Next-Generation 3D Electronics: Merging Polymer Printing with Molten Metal Droplet Deposition

A novel multi-material additive manufacturing (MMAM) platform is developed within this thesis to advance next-generation 3D electronics. Producing 3D-electronic devices digitally by additive manufacturing remains a unique challenge in 3D printing. In addition to the structural material, conductive tracks and functional components have to be integrated into the printed parts to provide a functional electronic circuit. Achieving a low electrical resistance together with low material costs and a simple printing setup for industrial applications remains a challenge in the field of 3D electronics. Within this thesis, for the first time, fused filament fabrication (FFF) of polymer is combined with precise, non-contact printing of molten metal droplets. The drawback of high electrical resistance of printed conductive tracks from conventional nanoparticle-based inks is eliminated by using the StarJet printhead, a patented technology from the University of Freiburg. Tracks printed from molten metal microdroplets within this thesis show 12 times lower electrical resistance compared to typical nanoparticle-based inks. This thesis presents the construction, process development, and demonstration of advanced prototypes for MMAM of 3D electronics. In the first part of the thesis, the developed MMAM platform closes the existing gap between the conventional and emerging printed electronics platforms, which often rely on nanoparticle-based inks that require sintering at elevated temperatures. Typically, exceeding 150 °C sintering conditions for several hours to achieve acceptable conductivity. Such processing conditions add to production costs and energy use, limit substrate choices, and introduce extra machine steps that complicate the workflow. In contrast, the MMAM platform consists of a simple, space-saving desktop 3D-printer that keeps costs and processing steps low by using bulk metal wire feedstock instead of expensive nanoparticle inks. The process complexity is reduced compared to conventional 3D-electronic platforms by building a fully functional multilayer electronic circuit in a single build cycle using a single machine script (G-code). In the second part of the thesis, the MMAM platform enables single-process, layer-by-layer fabrication of complete multilayer circuits by establishing three key process development stages. These stages are: sweep deposition of conductive tracks on flat polymers, embedding tracks within polymer layers, and forming 3D interlayer vertical interconnect access (via). Once established, all three stages are fabricated in a continuous process to produce functional demonstrators in the next part of the thesis. In addition, the versatility that additive manufacturing provides in diverse material selection is explored through material compatibility tests. The main conductive material used in this work is Sn96.5Ag3Cu0.5 (SAC305), low-cost solder alloy, with a low melting temperature. Furthermore, five different polymer materials are selected to explore the potential for multi-material electronics printing. The identification of process parameters for different process development stages are guided by scientific hypotheses. Key findings in the thesis include: 1) molten metal droplets from SAC305 were successfully printed, exhibiting a coefficient of variation (CV) in droplet diameter of 1.2% for N = 10,000 droplets. The CV confirms the high reproducibility of droplet generation, establishing a solid foundation for printing conductive tracks, and vias. 2) The deposition frequency, droplet overlap, and stand-off distance strongly influence both track morphology and resistance. Increasing droplet overlap and deposition frequency widens the track and reduces its resistance. Changing the stand-off distance from 8 mm to 12 mm causes a significant increase in the track resistance, whereas distances below 8 mm show no notable effect. 3) The conductive tracks have exemplary shape fidelity in track width (266±11 µm) using process parameters such as 68% droplet overlap percentage, a deposition frequency of 18 Hz, and a stand-off distance of 4 mm. 4) Shape fidelity and electrical resistance of conductive tracks printed on different polymer material substrates are comparable. 5) A high adhesion strength of (16±4) N per sqaure-mm between metal and polymer was achieved without surface treatment of the polymer substrates, as determined by the pin-pull test. 6) To evaluate the environment robustness and service life of the conductive tracks printed on polymer substrates, thermal shock tests under alternating thermal loads were performed. The cold chamber was set to −40 °C and the hot chamber to 80 °C. The conductive tracks printed on polyethylene-glucose (PETG) polymer did not show any loss of conductivity after 1000 cycles in the thermal shock tests. 7) Embedding conductive tracks in polymer channels does not alter their resistance. For example, the length-specific resistances on the PETG substrate before and after embedding are (3.6±0.4) mΩ per mm, and (3.6±0.7) mΩ per mm, respectively. 8) Vias embedded in polymer layers can be built layer-by-layer and are successfully printed up to, but not limited to, 10mm in height. In the last part of the thesis, three functional prototypes demonstrate the combined capability of all the process development stages. For example, a functional multilayer 555 timer oscillator circuit is built with three polymer layers, two conductive layers, and two vertical vias, including an integrated USB power connector and direct interconnect of all surface mounted devices (SMDs) and integrated circuits (ICs). All the layers including the interconnecting of SMDs to conductive tracks are printed by the MMAM printer within a single build cycle. A second example is a smart eyeglass frame with integrated sensors which shows the platform’s potential for fabricating wearable devices. And finally, an interdigitated 3D-capacitor is printed with a high aspect ratio of 16:1. Together, these results prove that the developed MMAM platform can produce compact, multilayer 3D electronics in a single process step, streamlined workflow at low material cost without the need for post-processing.