PCB Technical - Influence of PCB structure on millimeter wave radar

The dielectric layer of common composite printed circuit board (PCB) mostly uses glass fiber as filler. However, due to the special braided structure of glass fiber, the local dielectric constant (DK) of PCB will change. Especially at the millimeter wave frequency, the glass braiding effect is more obvious than that of the thin laminate, and the local inhomogeneity of DK will lead to obvious changes in RF circuit and antenna performance. The influence of PCB structure on transmission line performance was studied by using glass braided polytetrafluoroethylene (PTFE) laminate with thickness of 100 μ M. according to different types of glass braided structure, the dielectric constant of PCB board fluctuated between 0.01 and 0.22. In order to study the effect of different glass braided structures on the antenna performance, a series fed microstrip patch array antenna was fabricated on Rogers' commercial laminate ro4835 and ro4830 thermosetting laminate respectively, and the experimental results show that the electrical properties of the antenna fabricated with ro4830 laminate in accordance with the normal tolerance are more consistent with the calculated values, and the changes are smaller It has good reflection coefficient (S11 < – 10dB) and Los gain performance.

Autopilot is a hot research topic at present. It can help drivers and pedestrians avoid potential fatal accidents, and requires high reliability. Therefore, it is required that the circuit must be highly reliable. Because of its compact structure and high sensitivity of environmental detection, mmwave radar provides a reliable and reliable solution for target detection in automatic driving. In commercial millimeter wave radar systems at 76-81 GHz, series fed microstrip patch antennas have become popular because of their easy design, compact structure, mass production and low cost. The higher the frequency, the smaller the wavelength. Therefore, compared with the low frequency, the size of transmission line and antenna working at millimeter wave frequency will be smaller. In order to ensure the ideal performance of vehicle borne radar, it is necessary to study the influence of PCB on transmission line and microstrip patch antenna. For the millimeter wave frequency circuit [2] which works in outdoor environment for a long time (affected by temperature and humidity), the consistency of material performance index is the primary consideration when selecting PCB circuit laminate. However, the copper foil, glass fiber reinforced material, ceramic filler and other materials that constitute the laminate will have a greater impact on the consistency of the indicators at high frequency.

Application of millimeter wave radar

This paper mainly studies the influence of PCB structure on the performance of millimeter wave radar. The dielectric layer of most PCB laminates is usually formed by coating polymer resin on glass fiber cloth. At millimeter wave frequency, the influence of glass fiber cloth on the uniformity of material properties is very obvious, because the width of glass bundle is equal to that of transmission line. In addition, when thin (for example, 100 μ m) PCB line laminates are used to design microstrip antennas, the glass woven cloth will cause significant changes in antenna performance and reduce the processing yield.

Composition of laminates

The laminate is usually made of glass fiber cloth and polymer resin to form a dielectric layer, and then covered with copper foil on both sides. The typical dielectric constant (DK) of glass cloth is higher, about 6.1, while that of low loss polymer resin is between 2.1 and 3.0, so there is a certain difference in DK in a small area. Figure 1 shows the microscopic top and cross-sectional views of the glass braided fibers in the laminate. The circuit above the knuckle bundle has a higher DK due to its higher glass fiber content, while the circuit on the bundle open has a lower DK due to the higher resin content. In addition, the properties of glass woven fabric are affected by the thickness of glass fabric, the distance between fabrics, the flattening method of fabric and the glass content of each axis.

Two typical weaving patterns of thin glass cloth, 1080 and 1078, are often used in thin laminates for millimeter wave applications, as shown in Figure 2. The unbalanced glass cloth is used in 1080 standard weaving. The glass content of one axis is higher than that of the other. Compared with 1080 woven fabric, 1078 open fiber glass braid has more uniform glass fiber plane, so the change of DK on the whole laminate is small. Compared with the laminate with multi-layer glass cloth, the change of DK value of single-layer glass cloth laminate is more significant. In addition, the laminate material with ceramic filler can reduce the DK change caused by different weaving methods of glass cloth.

Microscopic view of the structure of 1080 (open unbalanced braiding) and 1078 (open fiber) glass cloth

Influence on transmission line circuit

This test experiment uses microstrip transmission line circuit, using 1 mm termination connector. The connector is first connected to a 50 ohm grounded coplanar waveguide (GCPW) and converted into a high impedance microstrip transmission line through an impedance converter. As shown in Figure 3, the length of the microstrip transmission line is 2 inches, which ensures that the experimental circuit can test the effect of glass braided structure. The circuit is made of glass braided polytetrafluoroethylene (PTFE) laminate, and calendered copper and single glass cloth are used. In order to compare the effects of different glass braiding structures, transmission line circuits were made on three different PCB laminates, which were PTFE Teflon with 1080 glass cloth, PTFE polytetrafluoroethylene with 1078 glass cloth, and non PTFE laminate filled with 1080 glass cloth. Carefully check the processed circuit, select the appropriate transmission line for testing, and measure the amplitude and phase angle characteristics of the circuit. The dielectric constant of the laminate is determined by three parameters: phase angle (expanded phase value), group delay (based on phase angle varying with frequency), and propagation delay (calculated according to phase angle).

Influence on antenna performance

Series fed microstrip patch antenna array is a typical antenna for millimeter wave automotive radar. In order to study the effect of glass fiber effect on the antenna performance, a 1 * 4 series fed microstrip patch antenna is designed, and its operating frequency range is 76-81 GHz [3]. As shown in Figure 4, the antenna is made of two different glass cloth laminates, ro4835 and ro4830. The antenna is made of grounded adjacent elements to study its coupling effect.

Series fed microstrip patch arrays fabricated on rogers ro4835 and rogers ro4830 laminates

The dielectric constant of the laminate at 10 GHz is 3.48 and the loss tangent is 0.0037 (based on IPC TM-650 2.5.5.5 Standard Test). In addition, the dielectric constant of ro4830 laminate is 3.24 and the loss tangent is 0.0033 (based on ipctm-650 2.5.5.5 Standard Test). Ro4835 laminates are made of 1080 standard woven unbalanced glass cloth and reinforced with ceramic filler. In contrast, the ro4830 laminate was reinforced by 1035 flat open fiber glass braiding and ceramic filled with smaller particles. Table 3 further compares the properties of laminates based on ro4835 and ro4830.