# Additive Manufacturing for Automotive Radar Sensors Using Copper Inks and Pastes
## 1 Introduction  
Additive manufacturing (AM) technologies have greatly developed during the past 20 years, increasing structuring accuracy and throughput and reducing cost [1]. AM is moving from prototyping into the manufacturing of plastic or metal parts for end-users [2]. At first, AM was especially used for single part or small batch production, like manufacturing tools and parts for prototypes, or personalized products like medical implants or jewelry. Today, AM is also increasingly used for series production within diverse industrial fields like aerospace, automotive, or sportswear, e.g., shoe manufacturing. In addition, active electronic components like transistors can be realized by printed electronics [3]. These developments inspire researchers to attempt to fully print a microelectronic device like a mobile phone or a radar sensor. However, performance- and size-wise, printed electronic transistors cannot compete presently with the semiconductor technology needed for high-performance devices. But stepwise, AM technologies are also introduced in electronic system production. The AM of printed circuits boards (PCBs) has become popular in prototyping, and advanced equipment is available. For simple single-layer PCBs, additive manufacturing is already competitive, i.e., reducing the manufacturing cost by 50% [4]. This holds if the pattern density is small, i.e., in the range of 10%, because AM is driven by the material cost of the conductive paste. Therefore, the development of low-cost conductive materials is most important. However, when multilayer PCBs, high-speed or high-frequency application are considered, AM is presently not compatible because the inner layers have a large metal coverage [5]. In terms of the reliability required for high-end markets, conventional PCB technologies are predominant in flat electronic assemblies, i.e., laminating copper foil on FR4 (woven glass-reinforced epoxy resin sheets) and structuring the copper by lithography and back-etching. In contrast, AM processes are the predominant solution for manufacturing electronic circuits on complex 3D geometries [6]. Radar technology is one of the most important sensing methods for many application fields like air traffic control or geology [7,8]. In automotive, a radar sensor belongs to the important Advanced Driver Assistant System (ADAS) and enables functions like cruise control, collision warning, emergency breaking by distance and velocity measurement, and object detection [9,10]. Front- and back-looking radar sensors are mounted in the front or rear bumper or behind the car's front grille. Side radar sensors, used for blind-spot detection and lane-change assistance, are often installed in the side mirrors or doors. Therefore, it is important to note that radar sensors must be able to operate under harsh environmental conditions, e.g., low temperatures in winter, rain, snow, and saltwater conditions. They need to be sealed and withstand many large temperature cycles from \(-40\)\({}^{\circ}\)C to \(85\)\({}^{\circ}\)C, in accordance with ISO 16750 [11]. A typical modern radar sensor design is depicted in Figure 1a. A central element is the PCB which includes the low- (LF) and high-frequency (HF) parts of the electronics. On the top side of the PCB, a metallized volume antenna is attached [12,13]. The usage of the volume antenna allows the size of the PCB to be reduced and the HF and LF parts to be combined on one small PCBs, unlike earlier patch antenna designs [14]. The PCB board is inserted into the sensor housing, with the bottom side facing forward. Depending on the range and the integrated data processing, typically between 5 W and 20 W power is dissipated in a radar sensor--with falling tendency due to increasing efficiency. One major driver for power consumption in the radar sensor is data processing for object recognition and tracking [15]. Therefore, thermal management for heat dissipation to keep the operation temperature of the semiconductors below 150 \({}^{\circ}\)C is a challenging part of the sensor's housing design.