# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 1 Introduction
The heat-producing components, which are located at the back, are connected for heat management via insulating, thermal-conducting heat pads to a metal insert that spreads the heat before it is dissipated by the plastic housing. The metal insert also functions as an HF shield to block electromagnetic waves from the surrounding environment interfering with the HF part of the radar sensor. HF shielding can also be performed by AM, e.g., by metallizing the backside of the electronic board after encapsulating the electronic components using an electric non-conductive material. The primary radome is mounted on top of the antenna and sealed to the housing, protecting antenna, and electronics. Often, a secondary radome, protecting the primary radome from water and ice, is used with integrated heating coils for de-icing in winter. Figure 1b shows the Cu heating structure realized through AM on the inner side of the primary radome. Figure 1b also shows a multilayered PCB with a shielding layer prepared through AM and an SEM image of a laser-sintered Cu layer. This paper explores the potential of additive manufacturing (AM) for automotive radar sensors, with the primary research focus on the printing of conductive copper traces. The case study selected for investigation was radome heating, chosen due to the significant benefits that can be derived from AM. For optimal radar signal transmission under winter conditions, the de-icing of the radome is achieved via joule heating. Around 5 W of heating power is typically required to defrost the radome in a short span of time, i.e., approximately three minutes. Conventionally, thin metal wires are inserted into grooves on the exterior of the radome. To prevent any potential disturbance to microwave radiation and potential destructive interference, the wires need to maintain a minimal distance of 4 mm. Essentially, as soon as thin metal wires are routed in parallel with a 4 mm distance separation, any possible disturbance is greatly minimized, resulting in a dampening effect of only 1.3 dB [16]. This paper describes an innovative approach, which is the AM of heating coils on the inside of the primary radome. This approach can significantly simplify the manufacturing process and reduce the number of components involved. Implementing the traces on the inner side of the radome makes it possible to connect them directly to the main PCB through standard pins, thus eliminating the need to contact the thin wires. Additionally, the electronic driver board for the heating coil can be eliminated, as this function can be  
Figure 1: (**a**) Burst schematic of a modern radar sensor. (**b**) Additively manufactured component of radar sensor (radome with Cu layer using FDM, heating structure using material deposition, and multilayered PCB with shielding layer). integrated into the main board. Consequently, the number of parts and processes can be significantly reduced, leading to cost savings. In accordance with the cost estimation in reference [4], the AM metallization process is a very competitive process for realizing metal wiring on the surfaces. Different AM technologies for printing the heating traces on the inner side of the radome are investigated in this paper: FDM deposition using conductive filaments, inkjet printing of metal particle-free inks, and dispensing metal pastes [17]. Due to the economic implications of using silver conductive paste, this paper's approach prioritizes low-cost copper materials. Important factors for the manufacturing of the radome heating structure are the process temperature, which needs to be below 200 \({}^{\circ}\)C, and the electric resistance of the coil. Table 1 lists the state-of-the-art materials along with their applicable AM technology, process temperature, and specific resistance. In particular, the copper inks and pastes enable a low-temperature sintering process after printing, which is facilitated by the use of complexed metal salts.