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This paper presents a novel approach for removing out-of-plane deformation in stretched metal interconnects by adding a fractal structure to the original meander shape and using an optimized fabrication stack. In thermoformed electronics, in cases where copper is used as conductor, the twisting of meander-shaped structures caused by excessive mechanical stress can cause a non-uniform surface, delamination of the metal interconnect from the substrate, and in some cases, a short circuit to the adjacent tracks. Typically, designers of stretchable electronics use various shapes and widths of the copper interconnect to tackle this issue. Using conventional meander shapes such as horseshoes and U-shapes is not universally practical, especially when stretching is higher than 30 percent leading to significant out-of-plane buckling. Limiting this out-of-plane buckling by reducing the track width is not always applicable, as a minimum width is needed from a technology and conductivity perspective. The presented approach is inspired by computational and experimental studies of multiple meander shapes and fabrication methods. A geometry- and fabrication-based approach is presented, reducing the mechanical stress of almost all possible meander shapes by increasing the meander's path length to accommodate the metal track's produced torque during stretching. An analytical approach is provided for calculating the optimal meander parameters and the optimal fabrication stack is achieved based on simulation results. Experiments and finite element modeling for a case study show the improvement in the stress distribution and reduction of out-of-plane buckling.

The integration of assembled foils in injection-molded parts is a challenging step. Such assembled foils typically comprise a plastic foil on which a circuit board is printed and electronic components are mounted. Those components can detach during overmolding when high pressures and shear stresses prevail due to the injected viscous thermoplastic melt. Hence, the molding settings significantly impact such parts’ successful, damage-free manufacturing. In this paper, a virtual parameter study was performed using injection molding software in which 1206-sized components were overmolded in a plate mold using polycarbonate (PC). In addition, experimental injection molding tests of that design and shear and peel tests were made. The simulated forces increased with decreasing mold thickness and melt temperature and increasing injection speed. The calculated tangential forces in the initial stage of overmolding ranged from 1.3 N to 7.3 N, depending on the setting used. However, the experimental at room temperature-obtained shear forces at break were at least 22 N. Yet, detached components were present in most of the experimentally overmolded foils. Hence, the shear tests performed at room temperature can only provide limited information. In addition, there might be a peel-like load case during overmolding where the flexible foil might bend during overmolding.

The integration of structural electronics in injection-molded parts is a challenging step. The films—comprising of laminated stacks with electronics—are exposed to shear stresses and elevated temperatures by the molten thermoplastic. Hence, molding settings have a significant impact on the successful, damage-free manufacturing of such parts. In this paper, test films with polycarbonate (PC) sheets as outer and two different thermoplastic polyurethanes (TPUs) as middle layers incorporating conductive tracks on a flexible printed circuit board (flexPCB) are manufactured and overmolded with PC. Parameter studies investigating the influence of the melt temperature, mold temperature, injection speed and used TPU layer were performed. The molded parts were inspected visually and compared with a numerical simulation using injection molding software. A shear distortion factor for the TPU layer was derived based on the simulations that linked the shear stresses with the injection time and the softening (melting) of the TPU. The distortion of the films was found to reduce with higher melt temperature, lower mold temperature and faster injection speed. Films using the TPU with the higher melting temperature yielded significantly better results. Moreover, distortion on the films reduced with the increasing distance to the gate and a larger cavity thickness was found to be beneficial. All those relations could be correlated with the shear distortion factor.

This research presents an innovative method to accurately and repeatedly position electronic components in thermoformed electronics. The paper focuses on 3D shaped electronics, which are made up of stretchable metal structures integrated into thermoformable material. We used the degree of freedom theory to develop a design method that allows us to build the circuit in 2D in such a way that there is only one possible position for electronic components in the final 3D shape after thermoforming the sample. As a result, we have a reproducible design process for thermoformed electronics. We have assessed the result of the fabricated samples by measuring the 3D coordinates of the components on the 3D shaped design using a 3D scanner. The results proved that we have a repeatable component positioning methodology.