Automotive mmWave radar design trends and PCB material solutions

Autonomous vehicles and advanced driver assistance systems (ADAS) technologies have facilitated the rapid development of automotive millimeter-wave radar sensors and the iterative updates of technologies, making car driving and travel safer. Millimeter-wave radar has become an indispensable sensor in automotive autonomous driving and ADAS systems due to its advantages of high resolution, strong anti-interference performance, good detection performance, and small size. With the increasing design of domestic millimeter-wave radar and the installation rate of domestic models, the application of millimeter-wave radar has also been expanded to more aspects. This article will briefly explain some application scenarios and design trends of millimeter-wave radar; and discuss the selection of key PCB materials in the design of millimeter-wave radar antennas, and the key characteristics of PCB materials.

Application scenarios

With the development of technology, the evolution of millimeter-wave radar also follows the direction of meeting the needs of users, realizing the detection range from near to far, and the measurement accuracy is gradually improved. From the earliest speed measurement and distance measurement, to the realization of speed measurement, distance measurement, angle measurement, and now to achieve higher resolution image imaging. In the ADAS system, the application of millimeter-wave radar can be divided according to the needs and functions of the vehicle. For example, it can be divided into forward radar, backward radar and angle radar according to the different installation positions on the car; it can also be divided according to the detection distance. The distance is divided into long-range radar, medium-range radar and short-range radar. The application of millimeter wave radar in ADAS includes such as AEB automatic braking, FCW forward collision warning, LCA lane change assist, ACC adaptive cruise, BSW blind spot monitoring and so on.

Figure 1 Millimeter-wave radar sensor in ADAS

In addition to assisting the driving and driving safety of cars, the application of automotive millimeter-wave radar has also expanded to the application of obstacle detection when parking or opening the door, reducing the collision damage of the door when parking or opening the door.

Various other applications increase the diversity of millimeter-wave radar applications and actively expand new scenarios for millimeter-wave radar applications. For example, the driver’s vital signs monitoring radar sensor can realize non-contact monitoring of the driver’s vital signs, such as heart rate and breathing rate, so as to sense the driver’s fatigue state and achieve the purpose of safe driving. The passenger member monitoring radar sensor also realizes the reliable detection of occupants (adults, children, pets) in the vehicle in a non-contact way, avoiding the occurrence of accidental detention during travel, and providing consumers with safe travel guarantees.

design trends

The operating frequencies of automotive millimeter-wave radars mainly include the 24GHz band and the 77GHz band. The 24GHz frequency band is mainly used for short-range radar, with a detection distance of about 50m, which can be used for systems such as blind spot detection. However, due to its narrow bandwidth, the resolution and performance of the radar are greatly limited.

Relatively speaking, 77GHz radar has broad prospects, and its great advantages are high precision, high resolution and excellent scalability from short to long distances. The two frequency bands of the 77GHz radar are 76-77GHz and 77-81GHz, and the bandwidths are 1GHz and 4GHz respectively. The huge bandwidth advantage significantly improves the resolution and accuracy. On the other hand, the 77GHz radar, due to its high frequency and short wavelength, makes the design of components such as radar transceivers or antennas smaller, which reduces the overall size of the radar and is easy to install and hide in the body. The 77GHz band has gained significant traction both in terms of global regulation and industry adoption.

The application of 77GHz millimeter-wave radar corresponds to the advanced stage of automobile automation. With the development of autonomous vehicles and the increase of ADAS installation rate, most 24 GHz automotive radar sensors will turn to the 77 GHz frequency band, and its demand and application are gradually rising. .

Figure 2 Market trends of radar sensors in different frequency bands

The 77GHz millimeter-wave radar system module is based on the design scheme of FMCW radar, and most of them use complete single-chip solutions such as TI, Infineon or NXP. receive channel. The PCB board design of the radar module varies according to the antenna design of the customer, but there are mainly these methods.

The first type uses ultra-low loss PCB material as the carrier board for the uppermost antenna design. The antenna design usually uses a microstrip patch antenna, and the second layer of the stack is used as the ground layer for the antenna and its feeder. The other PCB materials of the stack are made of FR-4. This design is relatively simple, easy to process, and low cost. However, due to the thin thickness of the ultra-low loss PCB material (typically 0.127mm), it is necessary to pay attention to the effect of copper roughness on loss and consistency. At the same time, the narrow feeder of the microstrip patch antenna needs to pay attention to the precision control of the line width of the processing.

The second design method uses a dielectric integrated waveguide (SIW) circuit to design the radar antenna, and the radar antenna is no longer a microstrip patch antenna. Except for the antenna, the other PCB stack-ups still use the same FR-4 material for the radar control and power planes as in the first method. The PCB material used in the antenna design of this SIW still uses ultra-low loss PCB material to reduce the loss and increase the antenna radiation. The thickness of the material is usually thicker than the PCB to increase the bandwidth, which can also reduce the influence of the roughness of the copper foil, and there are no other problems when processing narrower line widths. However, the via processing and position accuracy of SIW need to be considered.

The third design method is to design the stack structure of multilayer boards with ultra-low loss materials. Depending on the needs, it may be possible to use ultra-low-loss materials for several layers, or it may be possible to use ultra-low-loss materials for the entire stack. This design method greatly increases the flexibility of circuit design, can increase the integration, and further reduce the size of the radar module. But the disadvantage is that the relative cost is relatively high and the processing process is relatively complicated.

Figure 3 Different PCB designs for radar sensors

Material Considerations

For different PCB designs of millimeter-wave radar sensors, a common feature is that they all need to use ultra-low loss PCB materials, thereby reducing circuit loss and increasing antenna radiation. PCB material is a key component in radar sensor design. Choosing the right PCB material can ensure high stability and performance consistency of the millimeter-wave radar sensor.

Figure 4 Microstrip antenna for automotive radar sensor

The performance of PCB materials suitable for 77GHz millimeter-wave radar needs to be considered from the following aspects:

The first is the electrical properties of the material, which is the number one factor in designing radar sensors and selecting PCB materials. Selecting PCB materials with stable dielectric constant and ultra-low loss is critical to the performance of 77GHz mmWave radar. The stable dielectric constant and loss can make the transceiver antenna obtain accurate phase, thereby improving the antenna gain and scanning angle or range, and improving the radar detection and positioning accuracy. The stability of the dielectric constant and lossy properties of the PCB should not only ensure the stability of different batches of materials, but also ensure that the variation within the same board is small and has very good stability.

The surface roughness of the copper foil used in the PCB material has an impact on the dielectric constant and loss of the circuit. The thinner the material, the greater the impact of the surface roughness of the copper foil on the circuit. The rougher the copper foil type, the greater the change in its own roughness, which will also cause a greater change in the dielectric constant and loss, affecting the phase characteristics of the circuit.

Secondly, the reliability of the material needs to be considered. The reliability of the material not only refers to the high reliability of the material in the PCB processing, the influence of the processing process, the via, the bonding force of the copper foil, etc., but also the long-term reliability of the material. Whether the electrical properties of the PCB material can remain stable over time, and whether it can remain stable in different working environments such as different temperatures or humidity, which is important for the reliability of automotive radar sensors and the application of automotive ADAS systems it goes without saying.

In general, for the antenna design of the 77GHz radar sensor, it is necessary to consider selecting a material with a stable dielectric constant and ultra-low loss, and choosing a smoother copper foil can further reduce circuit loss and dielectric constant tolerance variation; At the same time, the material should have reliable electrical properties and mechanical properties with time, temperature, humidity and other external working environments.

material selection

Rogers has maintained cooperation with the world’s top radar module manufacturers from the early stage of automotive millimeter-wave radar development. The RO3003TM material launched without glass cloth has been rigorously verified in all aspects, and can meet the needs of 77GHz radar sensors. RO3003TM is widely used in 77GHz millimeter-wave radar. RO3003TM has a very stable dielectric constant and ultra-low loss characteristics (the loss factor of 0.001 at 10GHz in conventional tests); at the same time, the structure without glass cloth further reduces the bandwidth in the millimeter-wave frequency band. The local dielectric constant change can eliminate the glass fiber effect of the signal, and further increase the phase stability of the radar sensor. RO3003TM also has ultra-low water absorption characteristics (0.04% @D48/50), extremely low dielectric constant with temperature change (TCDk) stability (-3ppm/°C), these characteristics also ensure the RO3003TM-based mmWave radar sensor Excellent performance over time, temperature and environmental changes. The variety of copper foil types and low copper thickness options provided by the product also help improve processing accuracy and product yield, enabling radar sensors to achieve better performance.

With the development of radar sensors in the 79GHz band (77-81GHz), they have a wider signal bandwidth, which can further improve the resolution of radar sensors, increase the scanning angle, and even achieve 4D imaging. Based on the RO3003TM material, Rogers has developed and launched the RO3003G2TM material to match the higher requirements of the radar sensor for the performance of the PCB material. Compared with RO3003TM material, RO3003G2TM optimizes a special filler system in the material system, reduces filler particles, improves the uniformity of the material system, and further reduces the dielectric constant tolerance between the whole board and batch; smaller and The uniform packing system also enables the design of smaller via holes in the PCB processing process; the RO3003G2TM material selects a smoother copper foil, which reduces the insertion loss in the circuit, and its performance is very close to the RO3003TM rolled copper insertion loss performance.

Figure 5 Comparison of RO3003G2TM and RO3003TM materials

In addition, Rogers’ CLTE-MWTM and RO4830TM materials can also meet the different needs of customers in 77GHz radar sensor design. CLTE-MWTM is a material based on PTFE resin system with very small loss factor characteristics (Df 0.0015@10GHz), reinforced with special low-loss fiber-opened glass cloth, and together with uniform filler, it provides excellent dimensional stability, Minimize the effect of glass fiber effects. Multiple thickness options from 3mil to 10mil make the CLTE-MWTM material ideal for 77GHz radar sensor RF multilayer board applications.

Figure 6 Loss characteristics of different materials

RO4830TM is specially developed based on the Rogers RO4000 series material system, and the dielectric constant matches the low dielectric constant (Dk) most commonly used in 77GHz radar sensors. At the same time, it has the characteristics of extremely low insertion loss and the same ease of processing as the RO4000? series products. The selection of special low-loss fiberglass cloth also improves the performance consistency of the material in the millimeter wave frequency band, so that the antenna can obtain more Consistent phase characteristics and higher antenna gain. The low cost and high cost performance of RO4830TM are the first choice for cost-effective materials in 77GHz/79GHz radar sensor design.

Figure 7 The fiberglass cloth of RO4830

Summarize

The unique advantages of the 77GHz millimeter-wave radar sensor make it an indispensable component for autonomous vehicles. Wider bandwidth and higher resolution 77GHz/79GHz radar sensors have gradually become the mainstream. For various radar sensor design schemes, the characteristics of PCB circuit materials determine the performance of radar sensor antennas to a large extent. As a global leader in advanced engineering materials, Rogers Technology is committed to developing a variety of materials to meet customer design needs. RO3003G2TM/RO3003TM/CLTE-MWTM/RO4830TM and other material solutions solve design problems for customers in a timely manner. At the same time, Rogers’ global customer and technical support teams can ensure closer cooperation with customers, jointly solve a series of problems in design, processing and testing, and speed up customer design cycle.

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