The Effect of Encapsulation Geometry on the Performance of Stretchable Interconnects

Abstract

The stretchability of electronic devices is typically obtained by tailoring the stretchable interconnects that link the functional units together. The durability of the interconnects against environmental conditions, such as deformation and chemicals, is therefore important to take into account. Different approaches, including encapsulation, are commonly used to improve the endurance of stretchable interconnects. In this paper, the geometry of encapsulation layer is initially investigated using finite element analysis. Then, the stretchable interconnects with a narrow-to-wide layout are screen-printed using silver flake ink as a conductor on a thermoplastic polyurethane (TPU) substrate. Printed ultraviolet (UV)-curable screen-printed dielectric ink and heat-laminated TPU film are used for the encapsulation of the samples. The electromechanical tests reveal a noticeable improvement in performance of encapsulated samples compared to non-protected counterparts in the case of TPU encapsulation. The improvement is even greater with partial coverage of the encapsulation layer. A device with a modified encapsulation layer can survive for 10,000 repetitive cycles at 20% strain, while maintaining the electrical and mechanical performance.

Mechanically driven strategies to improve electromechanical behaviour of printed stretchable electronic systems

“…As seen in previous publications regarding the presence of interconnects on stretchable electronic systems, it is possible to influence local strain through encapsulation and by modifying the geometry of the conductive ink interconnect 8,23 . In addition, vulnerable regions in the substrate, such as areas with rigid-to-soft connections can be reinforced by local enhancement of the stiffness 45,46 .…”

“…The aforementioned applications require the electrical system to deform together with the underlying materials: especially for health-patch applications, this means that such electrical devices have to seamlessly adapt to human skin deformations 4 , whose deformation levels can be highly heterogeneous 5 . These reasons make it difficult to restrict the working conditions for this set of systems 6 , while, on the other hand, they encourage the request for materials that have mechanical properties similar to the human skin, in order to improve comfort to the user and reduce obtrusiveness of such electrical systems 7,8 .…”

“…These classes of conductive composites offer high flexibility of properties and many advantages over the thin metallic films that are generally used in this field. Moreover, they can be printed on top of the desired substrate, which can range from thermoplastic polyurethanes (TPUs) 8,23 and polydimethylsiloxanes (PDMS) 24,25 to textiles 26,27 . However, conduction in these materials is based on deformation mechanisms that are radically different from the ones available for thin metallic films: in fact, for these materials conduction is carried through the filler particles, that are in contact with each other and act as rigid inclusions, while the polymeric binder typically carries the deformation transferred from the substrate.…”

“…The interconnects were connected to the Keithley measurement cables using pads of 3M 9703, a pressure-sensitive conductive tape. The schematic of the test setup can be seen from previous works 8 . Sets of 10 samples from each of the four groups were analysed.…”

Structural Innovations in Printed, Flexible, and Stretchable Electronics

“…Additional structural innovations, such as special junction shapes, encapsulation design, or a mixture of different material to form a gradient of mechanical rigidity, have been proposed to reinforce the island‐bridge structures. [ 98–100 ] However, such printed devices are limited by the resolution of the selected printing techniques, making it challenging to create devices requiring micron‐level features. In addition, as common to island‐bridge structures, such printed devices require larger footprints, as a considerable amount of area is delegated to the interconnecting bridges, which usually has intricate, tortuous shape designs, and lowers the effective area utilization rate.…”