Drop-on-demand printing of recyclable circuits by partially embedding molten metal droplets in plastic substrates

Abstract

Recyclable electronics are important in green manufacturing and sustainable development, but in infancy, printing strategies still need more exploration and improvement. Metal drop-on-demand (DOD) printing is an ideal candidate for fabricating recyclable circuits, owing to its advantage of inherent direct recyclable feedstocks. Liquid metals with melting points at room temperature have been utilized to print circuits. However, developing liquid metal-based composites or encapsulation is still needed to maintain shapes of printed patterns and improve the adhesion between liquid metals and substrates, restricting the directly recycling of printed circuits. Herein, tin solder is utilized to directly print recyclable circuits via metal drop-on-demand printing method. The molten tin solder droplets directly melt plastic substrates and partially embed the substrate to obtain good adhesion. Molten droplets solidify timely to maintain the shapes of printed 2D conductive patterns and 3D microstructures without encapsulation. Moreover, the electrical conductivity of printed lines is close to the homogeneous bulk metal due to the direct metallurgical bonding between droplets. The conductive traces can be directly recycled by mechanically peeling from the substrate and remelting in the crucible. To validate this concept, a 555 circuit is successfully printed and recycled to fabricate a new 3D cross circuit. This work opens a new direction to directly fabricate recyclable circuits by using metal drop-on-demand printing.