2, Department of Physics and Astronomy, University of Bologna, Bologna, , Italy
Inkjet printing is a relatively new and promising technology for fabrication; its major advantages, compared with traditional photolithography, rely on fabrication costs reduction, reduced material waste output, and simpler manufacturing steps. This technology has gained increasing interest in the fabrication of thin films for flexible and stretchable electronics using new functional nanomaterial inks. Despite the established advantages of this technology, the electromechanical reliability of the fabricated material systems is still a key point to achieve competitive devices. In particular, bendable and stretchable electronics require electromechanical reliable interconnects as they are subject to mechanical loading which jeopardize the integrity of the material itself (the electrical interconnects) or that of the interfaces.
This work is focused on the electromechanical reliability of bendable and stretchable interconnects obtained through ink-jet printing by using a Dimatix Fujifilm printer. PET and PDMS were chosen as substrates in order to realize flexible and stretchable interconnections, respectively. Conductive ink, containing Silver Nanoparticles (AgNPs), was printed directly on PET. Since the direct printing on PDMS is a challenging task due to its elastomeric, hydrophobic, and inert nature, a different fabrication procedure was assessed for the PDMS substrate, combining the inkjet printing of AgNPs ink with transfer printing technique. For each substrate the experimental protocol was defined: (i) pre-treatment of the surface, if needed; (ii) optimization of the printing parameters; (iii) definition of the sintering temperature to optimize the conductivity, while maintaining compatibility with the glass transition temperature of the substrate. The selection of the above parameters was optimized as to maximize the capability to withstand mechanical strain without significant loss of electrical resistivity. Furthermore, an investigation on the role of the geometrical features of printed silver interconnections was also performed. Printed rectilinear and serpentine shapes interconnections of various geometries were characterized in-situ electromechanical tensile tests with Confocal Laser Scanning microscope. The microscopy analyses were aimed at estimating the integrity of the AgNPs sintered material. The experiments confirmed that electromechanical reliability is strongly affected by the geometrical layout: serpentine interconnections exhibited higher electromechanical reliability than the linear ones; although the directionality of the printing process should be taken into account when planning the geometrical features. As for the interconnects on PDMS, the curing process introduces a residual stress filed in the AgNPs layers which may fracture. A suitable selection of curing temperature, components ratio and silicone thickness are key factors for a flowless and electro-mechanically reliable stretchable silver interconnects.