Two alternative prototyping techniques suitable for rapidly producing low-cost built-in-lab RFID UHF tags have been developed in our laboratory: SI_FPCB and CP_FACS.
Solid Ink Based Tag Prototyping on Flexible PCBs: SI_FPCB
Different techniques for production of high frequency circuits and antennas can be based on the use of flexible instead of rigid PCBs, thus guaranteeing the precious added value of the flexibility which is usually required in RFID tags development. Several types of flexible PCBs are on the market. Among them, without loss of generality, DuPont Pyralux samples have been taken into account. Typically, Pyralux is distributed as thin laminate sheets. Each sheet is composed of a flexible substrate of polyamide of thickness ranging from 12 to 45 µm and a copper-clad laminate of thickness varying between 12 and 35 µm.
Based on the use of such Pyralux laminates, a new very practical alternative prototyping solution, the SI_FPCB whose flow-chart is represented in Figure 1, can be introduced. As shown, only two main steps are needed. Indeed, once the antenna layout has been optimized, it is directly printed on the Piralux sheet by a so-called solid-ink printer which, differently from traditional office printers, deposit a layer of a wax-based ink which perfectly adheres on copper layers. Moreover, such solid ink at the same time shields the copper from subsequent acid etchings, which represents the second and last step of the procedure. In this
way, compared with the typical photolithography on rigid substrates, the prototyping process is greatly accelerated and simplified, as time consuming and tedious steps such as UV exposure and sodium hydroxide etching are not required anymore. Indeed, a tag is realized in less than 30 minutes, making use, de facto, only of the last step of the photolithography process. Moreover, flexibility is guaranteed and materials and costs are significantly reduced. A suitable printer costs less than 800 USD and guarantees thousands of prints with a few dollars of wax-based ink. As for the consumables, 15 USD are enough for a single A4 format Pyralux sheets, whilst the ferric chloride has a negligible cost per antenna.
It is worth highlighting that the use of wax-based solid ink printers (a Xerox Phaser 8550 in the case of Figure 1) is crucial, because common ink-jet and laser printers are not appropriate at all. The former because common liquid ink does not adheres on copper, the latter because laser printers cannot print on reflecting surfaces.
As for the better resolution achievable with this technique, it strongly depends on the resolution of the printer. Most of solid ink printers guarantees a resolution inferior to 250 µm. Nevertheless, in the author’s experience, antennas requiring resolutions better than 600 µm (very unusual in UHF RFID tag design) require a very careful realization in clean rooms and the etching time should be opportunely estimated. Vice versa, no particular precautions are needed for classical RFID antennas, where resolution higher than 1 mm are usually adopted.
Cutting Plotter Based Tag Prototyping on Flexible Adhesive Copper Sheets: CP_FACS
CP_FACS (Cutting Plotter Based Tag Prototyping on Flexible Adhesive Copper Sheets) is a novel and particularly time-saving technique useful to prototype extremely flexible and high-performing RFID tags through the use of a cutting plotter. This machine is regularly used in the graphic industry for cutting and shaping vinyl foils. Indeed, a cutting plotter is similar to a printer, but it is equipped with a precise cutting tip. In the antenna realization context, the use of cutting plotters is substantially new. The idea is to use flexible adhesive copper sheets instead of vinyl foils. In this way, in a first step the tag antenna layout can be straightforwardly shaped on the copper sheet surface. In the second and last step, the extra adhesive copper is manually removed. In Figure 2, for instance, the flow-chart of the CP_FACS procedure is reported and the two mentioned steps, the tag shaping and the extra copper removing, are highlighted.
The use of this prototyping technique promises immediate advantages especially in terms of average prototyping time. Indeed, only few seconds are required to incise the tag antenna on the support and about a couple of minutes to remove the extra copper, depending on the tag complexity. Therefore, less than 5 minutes including soldering are necessary to obtain a functioning tag. Moreover, this technique could be also attractive because it does not make use of chemicals and, moreover, guarantees very low cost of both installation and management. Indeed, the cost of suitable cutting plotter is less than 1000 USD and the only necessary consumables are the adhesive copper sheets, costing less than 5 USD each for the A4 format.
Finally, for the CP_FACS prototyping process, the achievable resolution mainly depends on two different factors. The first one is the cutting plotter resolution, which is around 250 µm also for entry level machines. The second one is the fragility of the copper. Indeed, once the antenna shape has been incised, the elimination of the extra copper becomes a crucial operation if lines 250 µm large are used. No problem have been experienced with very simple antennas requiring 500 µm of resolution, whilst for very complicated antenna shapes, lines larger than 1 mm should be preferred.
L. Catarinucci, R. Colella, L. Tarricone, “Smart prototyping techniques for UHF RFID tags: Electromagnetic characterization and comparison with traditional approaches,” Progress in Electromagnetics Research (PIERS), Vol. 132, doi:10.2528/PIER12080708, pp. 91-111, 2012. Scopus ID: 2-s2.0-84867072543.
L. Catarinucci, R. Colella, L. Tarricone, “Prototyping Flexible UHF RFID Tags Through Rapid and Effective Unconventional Techniques: Validation on Label-Type Sensor-Tag,” IEEE International Conference on RFID-Technologies and Applications (RFID-TA), p. 176-181, Nice, November 5-7 2012.