Microwave Tech - Automotive millimeter wave radar design trend and PCB material solutions

The rapid development of automotive millimeter wave radar sensors and the iterative update of technology also make driving and travel safer. Millimeter wave radar with its own high resolution, strong anti-jamming performance, good detection performance, small size and other advantages, has become an indispensable sensor in auto driving and ADAS system. With the increase of domestic millimeter wave radar design and the installation probability of domestic models, the application of millimeter wave radar is also expanded to more aspects. This article will briefly describe some application scenarios and design trends of millimeter wave radar; The selection of key PCB materials and the key characteristics of PCB materials in the design of millimeter wave radar antenna are discussed.

Application scenarios

With the development of technology, the evolution of millimeter wave radar is also along 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 range measurement, to the realization of speed measurement, range measurement and angle measurement, and now to the realization of higher resolution image imaging. In ADAS system, the application of millimeter wave radar can be divided according to different vehicle requirements and functions. For example, it can be divided into forward radar, backward radar and angle radar according to different installation positions on vehicles; It can also be divided into long range radar, medium range radar and short range radar according to the detection distance. The application of millimeter wave radar in ADAS includes AEB automatic braking, FCW forward collision warning, LCA lane changing assistance, ACC adaptive cruise, BSW blind area monitoring and so on.

Automotive millimeter wave radar

In addition to assisting vehicle driving and driving safety, the application of vehicle millimeter wave radar is also extended to the application of obstacle detection when parking or opening the door, so as to reduce 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 the new application scenarios of millimeter wave radar. For example, the driver vital signs monitoring radar sensor can realize non-contact monitoring of driver vital signs, such as heart rate and respiratory 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 the passengers (adults, children, pets) in the car in a non-contact way, so as to avoid the occurrence of accidental detention in the process of travel and provide safe travel protection for consumers.

PCB design trends

The working frequency of Automobile Millimeter Wave Radar mainly includes 24GHz band and 77GHz band. 24GHz band is mainly used for short-range radar with detection range of about 50m, which can be used for blind spot detection system. However, due to its narrow bandwidth, the resolution and performance of radar are greatly limited.

Relatively speaking, 77GHz radar has broad prospects. Its great advantages are high precision, high resolution and excellent measurability from short distance to long distance. The two frequency bands of 77GHz radar are 76-77ghz and 77-81ghz. The bandwidth is 1GHz and 4GHz respectively. The huge bandwidth advantage significantly improves the resolution and accuracy. On the other hand, due to the high frequency and short wavelength of 77GHz radar, the designed radar transceiver or antenna and other components are smaller, which reduces the overall size of the radar and makes it easy to install and hide in the car body. The 77GHz band has gained significant appeal in terms of global regulation and industry adoption.

The application of 77GHz millimeter wave radar corresponds to the advanced stage of automotive automation. With the development of automatic driving vehicle and the increase of ADAS installed rate, most of the 24 GHz vehicle radar sensors will turn to the 77 GHz band, and their demand and application will gradually increase.

Market trend of radar sensors in different frequency bands

The 77GHz millimeter wave radar system module is based on the FMCW radar design scheme, and mostly adopts the complete single-chip solution such as Ti, Infineon or NXP, which integrates the RF front-end, signal processing unit and control unit, and provides multiple signal transmitting and receiving channels. The PCB design of radar module varies according to the antenna design of customers, but there are mainly these ways.

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

The second design method uses SIW circuit to design radar antenna, which is no longer microstrip patch antenna. In addition to the antenna, the other PCB stacks still use FR-4 material as the radar control and power layer as the first way. The PCB material used in this SIW antenna design is still ultra-low loss PCB material, which can reduce the loss and increase the antenna radiation. The thickness of the material is usually thicker than that of the PCB to increase the bandwidth, which can also reduce the influence of the copper foil roughness, and there are no other problems when processing narrow line width. But we need to consider the problem of SIW through hole machining and position accuracy.

The third design method is to design multilayer structure with ultra-low loss materials. According to different requirements, some of them may use ultra-low loss materials, or all of them may use ultra-low loss materials. This design greatly increases the flexibility of the circuit design, increases the integration, and further reduces the size of the radar module. But the disadvantage is relatively high cost, the processing process is relatively complex.

Different PCB design of radar sensor

PCB material considerations

For different PCB designs of millimeter wave radar sensors, there is a common feature that ultra-low loss PCB materials are needed to reduce circuit loss and increase antenna radiation. PCB material is the key component of radar sensor design. Choosing appropriate PCB material can ensure the high stability and performance consistency of millimeter wave radar sensor.

The performance of PCB material for 77GHz millimeter wave radar should be considered from the following aspects

The first is the electrical characteristics of the material, which is the primary factor in the design of radar sensors and the selection of PCB materials. The selection of PCB materials with stable dielectric constant and ultra-low loss is very important for the performance of 77GHz millimeter wave radar. Stable permittivity and loss can make the receiving and transmitting antenna obtain accurate phase, so as to improve antenna gain and scanning angle or range, and improve radar detection and positioning accuracy. The stability of dielectric constant and loss of PCB not only needs to ensure the stability of different batches of materials, but also needs to ensure that the change in the same board is small and has very good stability.

The surface roughness of copper foil used in PCB material will affect the dielectric constant and loss of the circuit. The thinner the material is, the greater the influence of the surface roughness of copper foil on the circuit. The rougher the type of copper foil, the greater the change of its own roughness, which will also cause the larger changes of dielectric constant and loss, and affect the phase characteristics of the circuit.

Secondly, the reliability of materials should be considered. The reliability of materials not only refers to the high reliability of materials in PCB processing, such as lamination, influence of processing process, through hole, copper foil adhesion, but also includes the long-term reliability of materials. Whether the electrical performance of PCB material can remain stable with the increase of time, and whether it can remain stable in different working environment, such as different temperature or humidity, is self-evident for the reliability of automotive radar sensor and the application of automotive ADAS system.

Generally speaking, for the antenna design of 77GHz radar sensor, it is necessary to consider the selection of materials with stable dielectric constant and ultra-low loss. Choosing smoother copper foil can further reduce the circuit loss and the permittivity tolerance; At the same time, the material should have reliable electrical and mechanical properties with time, temperature, humidity and other external working environment.

PCB material selection

Rogers has been cooperating with the world's top radar module manufacturers since the early development of automotive millimeter wave radar. It launched ro3003 without glass cloth The performance of the material has been rigorously verified in various aspects, and can meet the requirements of 77GHz radar sensor. RO3003 The material is widely used in 77GHz millimeter wave radar, and has very stable dielectric constant and ultra-low loss characteristics (the loss factor is 0.001 at 10GHz by conventional test); At the same time, the structure without glass cloth further reduces the change of local dielectric constant in millimeter wave band, eliminates the glass fiber effect of signal, and further increases the phase stability of radar sensor. RO3003 The material also has the characteristics of ultra-low water absorption (0.04% @ d48 / 50), extremely low dielectric constant with temperature (tcdk) stability (- 3ppm / ° C), which also ensures the stability of ro3003 The millimeter wave radar sensor of the material can still maintain excellent performance with the change of time, temperature and environment. The selection of various copper foil types and low copper thickness also help to improve the processing accuracy and yield of products, and make the radar sensor achieve more excellent performance.

With the development of 79ghz (77-81ghz) radar sensor, it has a wider signal bandwidth, which can further improve the resolution of radar sensor, increase the scanning angle, and even realize 4D imaging. Rogers based on ro3003 Based on the materials, ro3003g2 was developed and launched ™ Materials to match the higher requirements of radar sensor for PCB material performance. Compared with ro3003 ™ Material, ro3003g2 ™ In the material system, the special packing system is optimized to reduce the filler particles, improve the uniformity of the material system, and further reduce the permittivity tolerance between the whole plate and batch; The smaller and uniform packing system also makes it possible to achieve smaller via design in PCB processing; RO3003G2 The more smooth copper foil is selected to reduce the insertion loss in the circuit, and its performance is very close to that of ro3003 The insertion loss properties of rolled copper were studied.

Comparison between ro3003g2 and ro3003

In addition, Rogers clte-mw And ro4835 The materials can also meet the different requirements of customers in 77GHz radar sensor design. CLTE-MW It is based on PTFE resin system and has very small loss factor (DF 0 0015@10GHz )It is reinforced by special low loss open fiber glass cloth, which, together with uniform filler, provides excellent dimensional stability and minimizes the influence of glass fiber effect. There are a variety of thickness options from 3mil to 10mil to make clte-mw The material is very suitable for 77GHz radar sensor RF multilayer applications.

Loss characteristics of different materials

RO4835 The material is based on Rogers ro4000 The dielectric constant is matched with the low dielectric constant (DK) commonly used in 77GHz radar sensors. At the same time, it has the characteristics of extremely low insertion loss and the same processability as ro4000 series products. The selection of special low loss open fiber glass cloth also improves the performance consistency of the material in the millimeter wave band, so that the antenna can obtain more consistent phase characteristics and higher antenna gain. RO4835 Low cost and cost-effective materials are the first choice in the design of 77GHz / 79ghz radar sensor.

Open fiber glass cloth of ro4835 material

The unique advantages of the 77GHz millimeter wave radar sensor make it an indispensable part of the automatic driving vehicle. The 77GHz / 79ghz radar sensor with wider bandwidth and higher resolution has gradually become the mainstream. For various radar sensor design schemes, the characteristics of PCB circuit materials determine the performance of radar sensor antenna to a great extent. As a global leader in advanced engineering materials, Rogers technology is committed to developing various materials to meet the design needs of customers. RO3003G2 / RO3003 / CLTE-MW / RO4835 And other material solutions, timely for customers to solve the design problems. At the same time, Rogers company's global customer and technical support team can ensure closer cooperation with customers, solve a series of problems in customer design, processing and testing, and speed up the customer design cycle.