4Layers RO4350B PCB

When selecting high frequency PCB boards, the following aspects are usually considered: low dielectric constant, low loss factor, frequency and temperature stability, and cost (including material cost, design-test-manufacturing cost). RO4350B is a low loss material for hydrocarbon and ceramic filler laminates and semi-cured sheets with excellent high frequency performance (generally applicable below 30GHz). Because the RO4350B is processed using the standard epoxy/glass (FR-4) process, the line processing cost is low. It can be said that RO4350B achieves the optimization in cost and high-frequency performance, and is the most cost-effective low loss high-frequency plate. In order to better meet the design requirements, the insertion loss of 24GHz microstrip transmission line based on RO4350B plate is studied when designing microstrip array antenna.

At frequencies below 10GHz, the conductor loss of microstrip lines is significantly higher than the dielectric loss; however, when the operating frequency reaches 24GHz, the dielectric loss exceeds the conductor loss. The insertion loss of microstrip lines primarily consists of conductor loss, dielectric loss, surface wave loss, and radiation loss, with the latter two being the main contributors. The skin effect causes high-frequency currents to concentrate in a thin layer directly in contact with the ground plane, resulting in an equivalent AC resistance that is much higher than the low-frequency case.

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 mil. As can be seen from the figure, the insertion loss of the microstrip line is about 17dB/m, of which the metal loss, dielectric loss and other losses are 4.47dB/m, 11.27dB/m and 1.26dB/m respectively. For comparison, Table 1 shows the insertion loss of the microstrip line calculated by MWI2016. It can be seen that under the same conditions, the MWI calculated value is 24.4dB, of which the dielectric loss value is close, but the conductor loss value differs by 7dB. The reason for the difference is that the surface roughness of the conduction strip and the ground plane is not considered in the HFSS model.

Measures to reduce microstrip line insertion loss
1) Reasonably select the board thickness and use green oil with caution
As can be seen from Table 1, for microstrip lines with the same characteristic impedance, the conductor loss decreases with the increase of dielectric thickness, while the dielectric loss remains basically unchanged. The reason is that the thicker the dielectric substrate, the narrower the microstrip line width, the more concentrated the high-frequency current, and thus the greater the conductor loss. It is worth noting that the loss tangent of the green oil medium at 24GHz is large, which will increase the insertion loss of the microstrip line. Therefore, when designing a 24GHz microstrip antenna, it is necessary to open a solder mask window in the antenna area.

2) LoPro copper foil is preferred
The surface roughness of the copper foil of the conductor and ground plane is also an important factor affecting the insertion loss of the microstrip line. The smoother the copper foil surface, the smaller the conductor loss. RO4350B provides two types of copper cladding: electrolytic copper foil (ED) and low roughness reverse processed 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 microstrip line insertion loss of these two 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 is 40% smaller than that of the ED copper foil at 24GHz.