High-Conductivity Crack-Free 3D Electrical Interconnects Directly Printed on Soft PDMS Substrates

Nanoparticle 3D printing and sintering is a promising method to achieve freeform interconnects on compliant substrates for applications such as soft robotics and wearable healthcare devices. However, previous strategies to sinter metallic nanoparticles while preserving the soft polymer substrate are rife with problems such as cracking and low conductivity of the metallic features. In this paper, the mechanisms of cracking in nanoparticle-based 3D printed and sintered stretchable interconnects are identified and architecture and processing strategies are demonstrated to achieve crack-free interconnects fully embedded in thin (<100 mu m in thickness) stretchable polydimethylsiloxane (PDMS) with external connectivity. Capillary forces between nanoparticles developed through rapid solvent evaporation in the colloidal ink is hypothesized to initiate cracking during drying. Additionally, the presence of oxygen promotes the removal of organic surfactants and binders in the nanoparticle ink which increases nanoparticle agglomeration, grain growth, and subsequently conductivity. An experimental step-wise variation of the thermal/atmospheric process conditions supports this hypothesis and shows that the presence of air during a low temperature drying step reduces the capillary stress to produce crack-free interconnects with high conductivities (up to 56% of bulk metal) while having an excellent compatibility with the underlying polymer materials. Finally, stretchable interconnects fully-encapsulated in PDMS polymer, with 3D pillar architectures for external connectivity are demonstrated, thus also solving an important "last-mile" problem in the packaging of stretchable electronics.