyond the fabrication of a 3D-‐printed device to include the demonstration of a UHF RFID tag within an application. An application called Survivable Security was developed to take advantage of the unique properties of a completely 3D-‐printed RFID tag. Unlike traditional retail security tags that are printed or adhesively mounted onto the target product, a 3D-‐printed RFID tag can be structurally embedded into the target product. By designing the security tag as an integral structural component, its removal is made disadvantageous and can even disable the product. Combined with an embedded computer system and standard RFID reader, the system provides unique advantages for high value items. To fulfill its role in a security tag, the RFID tag requires at least a one-‐meter readable range. To meet the low cost requirements, a commercial, low-‐end, 3D-‐printer was selected for the project: a Makerbot Replicator 2X. The Replicator 2X ships with two 0.4 mm extrusion nozzles for use with ABS or PLA filaments. The platform is capable of 100 micron layers. MATERIALS Several conductive materials were investigated, including conductive polymers, silver bearing inks, and low melting point solder alloys. There are conductive polymers available for purchase and use with consumer grade 3D printers. The conductive polymers are
achieved by mixing metallic impurities into the ABS or PLA base, achieving 5000-‐10000 Ohms/cm. The commercially available conductive polymers investigated are not suitable for a UHF antenna. Silver bearing inks were investigated, and shown by several previous researchers to be a viable option for UHF RFID applications. The inks have conductivities ranging from 1x105 to 1x107 depending on cure temperature. For maximum conductivity, inks required cure temperatures of 150°C to 170°C for 15 minutes. For this projects application, conductivity of at least 1x106 was desirable, and conductive ink required annealing temperatures that deform the structural ABS which is printed at 230°C, but starts melting at 105°C. In addition to the previously explored polymers and conductive inks, this project investigate the use of low melting point solder alloys to create conductive components. Standard Sn60Pb40 solder melts at 183°C, which is lower than the nozzle temperature used for ABS plastic. However, a specialty solder alloy, Bi58Sn42, melts at 138°C with a conductivity of approximately 2.898x106 S/m. The solder alloy is commonly available as a solder paste, suspended in a flux gel. Several similar alloys with varying ratios of Bizmuth are available and a Indium-‐Tin alloy achieves a lower melting point of 118°C. For this project, the Bi58Sn42 was selected because of availability and ease of handling. 3D-‐PRINTER MODIFICATIONS The commercial Makerbot Replicator 2X does not support extrusion of solder alloys as sold. Several modifications were designed to adapt one of the two extrusion nozzles for the Bi58Sn42 solder paste. The solder paste is widely available in 5 cc syringes for easy reflow applications. The syringe also provides a convenient mechanism for applying dispensing pressure. The modifications to the 3D-‐printer take advantage of the existing syringe and the existing stepper motor from the Replicator 2X. A custom designed mechanical assembly was designed that utilizes the stepper motor to drive the syringe pump using a thread-‐rod mechanism. All of the stepper motor drive circuitry from the existing Replicator 2X was reused in the new assembly. The syringe pump is a 5.1x5.2x2.4 inch3 L-‐shaped assembly manufactured symmetrically from two pieces of aluminum. The design was completed using SolidWorks and prepared for CNC cutting using Mastercam. The aluminum assembly, shown in Figure 2, incorporates a cavity for mounting the solder paste syringe. A steel lever arm was welded to a hex nut matching the threaded rod and positioned to drive the syringe plunger. Two drive gears were manufactured using the Replicator 2X out of ABS. The stepper motor torque required a 3:1 gear ratio in order to successfully drive the syringe plunger during operation. A plastic timing belt couples the two gears, and two printed, circular pieces of ABS are used to hold the timing belt in place during operation. The ends of the threaded rod are terminated with circular bearings, and an additional metal shaft was used to couple the thread rod to the ABS gear above.
Coupling the syringe to the extrusion nozzle of the Replicator 2X was the largest challenge. A first attempt used high-‐flexibility silicone rubber tubing pressure fitted to the syringe needle and then pressure fitted onto a rolled steel 2 mm outer-‐diameter rolled steel tube inserted into the print shaft on the Replicator 2X. However, the necessary pressure for extruding the solder paste exceeded the 10-‐psi rated tubing. A replacement tube made from ABS rated to 400 psi was able to withstand the necessary pressure. In addition to withstanding the necessary pressure, the coupling mechanism requires a sharp thermal profile in order to prevent solidification of solder paste in the tubing above the extrusion nozzle. The original design using a rolled steel tube inserted into the aluminum print shaft resulted in solidification of solder paste above the print shaft within minutes of starting operation at 150°C. Ideally, the interface is a thermal step junction where the extrusion nozzle is maintained at the print temperature and all the assemblies
Figure 1. Front view of the Replicator 2X with syringe pump assembly installed
Figure 2. Syringe assembly design
Figure 3. Close-‐up of the syringe coupling and entry point into the Replicator 2X
above are maintained at room temperature. To better approximate a step junction, the steel tubing insert was replaced with a Kapton polyimide plastic tubing with a thermal conductivity of 0.37 W/m°K, and order of magnitude lower than the steel tubing. The increased flexibility of the insert tubing required fabrication of a plaster stiffener structure using the ABS nozzle of the Replicator 2X. ANTENNA DESIGN The RFID tag was designed to be passive to reduce the size and cost of each tag. The SL900A RFID IC from AMS (Austria MicroSystems) was selected as the IC for the RFID tags for its reasonable performance and price. The SL900A has a complex impedance of 31 -‐ j320 Ω at 915MHz. Therefore, the target impedance of the antenna design was 31 + j320 Ω so that the antenna and IC would be complex conjugately matched. The SL900A has a differential antenna input, which is well suited for dipole antenna designs, so a half wavelength dipole design was chosen for the RFID tag. The basis for the antenna design was the meandered half wavelength dipole antenna presented by Rao et al. This design was first modeled in HFSS, and then modified to perform with the specified RFIC IC and at the desired operating frequency. The base antenna design parameters are shown below. The lengths, widths and spacing of the meandered segments of the dipole were adjusted to tune the impedance of the antenna to the desired value. The 3D printing method imposed several constraints on the physical dimensions of the conductive traces. The minimum trace width was 0.4mm due to the resolution of the printer head extruder nozzle. The thickness of the traces was limited to a minimum of 0.6 mm. The overall size of the final antenna design was 90mm x 15.5mm x 0.6mm and the size of the printed RFID tag was 100mm x 25mm x 1.6mm.
Figure 4. Parameters of the loaded meander tag antenna (mm)
RESULTS AND CONCLUSIONS Initial results from the project show that 3D-‐printing low melting point solder alloys is possible using commercial grade 3D printers. A test vehicle consisting of a single 0.6mm thick conductive trace, 0.8mm wide and 50 mm long, was printed using the modified printer and syringe assembly. Several iterations were necessary, since the syringe to printer interface is not completely reliable, even with the polyimide tubing. The third iteration produced a 1.4mm thick substrate with a single track of solder alloy in the center. Several attempts to print the complex meander line antenna geometry were only minimally successful so far.
Figure 5. Final antenna design shown in ANSYS HFSS
Figure 6. Plot of antenna impedance over operating frequency range
The first major problem stems from the limited shelf life of the solder alloy paste. The flux mixture evaporates within days after first opening the packaging. The evaporation was countered with addition of a liquid flux to the mixture before insertion into the syringe assembly. However, the volume of added flux was found to severely affect the ability of the system to extrude the solder paste. Without added liquid flux, the paste tended to solidify in the ABS tubing before reaching the heated nozzle. Conversely, a high concentration of added flux resulted in the nozzle leaking molten solder during ABS printing, with the solder paste stepper motor turned off. The results from the project so far prove the fundamental concept of printing a low melting point solder alloy is viable, but requires further development before it is reliable. Further experimentation is necessary to determine the concentration of liquid flux necessary and the exact 3D-‐printer CAM software parameters for the modified nozzle. In addition, exploration of an alumina or mullite ceramic tube to replace the Kapton polyimide tubing may increase reliability of the syringe to printer coupling. Mullite could also better approximate the necessary thermal step junction. ACKNOWLEDGEMENTS The authors thank Xiaoju Yu for her support in completing the HFSS antenna design, and Min Liang for characterizing the 3D-‐printed ABS dieletric properties.
Figure 7. Simple test vehicle after printing with leaked solder paste residue visible
REFERENCES Castillo, S., Medina, F., MacDonald, E., and R. Wicker, “Electronics integration in conformal substrates fabricated with additive layered manufacturing”, Proceedings of the 20th Annual Solid Freeform Fabrication Symposium, 2009, pp. 730-‐7.
“DSP 798LF (Sn42/Bi58) Lead Free Water Soluble Solder Paste”, Qualitek International Inc. Lopes, Amit, MacDonald, Eric, and Ryan Wicker, “Integrating stereolithography and direct print technologies for 3D structural electronics fabrication”, Rapid Prototyping Journal, vol. 18, no. 2, 2012, pp. 129-‐133.
Medina, F., Lopes, A.J., Inamdar, A.V., Henessey, R. Palmer, J.A., Chavez, B.D., and R.B. Wicker, “Integrating multiple rapid manufacturing technologies for developing advanced customized functional devices”, Rapid Prototyping & Manufacturing 2005 Conference Proceedings, 2005.
Rao, K.V.S., Kikitin, P.V., and S. Lam, “Antenna design for UHF RFID tags: A review and a practical application”, IEEE Trans. Antennas Propag., vol. 53, no. 12, pp. 3870-‐3876, Dec., 2005.