Hybrid assembly and joining methods for ADAS sensors
Radar and LiDAR sensors are crucial components in autonomous vehicles, providing essential data for navigation and obstacle detection. Vehicles, pedestrians, and cyclists are detected, and distances and speeds are measured, enabling the vehicle’s system to react and avoid collisions. The fusion of data from various sensors, including radar, LiDAR, cameras, and other technologies, forms a comprehensive perception system that helps autonomous vehicles navigate safely and efficiently in diverse environments.
Technical requirements of sensors for autonomous driving
Since it is mounted on the outside of the vehicle, radar and LiDAR sensors must be compact and robust and comply with defined installation dimensions. A minor error in the function of the complex sensor system can lead to major injuries or death. Hence, the reliability and accuracy of these sensors are mandatory. Therefore, the sensors and electronics need to be well protected against external influences such as rain, fog, extreme weather conditions or road dirt to ensure the performance.
Radar and LiDAR sensor systems are hybrid assembly packages, which consist of an aluminum carrier with an electronic control board inside and a plastic cover. The cover is glued with the carrier and the function of the adhesive is obviously to mate both materials, but at the same time it also seals the sensor housing hermetically.
Due to the need of a high number of sensors for a higher number of vehicles, radar and LiDAR sensors are produced in mass production with typical cycle times of less than 20 seconds. Consequently, fast and efficient assembly processes are mandatory, and this can be achieved with new technology.
Time reduction in adhesive bonding processes
Looking at the carrier of a sensor, the glue or Formed-in-Place Gasket (FIPG) needs to be dispensed very precisely (see Figure 1), otherwise inconsistent dispensing may lead to leaks and result in failures of the sensor. The cycle time of the dispensing process is determined by the speed with which the material can be applied into the rounded corners. In a traditional dispensing process, the glue bead will be applied constantly with consistent axis speed. With bdtronic’s speedUP system however, the speed of the axis and the material volume are intelligently linked and controlled. This way speedUP achieves the shortest possible cycle time with an optimum dispensing result. Long straight sections will be dispensed at high speed and critical curves at low speed. The result is a significant reduction in total cycle time.
Figure 1: Dispensing glue or FIPG into a sensor housing
Figure 2: Carrier of a Radar Sensor with dispensed sealing and TIM Material
Figure 3: Traditional dispensing speed vs speedUP à cycle time reduction of almost 4 seconds per unit (~50 %)
Thermal management in ADAS applications
Modern radar and LiDAR sensor units are smaller than smartphones. But heat dissipation and thermal protection is crucial to long term functionality. So thermally conductive liquid gap filler materials are designed and dispensed automatically in high volume manufacturing.
Figure 4: Dispensing machines B5200 with 2k duplex mixing head for applying gap fillers
Those gap fillers provide excellent thermal and mechanical performance and induce zero stress on the components during assembly. Because gap fillers are liquid, they can be freely dispensed in almost any desired pattern or shape thus providing extreme flexibility in manufacturing and design.
Dispensing highly filled materials for thermal conductivity
But thermally conductive pastes present fundamental challenges for the dispensing technology used:
- The abrasiveness of the gap fillers, that must be processed, without increasing wear of the components of the dispensing system.
- The low proportion of polymer matrix, which tends to separate from the filler portion of the formulation under high pressure and mechanical stress.
Commonly used dispensing pumps like gear pumps quickly reach their wear limit with such highly filled materials. Piston pumps, on the other hand, often undergo laborious and lengthy maintenance due to their complex design.
bdtronic has been successfully using eccentric screw pumps for the application of thermally conductive pastes for many years. Compared to other pump systems, it offers several advantages due to its robust and low-wear technology. bdtronic pumps are especially designed for abrasive media, for example with different stator geometries in various elastomers and rotors with special coatings such as ceramic or DLC diamond-like carbon.
Figure 5: Applying a gap filler into a radar sensor housing to ensure thermal conductivity
With this pump technology automotive suppliers of radar and LiDAR sensors worldwide profit from long durability of pumps and the high-quality dispensing results due to the gentle processing of the material.
Surface cleaning and activation before adhesive bonding
To achieve best results in adhesive bonding, clean surfaces with good wetting and bonding properties are essential. For the housing of radar and LiDAR sensors there are mostly two different materials used:
- An engineering thermoplastic with an excellent firmness and stiffness, like glass fiber reinforced PBT. Unfortunately, PBT has a relatively low surface energy (~32 mN/m), which results in poor wetting properties and is not ideal for adhesive bonding.
- Carrier bases often consist of aluminum. This surface can show traces of corrosive attacks or residual organic contamination such as oil, grease, rolling oils and cleaning fluid.
Treating plastic and metal surfaces with plasma results in clean surfaces with an increased surface energy and this improves their bond ability. Both materials can be treated with our atmospheric plasma VP4.
Dynamic plasma technology for different surface materials
Plasma is a mixture of free electrons, ions, radicals and molecular fragments and it is created when energy is applied to a gas. It removes organic contaminants and etches the surface of the material at a microscopic level. Secondly it activates the surface of the material, making it more wettable and more likely to react with an adhesive. This significantly improves the adhesive bond’s mechanical strength, securing a long-lasting adhesion and clean and polar surfaces with no residue.
Unique about bdtronic’s plasma technology VP4 is that it is a dynamic system with continuous and variable power adjustment, which can be used for metal and plastic surfaces:
- Metal surfaces require a very high plasma power to be cleaned from rolling oil deposits and to break down the oxide layer.
- Plastic surfaces however are at risk of getting burned when exposed too high plasma power.
The VP4 plasma system can adjust the power in real time during the process. Therefore, the plastic and metal surfaces can be treated with the same tooling head – simply be adjusting the power settings in the drive program.
Hybrid joining saves floor space and processing time
Typical sealants for radar and LiDAR sensors are single-part silicones, that are designed for applications which demand a strong but flexible bond, such as bonding materials with differing thermal expansion rates. Silicone sealants cure at room temperature and with 50 % relative humidity in the air. The curing takes roughly 24 hours, which is not ideal for mass production. Because it needs a large buffer storage before the assembled components can be packed and shipped to the customer.
Storage and drying time can be saved by using a fast-curing 2-part adhesive, a hybrid joining method, or a combination of the two. The cover can be fastened with screws until the glue is completely cured. But looking at the assembly process this is not the best solution – as the dust and chip formation while screwing is not desired in an electronics environment.
Heat staking can be a more cost-effective and cleaner alternative to screws. Since the housing material PBT is a thermoplastic, these pins can be heated and reformed to a dome-shaped rivet. The pins can be integrated into the injection mold of the plastic cover and no burrs or dust is generated during the process.
Heat staking is suitable especially for joining electronic components, because while processing the electronic parts are only subjected to low mechanical and thermal impact. So, joining the cover to the aluminum carrier after adhesive bonding ensures that the assembled unit is kept perfectly in place until the glue has cured completely. The components can be immediately packed and shipped and there is no need for buffer storage.
With two thermal riveting processes to heat pins and form rivets, while monitoring all process steps, the results are clean and reproducible with maximum rivet strength.
Case study: Joining a radome to a sensor housing
For joining a radome to the sensor housing, one automotive manufacturer opted for heat staking with the BHS HOT STAMP® process instead of the originally planned bolting process to meet the requirements for technical cleanliness and to save running costs. The pin material selected is a special, durable semi-crystalline thermoplastic. For designing the riveted joint, the engineer followed the bdtronic design guideline:
Figure 6: Design Guideline: Overview of solid pins
The final production step is carried out with four hot riveting points in the corners of the product, whereby the free area around the riveting points for forming the rivet heads and supporting the tools is very tight. In the production process the radome is joint to the housing after dispensing the liquid seal, which may be exposed to a maximum temperature of 80 °C during the curing time of approx. 24 hours.
The BHS HOT STAMP® hot riveting process produces a very low temperature input on the component and the tools have the smallest diameters, so it was the right method to use in combination with this sealing material. The process simulation and test results in the Technology Center showed that all requirements were met.
In the final production process, the assembly was brought into the machine with the rivet pins on the underside. Turning the assembly around after dispensing the sealant and bringing the entire assembly together was risky. As doing so could possibly negatively affect the cross-linking and bonding of the sealant. So, riveting against the direction of gravity was desired to eliminate the need to turn the assembly before riveting.
BHS HOT STAMP® enables this process direction, which reduces the investment costs and allows the production sequence to be reproduced safely. A special punch was designed for the series production line which can be guided through the hold-down mask. Diverse product variants, some with different material types, are now produced in many production lines worldwide with a cycle time of 15 seconds.
Figure 7: BHS HOT STAMP® machine B7000 for sensor production
Figure 8: Tools for joining with BHS HOT STAMP® technology
Case study: Joining a sensor housing
In another radar sensor project, a carrier is positioned precisely in the housing using the hot-air riveting process, while the joining partners are pressed together under defined pressure. In this application the rivet joint must ensure that the joining partners do not move in their position relative to each other over their service life. The BHS HOT AIR® heat staking process is used to ensure maximum holding force and strength even under vibration loads in the vehicle. The production line produces one component every five seconds in various riveting stations.
For other sensor projects, hot-air riveting is used to join and securely position antennas to radomes, printed circuit boards to housings, plastic-plastic pairings, or cooling plates in housings. The BHS HOT AIR® heat staking process ensures maximum holding force and secure gap filling to achieve movement-free positioning. The heat staking machine for joining two parts made of different thermoplastic materials must work in cycle with 4-cavity injection molding machines and rivets a component every three seconds.
Regina Körner, Marketing, bdtronic
