Dontoral Scholar IMTEK, Albert Ludwig's University of Freiburg, Germany
Soft, flexible, and stretchable electronics are ubiquitous for next-generation wearables, soft robotics, and biocompatible devices. These devices can conform to complex and dynamic surfaces, such as those found in biological systems, and bioinspired robotics based on the application. However, flexible electronics demand novel fabrication techniques that allow direct integration of high performance and scalable electronics onto flexible substrates. To maintain device performance, and reliability while staying flexible, these electronics require highly conductive electrical traces and interconnects to be deposited directly onto flexible substrates. Functional printing techniques via screen, inkjet, and aerosol jet printing use silver conductive traces. However, nano- or micro particle-based inks require sintering after printing, resulting in conductive traces with high resistance and low layer thickness (typical < 1 µm).
This work addresses poor conductivity in flexible electronics by printing metal droplets on flexible substrates. StarJet technology, developed at the Department of Microsystems Engineering (IMTEK) University of Freiburg, prints molten micro metal droplets from bulk metal (e.g. solder and aluminium alloy) in a contactless fashion.
In this work, SAC305 (Sn96.5Ag3Cu0.5) prints electrical traces with a high-aspect-ratio and exceptional conductivity of almost 80% of the bulk solder. Without any post-processing and potentially direct soldering electronic components, this method fills the niche for printing dense structures on various flexible substrates. Elemental and surface morphology via SEM reveals uniform rheological properties of micro metal droplets forming flexible and electromechanically robust electrical traces. Various bending flexion and resistance variation tests are done on a 2.5D 555 timer circuit printed on polyethene terephthalate sheets. The integrated circuits and surface mounted devices are directly interconnected on circuits via StarJet. With remarkable electrical and mechanical properties of the printed electrical traces, application in soft robotics, wearables, seems promising. Lower latency, inexpensive raw material, easy accessibility, and zero post-processing makes this technology easily adaptable for different markets.
