Alternative Manufacturing and Testing Methods of Stretchable Electronics

Abstract

Stretchable electronics are used in wearable applications to implement intelligent features. The main characteristic of stretchable electronics is stretchability enabling deformation required in wearable objects such as bandages and clothes. In this thesis, the stretchable electronics consist of elastic substrates, printed stretchable interconnections, adhesives and rigid modules with traditional electronic components. The modules on the elastic substrate form rigid islands that allow the substrate to stretch. Stretchable electronics can endure only a specific amount of elongation before their electrical interconnections fail. Adhesion and deformation mechanisms in the joint and in the joint area of the module and the substrate affect elongation. The durability of stretchable electronics can be improved by improving adhesion and controlling the deformations via optimizing the structure of the joint and the joint area. In this thesis, the stretchable electronics were studied on several levels. A thermoplastic polyurethane (TPU) film was used as the elastic substrate. Wettability and effectiveness of pre-treatments on wettability were examined. The substrate was investigated by measuring contact angles of droplets with a drop shape analyzer. Adhesion and peel behavior of non-conductive adhesives between the TPU-film and the rigid substrates were studied with a floating roller peel test setup. Finally, tensile testing was used to investigate deformations and elongation of the fabricated stretchable electronics samples. In the tensile test samples, width of the interconnection, the amount of the conductive adhesive and the use of a supportive frame structure were varied. The tests presented new results that can be adopted alone or as whole. The wettability of the TPU-film improved most with a plasma pre-treatment that decreased the contact angles up to 63 percent. The peel tests showed that the sample with one cyanoacrylate adhesive with a primer had the highest momentary bond strength (0,5 N/mm). The high bond strength made the TPU-film elongate during the peeling test. Unlike the tested structural adhesives, an elastic transfer tape adhesive had the most even peeling force during the tests (between 0,2 – 0,3 N/mm) and was the easiest adhesive to process. According to the stress peaking concept, in the tensile testing, when the samples elongated, stress concentrated close to the attached module and broke the samples. The strongest interconnection elongated 91,7 % before failure. The referred sample type had the supportive frame and conductive adhesive only under the contacts. Similarly, according to the concept, the stress exerted on this sample was more uniform compared to the other tensile test samples, which explains the good results.