In last years, it has been demonstrated that the paper substrates can be used to fabricate planar circuits up to microwave frequencies. Despite of its relatively high dielectric losses (due to moisture and filler content), the paper substrate, the most common natural polymer, is ecocompatible, flexible and very cheap, thus it represents the best choice for green or large-area electronic applications. The typical example is a disposable wireless sensor that must be distributed in the environment without causing pollution problems, potentially reaching the 24GHz ISM frequency band.
In the above applications, the ink-jet or screen printing technology has been used to fabricate the wiring network or the sensor antenna. To this purpose, a conductive ink, based on Silver (Ag) nano-particles, is adopted along with a post-print curing process step. Here, the complementary vision of an electronic that can be sticked on-demand is proposed. A new fabrication process for the wiring network is demonstrated starting from a copper (Cu) adhesive tape. Such a tape is etched by means of a photo-lithographic technology, but a direct layout shaping with cutting machines is, in principle, possible. Then the circuit features are transferred to a paper substrate via a sacrificial layer. The main advantages of the Cu tape are a superior conductivity with respect to cured Ag ink and the possibility to solder standard electronic components on it.
To validate the proposed technology, several structures of practical interest have been fabricated and transferred from the Cu tape to the paper foil. The photograph, for example, shows a layout composed of several non-connected shapes, actually the soldering pads of a SMT transistor with a SOT353 package. The first microwave characterization has been carried-out studying the propagation characteristics of simple microstrip lines. Experimental results show an insertion loss better than 0.6 dB/cm at 10GHz for a Kodak photo-paper.
Federico Alimenti, Paolo Mezzanotte, Marco Dionigi, Marco Virili, Luca Roselli, "Microwave Circuits in Paper Substrates Exploiting Conductive Adhesive Tapes," IEEE Microwave and Wireless Components Letters, 2013, forthcoming.