# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 2 Materials and Methods
### Preparation, Application, and Sintering  
A Cuf-A2P complex was prepared by mixing Cuf (6 g) and A2P (4.12 mL) in a specific molar ratio of 1:2, based on previous work [21; 22]. Stabilizers and application-dependent solvents were added to adjust the viscosity and wettability of the substrate. Both the Cu ink and commercial paste were mixed in a planetary rotary paste mixer (SR500, Thinky Corporation, Tokyo, Japan) for 15 min at 1000 rpm before application. The viscosity and contact angle of the prepared Cu complex ink were measured using a viscometer (Haake TM ViscotesterTM, Thermo Fisher Scientific, Waltham, MA, USA) and a drop shape analyzer (DSA 30E, Kruss Scientific GmbH, Hamburg, Germany).  
Before the dispensing or spray-coating of the Cu ink or Cu paste, the substrates were first cleaned using isopropanol, followed by Ar-O\({}_{2}\) plasma pretreatment (Zepto PLS, Diener electronic GmbH & Co.KG, Ebhausen, Germany) to remove organic residues. Additionally, plasma pretreatment varied from 1 to 5 min to improve the wettability of the Cu complex ink on the polymeric substrates. The Cu complex ink and Cu paste were dispensed onto the polymeric substrates using a dispenser from Musashi Engineering Europe GmbH, Munich, Germany. The CAD design of the printed structures was added to the Musashi Image Master 350 PC. For the Cu complex ink, the dispensing and spray-coating parameters were as follows: dispensing pressure: 45 kPa; nozzle diameter: 100 um; spraying distance: 500 um; tank pressure for spraying: 5 kPa, achieving a uniform trace and film thickness of \(\sim\)10 um. For the Cu paste, the dispensing pressure was 150 kPa and the nozzle diameter was 200 um. The difference in application parameters is mainly due to the difference in viscosity between the Cu ink (Cu is present in the form of Cu ions; particle-free system) and the paste (Cu microparticles).  
The RSS-160-S reflow oven from UniTemp GmbH, Pfaffenhofen an der Im, Germany, was used for the predrying and sintering of the Cu complex ink and Cu paste. The corresponding predrying and sintering profiles for the Cu complex ink and Cu paste are shown in Figure 2. For the Cu complex ink, the peak sintering temperature was 150 \({}^{\circ}\)C to facilitate sintering onto low-melting substrates such as PET. Formic acid-enriched N\({}_{2}\) was used to prevent surface oxidation in the in situ-formed Cu nanoparticles, since these nanoparticles were \(<\) 100 nm, making them highly reactive due to the high surface-volume ratio. For the Cu paste, the peak sintering temperature was 200 \({}^{\circ}\)C. Additionally, from the supplier, it was known that the deformation temperature of the radome substrates was \(<\)225 \({}^{\circ}\)C. Therefore, both materials needed to form conductive sintered Cu metallic traces at a temperature lower than the deformation temperature.  
Additionally, the laser sintering of the Cu complex inks was performed, ensuring effective sintering without damaging the polymeric substrate, as observed in previous work [23]. These trials were carried out on polyimide substrates. As with oven sintering, the printed traces were first predried at 100 \({}^{\circ}\)C for 5 min under air to remove volatile organic solvents that may cause rapid bubbling or degassing during the sintering process. The predried traces were then laser-sintered using an infrared (1064 nm) spot laser (G3 20 W nanosecond pulsed spot laser) supplied by SPI Laser, Rugby, UK. The spot laser can work in continuous-wave and pulsed mode (from 9 ns to 200 ns) with 22 W of maximum power. The expanded laser beam was linked to a galvanometer mirror scanner (Extreme-15-YAG, Nuffield Technology, Londonderry, NH, USA), controlled using SAMLight v3.05 software (SCAPS Gmbh, Oberhaching, Germany). The galvanometer mirror scanner allows the laser beam to move with speeds from 0.01 mm/s to 20,000 mm/s. For FDM (fusion deposition modeling), a 5-axis printer from 5 axismaker, London, UK, was used. Commercial filaments were sourced as listed under 3.2.