3D Printed Alumina for Geometrically-Complex Electronic Substrates With High-Performance Printed Conductors

Traditional high performance electronic substrates are currently fabricated with molds and screens by stacking punched green ceramic tapes and selectively screen printing electrical interconnects with silver-particle pastes and inks. The structures are spatially registered and stacked, then subsequently fired with pressure to provide electrically-conductive traces in thermally-conductive high-performance dielectric substrates. However, Low Temperature Co-Fired Ceramics (LTCC) processes require tooling and are geometrically limited as opposed to the fully-digital 3D nature of additive manufacturing. Printing alumina substrates is now possible with high spatial resolution, superior surface finish and complex geometries (overhangs, density-varying lattices, internal microfluidics). This facilitates the production of distinctive circuitry that would otherwise be constrained by current manufacturing limitations, paving the way for novel sensing capabilities, secure electronics, heat exchangers, placement in extreme environments, and the integration of circuitry into otherwise inaccessible locations. This work is focused on thermal processing of dielectric ceramics and conductive inks, with a three-step furnace profile accounting for the disparate temperatures of the different materials: high temperature ceramics, mid temperature metal inks and finally adhesion of active electronic components. In the proof of concept demonstration, complex alumina substrates were fabricated which can accommodate miniaturized electronic components (including known good silicon die) and channels for the protection of deposited conductive inks to serve as interconnects. The electrical conductivity of commercially-available silver inks was explored for a range of thermal processing treatments (up to 1000°C) given the thermal stability of 3D printed alumina. An electrical conductivity of $3.13\times 10^{7}$ S/m which is within 53% of bulk copper plated in traditional printed circuit boards (PCB), the theoretical conductivity of bulk silver was achieved at about half the melting temperature.