# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 3 Results and Discussion
### Heating Structure Formed by Copper Ink and Paste
(**b**) Cu complex ink sintered onto a PET substrate. (**c**) Printing and processing parameters with Cu trace properties. smooth edges, and a height of approximately 5-6 \(\upmu\)m were measured, which aligns with the dimensional requirements for our heating structure. Assuming a 100% duty cycle for a typical voltage of 10 V, the target electrical resistance should fall within the range of 5-30 \(\Omega\) to achieve a heating power between 3 W and 20 W. To attain these resistance values, heating structures with specific dimensions were fabricated onto the radome substrate using the copper complex ink and copper paste, as shown in Figure 8a-c, respectively. As previously mentioned, the Multi3D Electrifi conductive copper filament exhibited a resistance of 960 \(\Omega\). In the case of the copper complex ink, the sintered structure was formed using five layers of ink, resulting in an overall resistance of 17.43 \(\Omega\), which is within the targeted electrical resistance range needed for heating purposes. The bulk resistivity of the sintered traces was 8.8 \(\upmu\Omega\)cm, which is 5.2 times that of bulk copper. However, the adhesion to the radome substrate after sintering was rated as 4B. During  
Figure 6: Effect of substrate plasma pretreatment on ink wettability and Cu metallization after sintering: (**left**) without plasma treatment; (**middle**) with plasma treatment for a duration of 5 min; (**right**) with plasma treatment for a duration of 10 min. Figure 7: Microscopy and profilometry analysis of the dimensions of a printed and sintered Cu ink. the peel test, it was observed that some copper particles on the top layer of the sintered structure did not adhere sufficiently to the trace. This was primarily due to the oven heating process, where heating energy is introduced conductively from the bottom to the top. A similar observation was made after sintering the copper paste, which yielded a resistance of 0.10 \(\Omega\) and a bulk resistivity of 4.0 \(\mu\Omega\)cm; however, the adhesion was worse (3B). It is important to note that the copper paste is developed for power electronics applications, involving sintering under low pressure, and not for fully pressureless sintering. Therefore, the results are promising, but further development is necessary. In both cases, using the ink or the paste, applying pressure would be beneficial, as it would allow the copper metallic layers to be sintered from both the top and bottom. This would also improve the densification of the sintered layers and further reduce the specific resistance of the sintered copper. The radome itself has a flat surface where the heating coils are printed, enabling the application of pressure. However, this would introduce additional manufacturing complexity due to the implementation of specific pressure tools, significantly increasing production costs. Furthermore, the application of pressure is generally not feasible for complex 3D applications. To address the adhesion and resistivity issues, pressureless sintering was performed using a laser. Laser sintering significantly enhanced the properties of the copper complex ink, resulting in lower resistivity (3.25 \(\mu\Omega\)cm) and optimal adhesion (5B), as depicted in Figure 8d. Laser sintering offers the advantage of achieving higher local temperatures within the copper material. This, coupled with the rapid processing, leads to the formation of smaller particles that are sintered into a denser copper trace. Various processes were investigated: (1) Spraying the copper complex ink onto a polymeric substrate (polyimide) using the Musashi dispenser equipped with a spray coating tool head. Following drying, the copper trace was sintered using a spot laser with a galvanometric scanner. This method facilitated the direct patterning of copper traces using a CAD file, leveraging the combination of a galvanometer mirror scanner and a laser beam.