Design and evaluation of LET-based soft hinges for stress mitigation in flexible copper interconnects

Copper interconnects on polyimide substrates are widely used in flexible electronics but are highly susceptible to stress concentration and early yield under mechanical deformation. This study investigates the integration of lamina emergent torsional (LET) joints into flexible printed circuit boards (FPCBs) to create kirigami-inspired soft hinges that can significantly reduce local stiffness and improve mechanical resilience. Five interconnect geometries, including serpentine and horseshoe layouts, were evaluated through finite element simulations under identical applied strain conditions. Results showed that adding LET-based hinges to a serpentine or horseshoe design improved compliance, increasing the yield onset by ∼ 17 %. The horseshoe-shaped interconnects with LET-based hinges exhibited the best mechanical performance, delaying yield onset until 92.5% of the applied strain and limiting plastic deformation to under 4% of the copper volume at maximum strain. Furthermore, the horseshoe design reduced the average elastic energy density by about 10% compared to the serpentine design, indicating a more uniform stress distribution. These findings demonstrate that integrating LET joints in FPCBs represents an effective strategy for regional stiffness reduction and strain redistribution, while improving the interconnect geometry can lead to further reduction in localized stress. Overall, this work highlights the potential of developing compliant substrate architectures to enhance reliability in flexible electronic systems.