PCB Tech - Influence of PCB structure on millimeter wave radar performance

The dielectric layer of common composite printed circuit board (PCB) mostly uses glass fiber as filling material, but due to the special woven structure of glass fiber, the local dielectric constant (Dk) of PCB board will change. Especially at millimeter wave (mmWave) frequencies, the glass braiding effect of thinner laminates will be more obvious, and the local inhomogeneity of Dk will lead to significant changes in RF circuits and antenna performance. The influence of PCB structure on transmission line performance was studied by 100μm thick glass woven polytetrafluoron (PTFE) laminate. The dielectric constant of PCB was found to fluctuate between 0.01 and 0.22 according to different glass woven structure. In order to study the influence of different glass braided structures on the antenna performance, a series fed microstrip patch array antenna was fabricated on Rogers' commercial laminates RO4835 and RO4830, respectively, and the experimental results showed that: The electrical properties of the antenna machined with RO4830 laminate according to normal tolerance are more consistent with the calculated values, with smaller changes and better reflection coefficients (S11 & LT; -- 10dB) and view-axis gain performance.

Autonomous vehicle is a current research hotspot. It can help drivers and pedestrians avoid potentially fatal accidents, and it requires high reliability. Therefore, it also requires its circuit to have high reliability. Millimeter wave (mmWave) radar provides a reliable solution for target detection in automatic driving because of its compact structure and high environmental detection sensitivity. In commercial millimeter-wave radar systems at 76 to 81GHz frequencies, the series-fed microstrip patch antenna is famous for its ease of design, compact structure, and ability to manufacture in large quantities and at low cost [1]. The higher the frequency, the smaller the wave length, so transmission lines and antennas operating at millimeter-wave frequencies will be smaller than those operating at low frequencies. In order to ensure the ideal performance of on-board radar, it is necessary to study the influence of PCB on transmission line and microstrip patch antenna. For the millimeter-wave frequency circuit that works in outdoor environment for a long time (affected by temperature and humidity) [2], the consistency of material performance index is the primary consideration when selecting PCB line laminate. However, copper foil, glass fiber reinforced materials, ceramic fillers and other materials that constitute the laminate will have a great influence on the consistency of the index at high frequency.

This paper mainly studies the influence of PCB structure on MMW radar performance. The dielectric layer of most PCB laminates is usually formed by coating a fiberglass cloth with a polymeric resin. At millimeter wave frequency, the effect of glass fiber cloth on the consistency of material properties is very obvious, because the width of the glass bundle is equivalent to the width of the transmission line. In addition, when thin (e.g., 100μm) PCB line laminates are used to design microstrip antennas, glass braided fabric can cause significant changes in antenna performance and reduce the machined yield.

Composition of laminate

Laminates are usually made by combining fiberglass cloth with polymeric resin to form a dielectric layer, which is then covered with copper foil on both sides. The typical permittivity (Dk) of glass cloth is high, about 6.1, while that of low loss polymer resin is between 2.1 and 3.0, so that the Dk varies within a small area. Figure 1 shows a microscopic top view and cross section view of glass braided fibers in a laminate. The circuit above the "knuckle-bundle" has a high Dk due to its high fiberglass content, while the circuit above the "bundle-open" has a low Dk due to its high resin content. In addition, the characteristics of glass woven fabric are affected by the thickness of glass fabric, the distance between fabrics, the way the fabric is flattened and the glass content of each axis.

The laminate has a dielectric constant of 3.48 and loss Angle tangent of 0.0037 at 10GHz (based on IPC TM-650 2.5.5.5 standard test). In addition, the dielectric constant of RO4830 laminates is 3.24 and the loss Angle tangent is 0.0033 (based on ipCTN-650 2.5.5.5 standard test). RO4835 laminates are made of type 1080 standard woven unbalanced glass cloth and reinforced with ceramic fillers. In contrast, RO4830 laminates are reinforced with type 1035 flat open fiber glass braided and ceramic filled with smaller particles. Table 3 further compares the characteristics of laminates based on RO4835 and RO4830.

The antennas that meet the design size after processing and whose antenna transmission line is aligned with the "knuckle beam junction zone" and "beam opening zone" of RO4835 laminate are selected, as shown in FIG. 5 (a) and (b). As the RO4830 laminate adopts the glass braided structure of flat open fiber, it is unnecessary to consider whether the conductor is aligned with the glass fabric in RO4830 laminate, as shown in FIG. 5 (c). The reflection coefficient (S11) and optic axis gain of the machined antenna were measured respectively.

FIG. 5 Antenna aligned with "knuckle beam junction zone" and "beam opening zone" on RO4835 laminate, and antenna sample on RO4830 laminate

For the sake of simplicity, the results presented in this paper are from the average of the test data of several antennas under test, and the measurement results are compared with the simulation results. FIG. 6 shows the antenna test results (five samples) on the RO4835 laminate. The reflection coefficient (S11) and axial gain of the "knuckle beam crossing region" and the "beam opening region" are significantly changed. The performance of the antenna on the RO4835 depends on the alignment of the wire with the "knuckle junction zone" and the "beam opening zone". In addition, the antenna gain also varie

s with frequency, indicating that the dielectric constant is also changing. Moreover, a shift towards higher frequencies indicates a lower permittivity.

FIG. 6 Comparison between measurement results and simulation results of "knuckle beam junction zone (KB)" and "beam opening Zone (BO)" antenna samples of RO4835 laminates

By comparing the antenna performance on the RO4830 laminate shown in Figure 7, the antenna performance obtained in the test is very consistent and more consistent with the simulation value of RO4830 laminate. The consistency between the measured results and the simulation results shows that the dielectric constant of the laminates changes. In contrast, the apparent axis gain changes by 4 dB in the standard braided RO4835 laminate and only 2 dB in the flat open braided RO4830 laminate. By such simple experiments, more consistent antenna performance such as reflectivity and axial-gain can be obtained by using a Rogers RO4830 laminate with a flat open fibreglass braided construction style.

FIG. 7 Comparison of measurement results and simulation results of antenna samples on RO4830 laminate

conclusion

The structure of laminates can affect transmission lines and antenna performance. The construction of glass cloth will also change the dielectric constant of the laminate, which will reduce the product performance and affect the yield of the product. Compared with RO4835 laminate, the antenna processed with RO4830 laminate has better consistency of performance. The improvement of antenna performance and processing yield is mainly attributed to the structure of laminate material, i.e. glass weaving of flat open fiber, less glass content (conductor away from glass fiber), thicker substrate, etc. The improvement of antenna performance is also related to the electrical properties of the material, such as RO4830 laminate, which has a lower dielectric constant and a lower loss Angle tangent value. Therefore, in the application of millimeter wave frequency radar with small wavelength, the performance and consistency of the antenna processed with Rogers RO4830 laminate is better than that processed with RO4835 laminate.