The close relationship between pcb and automobiles

The close relationship between pcb and automobiles

Three major technical characteristics of 77GHz automotive radar system PCB board

Three major technical characteristics of 77GHz automotive radar system PCB board

As electronic systems in cars and other vehicles change, many former military technologies are slowly being applied to ordinary cars.

For example,

more models on the market are beginning to provide 24-GHz millimeter-wave short-range radar (SRR) systems.

But automotive designers and car manufacturers are taking a longer-term view and are already considering 77-GHz and 79-GHz automotive radar systems. In order to truly promote changes in automotive electronic systems, printed circuit board (PCB) materials  will be a very important component, and the use of radar systems will greatly enhance the safety performance automobiles.

Compared with 24-GHz automotive radar, the shorter wavelength of 77-GHz and 79-GHz systems allows for the use of smaller antenna sizes. Since the Doppler shift effect is more pronounced in the millimeter wave band than at 24-GHz, these high-frequency systems will be more accurate in testing the distance and relative speed between cars and other objects. High-resolution 77-GHz and 79-GHz radar systems can detect more hazardous road conditions, including ice and snow.

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1. Stable dielectric constant across the entire temperature range

 Since any change in the dielectric constant of the circuit board material will have an impact on the performance of the millimeter wave automotive radar system, another important parameter that needs to be considered is the effect of temperature on the dielectric constant TCDk. This parameter refers to the change in dielectric constant within the entire test temperature range in a short period of time. Because typical commercial vehicles face the challenge of a wide operating temperature range, the stable operation of 77GHz and 79GHz automotive radar systems is a very important parameter. Some laminates exhibit temperature time constant errors TCDk in excess of +200 ppm/°C at a given temperature and frequency.

Our RO3000 series PCB boards, specially designed for high-frequency antennas and circuits, have a temperature time constant error TCDk better than +11ppm/°C in the temperature range of -50°C to +150°C at 10GHz frequency.

2. Stable mechanical properties in various environments

Considering that automotive radar systems need to endure various operating conditions, especially for high-precision 77GHz and 79GHz systems, temperature mechanical stability is particularly important to maintain system reliability. The RO3003 laminate in the RO3000 series PCB boards is also a ceramic-filled polytetrafluoroethylene composite material PTEE specially designed for high-performance electronic systems, and exhibits excellent mechanical stability under changing environmental conditions. RO3003 laminate has a thermal elongation coefficient CTE of 17ppm/°C in the x-axis and y-axis directions in the wide temperature range of -55°C to +288°C. It is very close to the copper foil elongation system and has very excellent PCB dimensional stability. The PCB laminate also has a very low CTE in the Z-axis direction, 24ppm/°C. It ensures the stability of plated through holes.

3. Good thermal conductivity

Another characteristic that needs to be considered in 77GHz and 79GHz automotive millimeter radar applications is good thermal conductivity. Although power tends to lower and lower levels in high frequency circuits, it is also recommended to improve the thermal conductivity of PCB boards, as this means a reduction in the maximum board temperature for a given power load. PCB boards with good thermal conductivity can also enhance the thermal stability of the dielectric constant, because heat will be better distributed throughout the PCB board, reducing the occurrence of hot spots in the PCB.

4. Circuit boards suitable for high-frequency automotive radar systems

PCB boards for high-frequency automotive radar systems operating at 77GHz and 79GHz must meet a more stringent set of requirements than 24-GHz systems. For example, the consistency of the relative dielectric constant εr across the entire PCB board of a 77GHz system will be very critical, because changes in the dielectric constant Dk will show impedance changes and frequency drift in the transmission line. Changes in these frequencies can cause erroneous measurements by automotive radar systems, jeopardizing vehicle safety. In general, changes in the dielectric constant Dk of circuit materials will cause changes in the impedance of the transmission line, resulting in greater reflected energy, return loss and insertion loss.

RO4000 series PCB boards are suitable for use in 24-GHz automotive radar systems. For 77GHz and 79GHz automotive radar system applications, RO3000 series PCB boards have good characteristics in millimeter wave circuits.

RO3000 series PCB high-frequency laminated sheets have been successfully used in the manufacturing of ACC adaptive cruise control circuits at 77GHz. Its strict dielectric constant error characteristics are very helpful for the stability of the operating frequency, although 77GHz is already in the spectrum. Very high.

series PCB boards have a dielectric constant of 3.00 at 10GHz frequency and an error range of ±0.04. Since the receiving and receiving power is limited at the 77 GHz and 79 GHz frequencies, it is also very important to minimize the loss of the millimeter wave circuit board. The antenna-grade

RO3000 series PCB board has an extremely low dielectric dissipation factor of 0.0013 at 10GHz frequency, indicating that it still has very low dielectric loss at 77GHz and 79GHz frequencies. Although other PCB boards can also meet the performance requirements of 77GHz and 79GHz automotive millimeter radar systems, such as RT/duroid 5880 laminates, the RO3000 series meets excellent electrical and mechanical characteristics at a lower cost through the above three A key feature of this product is that it is very suitable for the emerging 77GHz and 79GHz automotive millimeter radar market.

RO3000 series boards can also use standard PTFE-based circuit board processing technology to minimize processing costs, even if it is used in the high-frequency millimeter wave band.

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Automotive millimeter wave radar design trends and PCB material solutions

Selection of key PCB materials in millimeter wave radar antenna design

Self-driving cars and advanced driver assistance systems (ADAS) technology have promoted the rapid development of automotive millimeter-wave radar sensors and iterative updates of technology, and have also made car driving and travel safer. Millimeter-wave radar has become an indispensable sensor in automobile autonomous driving and ADAS systems due to its advantages of high resolution, strong anti-interference performance, good detection performance, and small size. As domestic millimeter wave radar design and the installation rate of domestically produced models increase day by day, the application of millimeter wave radar has also expanded to more aspects. This article will briefly explain some application scenarios and design trends of millimeter wave radar; and discuss key PCB material selection considerations in millimeter wave radar antenna design, key characteristics of PCB materials, etc.

1. Application scenarios

With the development of technology, the evolution of millimeter wave radar has also been in the direction of meeting user needs, achieving a detection range from near to far, and the accuracy of measurement has gradually improved. From the earliest speed measurement and distance measurement, it can now achieve speed measurement, distance measurement, and angle measurement, and now it can achieve image imaging with higher resolution. In the 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 the installation position on the car; it can also be divided according to the detection range. The distance is divided into long-range radar, medium-range radar and short-range radar. The applications of millimeter wave radar in ADAS include AEB automatic braking, FCW forward collision warning, LCA lane change assist, ACC adaptive cruise, BSW blind spot monitoring, etc.

 Millimeter wave radar sensor in ADAS

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

Various other applications increase the diversity of millimeter wave radar applications and actively expand new millimeter wave radar application scenarios. 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 respiratory rate, so as to sense the driver’s fatigue status and achieve safe driving. The passenger member monitoring radar sensor also reliably detects vehicle occupants (adults, children, pets) in a non-contact manner, avoiding accidental detentions during travel and providing consumers with safe travel guarantees.

2. Design trends

The working frequencies of automotive millimeter wave radar mainly include the 24GHz frequency band and the 77GHz frequency band. The 24GHz frequency band is mainly used for short-range radar, with a detection range of about 50m, and can be used for blind spot detection and other systems. 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 accuracy, high resolution, and excellent scalability from short to long distances. The two frequency bands of 77GHz radar are 76-77GHz and 77-81GHz, with bandwidths of 1GHz and 4GHz respectively. The huge bandwidth advantage significantly improves resolution and accuracy. On the other hand, due to the high frequency and short wavelength of 77GHz radar, components such as radar transceivers or antennas are designed to be smaller, thereby reducing the overall size of the radar and making it easy to install and hide in the car body. The 77GHz band has gained significant traction both in terms of global regulatory 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 in ADAS installation rate, most 24 GHz automotive radar sensors will shift to the 77 GHz frequency band, and its demand and application are gradually on the rise. .

Market trends of radar sensors in different frequency bands

The 77GHz millimeter wave radar system module is based on the design of FMCW radar. Most of them adopt complete single-chip solutions such as TI, Infineon or NXP. The RF front-end, signal processing unit and control unit are integrated into the chip to provide multiple signal transmission and receive channel. The PCB board design of the radar module varies according to the customer’s antenna design, but there are mainly these methods.

The first uses ultra-low-loss PCB material as the carrier board for the top-layer antenna design. The antenna design usually uses a microstrip patch antenna, and the second layer of the stack serves as the ground layer for the antenna and its feeder. Other laminated PCB materials are all made of FR-4. This design is relatively simple, easy to process, and low cost. However, due to the thin thickness of ultra-low loss PCB materials (usually 0.127mm), attention needs to be paid to the impact of copper foil roughness on loss and consistency. At the same time, the narrow feeder line of the microstrip patch antenna requires attention to the line width precision control of processing.

The second design method uses dielectric integrated waveguide (SIW) circuits to design radar antennas. The radar antenna is no longer a microstrip patch antenna. Except for the antenna, other PCB stacks still use FR-4 material as the radar control and power layer as in the first method. The PCB material used in this SIW antenna design still uses ultra-low-loss PCB material to reduce loss and increase antenna radiation. The thickness of the material is usually chosen to increase the bandwidth of a thicker PCB, which can also reduce the impact of copper foil roughness, 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 laminated structure of multi-layer boards with ultra-low loss materials. Depending on different needs, some layers may use ultra-low loss materials, or all stacks may use ultra-low loss materials. This design method greatly increases the flexibility of circuit design, can increase the integration level, and further reduce the size of the radar module. But the disadvantage is that the cost is relatively high and the processing process is relatively complicated.

Different PCB designs for radar sensors

3. Material considerations

For different PCB designs of millimeter-wave radar sensors, one common feature is that they all need to use ultra-low-loss PCB materials to reduce circuit losses and increase antenna radiation. PCB material is a key component in radar sensor design. Choosing the right PCB material ensures high stability and consistent performance of millimeter wave radar sensors.

 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 are the primary factors 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 millimeter wave radar. Stable dielectric constant and loss can enable the transmitting and receiving antenna to obtain accurate phases, thereby increasing the antenna gain and scanning angle or range, and improving radar detection and positioning accuracy. The stability of the dielectric constant and loss of PCB must not only ensure the stability of different batches of materials, but also ensure that the changes within the same board are small and have very good stability.

The surface roughness of the copper foil used in PCB materials will have 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 greater changes in 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 terms of lamination during PCB processing, influence by the processing process, via holes, copper foil bonding strength, etc., but also includes the long-term reliability of the material. Whether the electrical properties of PCB materials can remain stable over time, and whether they can remain stable in different working environments such as different temperatures or humidity, 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 77GHz radar sensors, it is necessary to consider selecting materials with stable dielectric constant and ultra-low loss. Choosing smoother copper foil can further reduce circuit losses and reduce dielectric constant tolerance changes; At the same time, the material must have reliable electrical and mechanical properties over time, temperature, humidity and other external working environments.

4.Material selection

Rogers Corporation has been cooperating with the world’s top radar module manufacturers since the early days of automotive millimeter-wave radar development. The performance of RO3003TM material, which does not contain 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 (loss factor 0.001 at 10GHz in conventional tests); at the same time, the structure without glass cloth further reduces the band loss in the millimeter wave frequency band. This results in local changes in dielectric constant, eliminates the glass fiber effect of the signal, and further increases 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 that the millimeter wave radar sensor based on RO3003TM changes with time, temperature and environment Still maintains excellent performance. The variety of copper foil type options and low copper thickness options provided by the product also help improve processing accuracy and product yield, allowing radar sensors to achieve better performance.

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

 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). It is reinforced with special low-loss fiberglass cloth and together with uniform fillers, provides excellent dimensional stability. Minimize the influence of glass fiber effect. A variety of thickness options are available from 3mil to 10mil, making CLTE-MWTM materials ideal for 77GHz radar sensor RF multilayer board applications.

Figure 6 Loss characteristics of different materials

RO4830TM is specially developed based on Rogers RO4000 series material system, and its dielectric constant matches the low dielectric constant (Dk) most commonly used in 77GHz radar sensors. At the same time, it has extremely low insertion loss characteristics and the same easy processability as RO4000® series products. The selection of special low-loss split fiber glass cloth also improves the performance consistency of the material in the millimeter wave frequency band, allowing the antenna to obtain more Consistent phase characteristics and higher antenna gain. RO4830TM’s low cost and high cost performance are the first choice of cost effective materials in the design of 77GHz/79GHz radar sensors.

 RO4830 fiberglass cloth

Summarize

The unique advantages of 77GHz millimeter wave radar sensors make them an indispensable component of autonomous vehicles. Wider bandwidth and higher resolution 77GHz/79GHz radar sensors are gradually becoming mainstream. For various radar sensor designs, 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 various materials to meet customers’ 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 customer problems in design, processing and testing, and speed up customer design cycles.

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