ORIGINAL ARTICLE Characterization of UHF RFID tags fabricated directly on convex surfaces by pad printing Sari Linnea Merilampi & Toni Björninen & Leena Ukkonen & Pekka Ruuskanen & Lauri Sydänheimo Received: 4 February 2010 / Accepted: 28 July 2010 / Published online: 24 August 2010 # Springer-Verlag London Limited 2010 Abstract The possibility of pad printing in RFID tag antenna manufacturing is investigated. Passive UHF RFID tags were printed on flat and on convex surfaces with two different polymer thick-film silver inks. The effect of the ink and substrate material properties on tag antenna performance was examined. The goal was to provide information which is needed in adapting the pad-printing technique in RFID tag manufacture. The results show that pad printing is suitable for tag manufacturing on flat and on convex surfaces. The curvature of the substrate did not significantly affect the tag performance. It was more important to take into consideration other substrate properties, ink characteristics, morphology, and printing parameters. The best practice is to take these matters into consideration in the initial tag design process to ensure proper tag performance at the desired frequency. Keywords Printable electronics . RFID . Pad printing . Polymer thick films . 3d substrate 1 Introduction The potential of different printing techniques in electronics manufacturing has been acknowledged. Printing is an additive, fast, and simple method. For example, conductors, components, passive radio frequency (RF) circuits, and radio frequency identification (RFID) tags have been fabricated using silk screen printing, gravure printing, flexography, offset lithography, and ink jet techniques [1–9]. Silk screen printing is a suitable method for RFID tag antenna manufacturing [2–4]. RFID tags printed with gravure printing are analyzed for example in articles [2, 10], and the use of a flexography technique in tag fabrication is investigated in, for example, articles [5, 9]. These articles suggest that, in addition to screen printing, RFID tag antennas can be fabricated by using these techniques. Ink jet techniques and offset lithography are also interesting methods. Ink jet printing is, for example, used for manufacturing components for RFID tags [11]. Offset lithography has also been proven to be suitable for printing RF components and circuits [8]. The success of gravure printing is especially encouraging for fabrication of RFID tags by pad printing. The benefit of gravure printing is the very high throughput. The drawback, however, is that it is not economically competitive for manufacturing small batches and there are limitations with ink film thickness [12]. Pad printing is an indirect gravure printing process. The pattern is transferred from a printing plate to the substrate with a silicon rubber pad. The engraved printing plate is called a cliché. After the cliché is filled with ink and excess ink removed, the pad lifts ink from the engravings and transfers it onto the substrate. The technique is based on the tackiness of the ink. The ink becomes tacky when the surface of the ink dries on the cliché and again on the pad. First, the tacky ink sticks onto the pad when the pad is pressed on the cliché, and then the ink sticks to the substrate when the pad is pressed onto it [12]. S. L. Merilampi (*) : P. Ruuskanen Tampere University of Technology, Pori, Electronics, Pohjoisranta 11 A, 28100 Pori, Finland e-mail: sari.merilampi@tut.fi T. Björninen : L. Ukkonen : L. Sydänheimo Department of Electronics, Rauma Research Unit, Tampere University of Technology, Kalliokatu 2, 26100 Rauma, Finland Int J Adv Manuf Technol (2011) 53:577–591 DOI 10.1007/s00170-010-2869-y