# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 3 Results and Discussion
### Heating Structure Formed by Copper Ink and Paste  
For initial trials to evaluate printability and low-temperature sinterability, the copper complex ink was printed onto a PET substrate. Previous investigations indicated that the onset of the copper reduction process (Cu\({}^{2+}\rightarrow\) Cu\({}^{1+}\rightarrow\) Cu\({}^{0}\)) is marked by the initiation of the decarboxylation reaction of the copper complex, occurring around 127 \({}^{\circ}\)C, where copper nuclei form and grow into copper nanoparticles [16]. Utilizing a Musashi dispenser, the copper complex ink, with a viscosity of 450 mPa\(\cdot\)s and a 15.8 wt.% copper metal content, was printed onto a PET substrate, as illustrated in Figure 5a. Predrying was performed at 100 \({}^{\circ}\)C for 5 min under a formic acid-enriched nitrogen atmosphere, followed by low-temperature sintering at 130 \({}^{\circ}\)C for 30 min in a  
Figure 4: FDM five-axis prototype equipment (5 axismaker) on the left side, a printed prototype of a meander heating structure in the center, and a parallel heating structure on the right side. The parallel structure was realized because the specific resistance of the conducting FDM filaments was low. A sufficiently small resistance, able to dissipate sufficient power using a 10 V power supply, is not feasible using a single meander structure. reflow oven. The sintered copper trace, after two printing and predrying passes, exhibited a bulk resistivity of 18 \(\upmu\Omega\)cm (10.65 times that of bulk copper) and an adhesion rating of 5B, as shown in Figure 5b. Following the successful realization of conductive copper traces on a PET substrate at a low sintering temperature of 130 \({}^{\circ}\)C, the copper complex ink was then printed onto a thicker radome substrate (1 mm). It was observed that the sinterability of the copper complex ink on the radome substrate differed from PET. The significantly greater thickness of the PBT-GF substrate (1 mm vs. 125 \(\upmu\)m) increased its thermal mass. This resulted in a longer time for the thicker PBT-GF substrate to heat up and reach a stable temperature during the sintering process. Consequently, the sintering temperature was raised to 150 \({}^{\circ}\)C for the radome substrate to achieve a comparable sinterability of the copper complex ink as when using PET. Another challenge encountered was the irregular wetting of the copper complex ink on the radome substrate, as depicted in Figure 6 (left), leading to a discontinuous sintered structure with rapid bubbling (degassing) during sintering. The uneven ink distribution caused gaps or voids in the sintered copper film, compromising its electrical conductivity. To address this issue, an argon-oxygen plasma treatment was performed on the radome substrate for 5 min to enhance the ink's wettability. The plasma treatment effectively removes contaminants from the radome substrate that impede ink wetting. Additionally, the plasma interacts with the radome substrate's surface, introducing polar functional groups (such as hydroxyl or carbonyl groups) to elevate the substrate's surface energy, rendering it more hydrophilic [25; 26]. This facilitated the improved spreading and adhesion of the copper ink, resulting in a more uniform and continuous film, as shown in Figure 6 (middle). However, extending the plasma treatment duration to 10 min resulted in the overspreading of the ink, creating unintended connections and short circuits in the sintered structure, as illustrated in Figure 6 (right). The prolonged plasma treatment significantly increased the radome substrate's surface energy, leading to the excessive wetting of the copper complex ink. Following the adjustment of wetting through plasma treatment, the dimensions of a five-layer printed and sintered copper metallic trace were analyzed using microscopy and profilometry. The results are presented in Figure 7. A width of 580 \(\upmu\)m, with notably  
Figure 5: Cu complex ink printed and sintered onto a PET substrate. (**a**) Cu complex ink printed onto a PET substrate.