Bond Strength and Failure Mechanisms of Nonconductive Adhesives for Stretchable Electronics

Over the past few years, there has been an increasing demand for techniques that allow the forming of stretchable electronics systems from the combination of rigid printed circuit board (PCB) modules and stretchable substrates. The durability issues between the module and interconnects have been solved by optimizing the module's geometry. However, the limiting factor is a reliable attachment method of the module on the substrate. The use of nonconductive adhesives (NCAs) for bonding is one of the most potential techniques due to their low costs and ability to form bonds fast and without a high-temperature cure. In this article, we focused on the testing of different stretchable electronics joints from readily available NCAs and different rigid module materials. The joint samples were tested by using a peel test setup. The fracture surface analysis was carried out by applying the Fourier transform infrared spectroscopy (FTIR). Three different classes of failure mechanisms were identified. The best results were achieved with a novel nonstructural adhesive joint. The nonstructural adhesive joints had a good (0, 28 N/mm) average maximum bond strength with the rigid and smooth FR4 substrate, which made the stretchable substrate elongate considerably (85%) during the peeling. The joint samples from structural adhesives, traditionally used in the electronics industry, were suboptimal.