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2021-08-24

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Author：Belle

The dielectric constant of** RO4350B **is relatively stable. The standard value is 3.48 at 10GHz. The dielectric constant will decrease as the frequency increases. At 24GHz, the dielectric constant decreases by 0.01 compared to the 10GHz frequency, which is 3.47.

Generally, **high-frequency PCB **plates are selected from the following aspects: low dielectric constant, low loss factor, frequency and temperature stability, and cost (material cost, design-test-manufacturing cost). ROGERS company RO4350B is a low-loss material for hydrocarbon resin and ceramic filler laminates and prepregs, with excellent high-frequency performance (generally applicable below 30GHz). Because RO4350B uses standard epoxy resin/glass (FR-4) processing technology for processing, it also has low line processing costs. It can be said that RO4350B has achieved the optimization of cost and high-frequency performance, and is the most cost-effective low-loss high-frequency sheet. In order to better realize the design requirements, the author studied the insertion loss of the microstrip transmission line based on RO4350B sheet at 24GHz when designing the microstrip array antenna.

Microstrip line insertion loss analysis

Microstrip line insertion loss mainly includes conductor loss, dielectric loss, surface wave loss and radiation loss, of which conductor loss and dielectric loss are the main ones. The skin effect makes the high-frequency current on the microstrip line concentrate on the thin layer where the conduction band and the grounding plate are in direct contact with the dielectric substrate, and the equivalent AC resistance is much greater than the low-frequency case. When working below 10GHz, the conductor loss of the microstrip line is much larger than the dielectric loss. When the working frequency rises to 24GHz, the dielectric loss exceeds the conductor loss.

Figure 1 shows the insertion loss of microstrip lines of different lengths calculated by HFSS. The dielectric substrates are all RO4350B with a thickness of 20 mils. It can be seen from the figure that the insertion loss of the microstrip line is about 17dB/m, and the metal loss, dielectric loss and other losses are 4.47dB/m, 11.27dB/m, 1.26dB/m, respectively. As a comparison, Table 1 shows the insertion loss of the microstrip line calculated by MWI2016. It can be seen that the calculated value of MWI is 24.4dB under the same conditions. The dielectric loss value is close, but the conductor loss value is 7dB different. The reason for the difference is that the surface roughness of the conduction band and the ground plate is not considered in the HFSS model. The HFSS calculation results of the insertion loss of the microstrip line are as follows:

Measures to reduce the insertion loss of the microstrip line

1) Choose the thickness of the board reasonably and use green oil with caution

As can be seen from Table 1, the conductor loss of the microstrip line with the same characteristic impedance decreases with the increase of the dielectric thickness, while the dielectric loss is basically unchanged. The reason is that the thicker the dielectric substrate, the narrower the microstrip line width, the more concentrated high-frequency current, and the greater the conductor loss. It is worth noting that the green oil medium has a large loss tangent angle at 24 GHz, which will increase the insertion loss of the microstrip line. Therefore, when designing a 24GHz microstrip antenna, it is necessary to open the window with solder mask in the antenna area. The MWI2016 calculation results of the insertion loss of the microstrip line are as follows:

2) Preferred LoPro copper foil

The surface roughness of the conductive strip and the ground plate copper foil is also an important factor affecting the insertion loss of the microstrip line. The smoother the surface of the copper foil, the smaller the conductor loss. RO4350B provides two types of copper clad: electrolytic copper foil (ED) and low roughness reversal treated copper foil (LoPro). The surface roughness of ED copper foil is about 3um, and LoPro copper foil can reach 0.4um, so it can Effectively reduce conductor loss. Figure 2 shows the comparison of the microstrip line insertion loss of these two types of copper foils. The thickness of the dielectric substrate is 0.1mm. It can be seen from the figure that the insertion loss of the LoPro copper foil microstrip line at 24GHz is 40% smaller than that of the ED copper foil. The comparison of the insertion loss of electrolytic copper and reversed copper is as follows:

3) Reasonable

Select surface treatment process

The surface treatment process is also one of the factors that affect the conductor loss. There are four common surface treatment processes, which are divided into immersion silver, immersion gold (non-nickel gold), nickel gold (nickel 3-5um, gold 2.54-7.62um) and immersion tin. Table 2 shows the electrical parameters of these metals. Nickel is a ferromagnetic material with a magnetic permittivity of 600. According to the skin depth calculation formula, the skin depth of nickel is an order of magnitude smaller than that of other metals, so the surface resistance of nickel is dozens of times larger than that of other metals, resulting in the conductor loss of the nickel-gold process being much larger than other processes. Figure 3 compares the insertion loss of bare copper, immersion silver and nickel-gold surface treatment processes, and the substrate thickness is all 20 mils. It can be seen from the figure that the insertion loss of the immersion silver process is similar to that of bare copper, but the insertion loss of the microstrip line after the nickel-gold surface treatment is 4dB/m (10GHz) larger. It is foreseeable that this difference will be even greater at 24GHz. Big. The electrical conductivity, magnetic permittivity and skin depth of different metals are compared with the insertion loss of nickel-gold process and bare copper as shown in the figure:

In summary, when we use **RO4350B PCB **dielectric substrates to design 24GHz microstrip antennas or microstrip circuits, we need to consider the thickness of the dielectric board, the type of copper coating, and the surface treatment process according to the performance and cost requirements. The conclusion is also applicable to most of the boards of **Rogers RO4000 **and **RO3000 series**.