Solid-Liquid Interdiffusion Bonding Enabling Heterogeneous Integration on Flexible and Shapeable Polymer Platforms

The integration of rigid microscale electronics onto soft and flexible polymer substrates remains a key challenge for flexible electronics, biointegrated systems, and body-first microrobotics, where the mechanical body, material architecture, morphology, and primary functions are designed first, while the electronic processing unit (e.g., a Si CMOS chiplet), acting as the brain to control the robot´s functionalities, is integrated later. Existing approaches, including anisotropically conductive adhesives and transfer printing, rely on non-patternable or particle-based interconnects, thick bonding layers, or complex processing, constraining interconnect density, scalability, and compatibility with mechanical transformations such as bending, rolling, or folding. Here, we present a patterned solid-liquid interdiffusion (SLID) bonding approach enabling deterministic, fine-pitch heterogeneous integration on ultrathin polymer substrates compatible with standard microfabrication. Electroplated Cu/Sn pillar bumps fabricated on 5 µm polyimide are systematically characterized to define geometric, electrical, and mechanical process window for reliable SLID bonding. The resulting interconnects exhibit controlled solder reflow, low electrical resistance, high mechanical robustness, and minimum inter-pillar spacings of 10 µm without adhesives or conductive particles. Microscale light-emitting diodes bonded by SLID remain functional during self-rolling, bending and folding-driven three-dimensional assembly with integrated silicon microchips. This work provides a scalable framework for mechanically reconfigurable microrobotic and flexible microsystems.