A recent article published in npj Advanced Manufacturing demonstrated the additive manufacturing (AM) of bulk metal and polymer on a single platform, enabling the iterative printing of multilayer three-dimensional (3D) electronic circuits.
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Background
Multi-material AM (MM-AM) combines the efficiency and flexibility of 3D printing with diverse material properties and complex geometries for enhanced functionalities. This technology allows for the customization of 3D electronics with multiple functions, reducing waste in the electronics industry and facilitating device miniaturization.
However, designing conductive inks for MM-AM is challenging as the ink performance depends on the printing conditions and substrate properties. While the substrates need pre-processing to enhance adhesion, the inks require post-processing to attain sufficient conductivity. Moreover, solutions in conductive inks may percolate between printed polymer layers and cause short circuits.
To address these challenges, researchers are exploring low-temperature solders or metal alloys for MM-AM in 3D electronics. Solder alloys containing tin, silver, bismuth, or indium are particularly promising, as they are compatible with thermoplastic materials printed at temperatures up to 250 °C or with glass transition temperatures up to 200 °C.
This study utilized a low-melting-point solder alloy, SAC305 (Sn96.5Ag3.0Cu0.5), to fabricate multilayer embedded 3D electronics.
Methods
A one-stop hybrid printing platform, Synkròtima, was developed to print polymer filament and molten metal microdroplets sequentially using two printheads on a linear x-axis. While fused filament fabrication (FFF) technology was employed to print polymers, a StarJet printhead in drop-on-demand (DoD) operation mode was used to generate molten metal microdroplets. The parameters for these configurations were optimized based on previous studies.
Two different polyethylene terephthalate glycol copolymer (PETG) filaments were used to print the polymer parts: Prusament PETG Prusa Orange and Fillamentum PETG Pink Lollipop Transparent. Alternatively, flux-free SAC305 solder alloy was used to print molten metal microdroplets.
Two methods were employed for the conformal deposition of electrically conductive traces within polymer channels (width: 250 μm, depth: 400 μm). The first was the conventional polymer-polymer-metal (PPM), which involved creating the channel and depositing the electrically conductive traces.
The second method was an ideal layer-by-layer MM-AM approach called polymer-metal-polymer (PMP), consecutively printing polymer and electrically conductive trace layers.
Four-point resistance measurements were performed on the printed 3D circuit with gold pad probes. In addition, the cross-section of metal structures enclosed in the polymer was analyzed by embedding the sample in a two-component epoxy. Thermal cycling tests were performed on these samples from −40 to 60 °C; each temperature extreme was maintained for 30 minutes.
Results and Discussion
The PPM and PMP methods yielded conformal and fully embedded electrically conductive traces within polymer channels. However, more significant unfilled spaces were observed in the PPM than in the PMP technique.
Notably, the PMP technique attained a high infill percentage of (92 ± 5)% for metal deposition as it smoothened any irregularity in the polymer printing by FFF technology. The infill percentages and resistance measured for each conductive trace before cross-section grinding did not vary even after 100 thermal cycles between −40 and 60 °C.
Cross-section grinding of the conductive traces printed via PMP along the channel length exhibited exemplary conformity of metal with the polymer channel. Moreover, unlike the polymer layer, no air entrapment was evident in the metal structures. These results are promising despite the difference in the coefficient of thermal expansion of metal and polymer.
Scanning electron microscopy (SEM) images of SAC305 metal droplets showed varying levels of contact with the PETG substrate depending on the deposition point, such as ridges or grooves. Despite these variations, the metal droplets consistently bonded well with the PETG substrate, forming a wedge-shaped ridge as the fusion structure.
A 555-timer circuit (two metal layers sandwiched between three polymer layers) with a blinking light-emitting diode (LED) was printed using the proposed workflow for MM-AM of embedded 3D electronics. Two different demonstrators, Squid and Bunny, were printed in a single run, validating the above results, highlighting the flexibility of the developed workflow, and showcasing easy scalability in the MM-AM process.
Conclusion
The researchers successfully combined the AM techniques for bulk metal and polymer printing on a single platform and demonstrated its efficacy in one-stop printing of multilayer 3D electronic circuits. The developed platform’s robustness, scalability, and customizability were illustrated through two vivid models.
The proposed workflow could print vertical bulk metal (up to 10 mm in height), expandable to different layers in a 3D circuit. Additionally, the exceptional (92 ± 5)% occupancy of electrically conductive traces in polymer channels is promising for embedded integration of electronic circuits.
The proposed MM-AM approach has significant potential for fabricating free-form 3D electronics. However, further investigation is required on long-term reliability, enhancing functionalities of 3D-printed devices, and exploring various substrate materials for widespread applications in the electronics industry.
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Journal Reference
Khan, Z., Gururajan, D., Kartmann, S., Koltay, P., Zengerle, R., Shu, Z. (2024). Iterative printing of bulk metal and polymer for additive manufacturing of multi-layer electronic circuits. npj Advanced Manufacturing. DOI: 10.1038/s44334-024-00001-0, https://www.nature.com/articles/s44334-024-00001-0
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