With the rise of the Internet of Things and the increasing content of mobile Internet content, people have put forward higher requirements on the transmission rate and service quality of mobile communication networks. The fifth generation (5G) wireless mobile communication technology has emerged and developed rapidly. At the same time, 5G will also penetrate into various other industry fields, and integrate deeply with industrial facilities, medical instruments, and car networking to effectively meet the diversified business needs of industries such as industry, medical care, and transportation, and realize a true “Internet of Everythingâ€. .
High-band millimeter waves have significant advantages in 5G communications, such as sufficient bandwidth, miniaturized antennas and devices, and higher antenna gain. The US Federal Communications Commission plans four high-band bands for 5G, including three licensed bands and one unlicensed band, but looking for high-performance and reasonably priced printed circuit-board (PCB) materials in these bands is a huge challenge. Therefore, how to correctly understand the key parameters and characteristics of PCB materials is crucial to select PCB materials suitable for applications in the 5G technology application band.
Below we discuss how to select PCB materials suitable for applications in the 5G technology application band from the aspect of thermal management.
Thermal management
When the high frequency/microwave RF signal is fed into the PCB circuit, the loss due to the circuit itself and the circuit material will inevitably generate a certain amount of heat. In 5G equipment applications, not only the frequency of use is increased, but also the equipment tends to be miniaturized, which is bound to generate more heat. Handling circuit thermal management and understanding the thermal characteristics of the PCB helps to avoid circuit performance degradation and reliability degradation due to high temperatures.
Thermal model
A simple representation of the basic thermal model of the circuit and the heat flow profile model of the microstrip line is shown in Figure 5. In the microstrip line circuit, the top signal plane is a circuit heat source, and the bottom ground plane is a low temperature region or a heat dissipation plane, and the dielectric material is filled between the two planes. In the thermal model, heat is dissipated from the signal plane, through the material to the low temperature region of the ground plane. Although the heat generation process of an actual microstrip line circuit is complicated, such a hypothesis is acceptable for a simple thermal model. In the heat flow equation in the figure, k is the heat conduction system of the material, A is the heat source area, L is the material thickness, and (TH-TL) is the upper and lower temperature difference. The heat flow equation and thermal model explain the choice of circuit materials with high thermal conductivity and thin thickness for better heat dissipation and thermal management.
Figure 1: Basic thermal model of the circuit
The left) is the basic heat flow model, and the right is the heat flow profile model of the microstrip line circuit.
Thermal management
Designers often evaluate temperature rise from the perspective of circuit efficiency and loss, but the closest thermal conductor of the PCB medium as a heat source is the part that has a greater impact on temperature rise. As shown in Fig. 6, we can find through simulation that in the commonly used sheet, the method of lowering the temperature rise by reducing the Df value of the sheet is effective without the method of selecting a higher thermal conductivity. Although the dielectric loss in different materials will eventually affect the insertion loss of the circuit, resulting in different heat, the thermal conductivity of the material is more pronounced for temperature changes. For the same thermal conductivity value, for example 0.4 W/m/K, the temperature rise caused by the dielectric loss Df from 0.001 to 0.004 is only about 0.22 ° C/W. However, even a material having a Df of 0.001, a thermal conductivity of 0.2 W/m/K to 1.5 W/m/K can cause a temperature drop of 0.82 ° C/W. If the input power of the circuit is 50W, the temperature can be reduced by about 40 °C.
Figure 2: Simulation calculation of temperature rise with changes in Tc and Df
In addition to the thermal conductivity of the material, other parameters of the material also have an impact on thermal management. To better understand the influencing factors related to the thermal performance of PCB circuits, Table 7 shows the temperature variations of circuits based on different materials, different material thicknesses, loss factors, thermal conductivity, copper foil roughness, and insertion loss. This table provides a reference for comparing the thermal effects of different circuit materials. Comparing the 1st and 2nd circuits, the difference between the two is the thickness of the circuit, so the change in the thickness of the PCB material will cause the difference in temperature rise. The thinner the thickness, the shorter the heat dissipation path and the lower the temperature rise under the same conditions. Compared with the No. 2 and No. 3 circuits, the difference between the two is mainly due to the difference in insertion loss caused by the roughness of different copper foils. The smaller the surface roughness of the copper foil, the lower the insertion loss and the smaller the temperature rise; the material of the circuit 4 is FR-4, which is basically not used in the microwave/millimeter wave band. As an example, it can be seen that FR-4 has deficiencies in many aspects, such as high dielectric loss, conductor loss and low thermal conductivity, resulting in the highest insertion loss under the same circuit, resulting in a significant increase in temperature rise. Circuit 5 is based on Rogers RT/duroid 6035HTC PTFE ceramic material, which has a thermal conductivity of up to 1.44 W/m/K, has the best thermal conductivity, has a very low loss factor, and has the lowest insertion loss at the same input power. It has the lowest temperature rise and is ideal for high power microwave applications.
Figure 3: Comparison of thermal tests for different materials and thicknesses
Therefore, the thermal management of the circuit should select a relatively thin circuit material, and at the same time, the material properties such as high thermal conductivity, smooth copper foil surface and low loss factor are beneficial to reduce the heating of the circuit in the microwave millimeter wave band.
Multilayer board design
The 5G technology requires not only a smaller base station device but also a smaller size of the antenna. At the same time, active antenna systems that combine active circuitry with antennas will be an important part of the upcoming 5G network. Both miniaturized designs and active antenna systems require more circuitry for the application of multi-layer boards.
Z-axis thermal expansion coefficient
The thermoplastic material commonly used for high frequency PCB boards is polytetrafluoroethylene, which can be reinforced by various forms of fillers such as fiberglass or ceramic materials. Compared to thermoset materials, PTFE thermoplastics generally have better electrical properties and lower electrical losses, but PTFE materials have a higher Z-axis thermal expansion coefficient than copper. When making a multi-layer board, the different expansion of the board due to the difference in thermal expansion coefficient between the material and the copper when the board is subjected to high temperature causes the reliability of the PTH via to fail.
The importance of choosing a material with a low coefficient of thermal expansion for the reliability of vias in high frequency multilayer board applications is self-evident. Rogers research has found that adding special ceramic fillers to PTFE thermoplastics can improve the coefficient of thermal expansion of materials. Combining the low temperature and electrical properties of PTFE materials, this material is ideal for high frequency millimeter wave multilayer boards. For example, Rogers' RO3000 series circuit board material has a Z-axis thermal expansion coefficient as low as 24ppm/°C, and only requires a simple plasma treatment process to achieve high-reliability vias; and it has extremely low dielectric loss. It is very suitable for the circuit design of high frequency multilayer boards.
Impedance matching
Via design and processing control in high frequency microwave/millimeter wave multilayer boards are also areas of concern. In the design and processing of vias, the size of the vias, the thickness of the vias in the vias, the size of the pad on the outside of the vias, and the spacing between the vias and the ground plane all affect the parasitic capacitance and parasitic inductance of the vias. This affects the distribution parameters of the vias, resulting in a mismatch in the overall line, which is more pronounced in the microwave/millimeter wave band. On the 7.3 mil Rogers RO4350B Lopro CCL, 8 mils of RO4450F hydrocarbon ceramic prepreg were laminated on both sides to form a four-layer microvia strip circuit. Through experiments, we found that the through-hole circuits have the same via length and copper thickness, but the smaller aperture and smaller hole pads have smaller parasitic capacitance, better broadband characteristics, and return loss. Figure 8 shows the measured data of the circuit's return loss and RF bandwidth by reducing the impedance step change caused by the via aperture and the hole pad.
Figure 4: Effect of through-hole impedance variation on RF performance
In general, the continuous development of 5G technology and the demand for microwave frequency bands put forward higher requirements for the performance of PCB materials. Selecting the appropriate thickness according to the frequency, selecting the PCB material with low loss factor, understanding the influence of the surface roughness of the copper material of the PCB material, and selecting different copper foils, and the appropriate surface treatment process are beneficial to reduce the insertion loss of the circuit. The high thermal conductivity PCB material facilitates smaller size in 5G applications, thermal management of higher integration circuits, and optimal heat dissipation. At the same time, the appropriate PCB material type, material thermal expansion coefficient, via processing and reliability will ultimately determine the material selection.
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