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[PCB design] Rogers: PCB material selection for rf high power applications

2020-07-29

Although there are some high-power PCB applications that are not base-station related, the majority of high-power PCB applications are base-station power amplifiers. In designing such high power RF applications, several considerations need to be made. This article focuses on the application of PCB-based base station power amplifiers, but the basic concepts discussed here also apply to other high-power applications.

Thermal management problems exist in most high-power RF applications, and some basic relationships need to be considered for good thermal management. For example, when the signal power is input into the circuit, the circuit with higher loss will generate higher heat. The other is related to frequency, and the higher the frequency, the more heat will be generated.
In addition, an increase in heat in any dielectric material causes a change in the dielectric material Dk (dielectric constant), namely the dielectric constant temperature coefficient (TCDk). With the change of loss, the change of circuit temperature leads to the change of Dk. This CHANGE in Dk caused by TCDk can affect the performance of RF circuits and may affect system applications.

For the heat loss relationship, a variety of different materials and corresponding PCB characteristics can be considered.
Sometimes, when designers choose low-loss materials for PCB applications, they may consider only dissipation factors (Df or loss Angle tangents). Df is only the dielectric loss of the material; however, there are other losses in the circuit. The total circuit loss related to rf performance is insertion loss, which consists of four losses and is the sum of dielectric loss, conductor loss, radiation loss and leakage loss.

Using a very low loss material with a Df of 0.002 and very smooth copper foil circuitry will have a relatively low insertion loss.
However, if the same circuit of the same low-loss material is still used, but rough electrolytic copper (ED) is used instead of smooth copper, the insertion loss will increase significantly.

The surface roughness of copper foil will affect the conductor loss of the circuit. It should be clear that the surface roughness associated with loss is the surface roughness of copper foil at the copper-medium interface during laminate processing. In addition, if the circuit USES a thinner medium, the copper foil surface will be closer, and the surface roughness of the copper foil will have a greater impact on the insertion loss than the relatively thick medium.

Thermal management is usually a common problem for high-power RF applications, and laminates with low Df and smooth copper foil are preferred. In addition, it is often wise to choose laminates with high thermal conductivity. A high thermal conductivity will facilitate and effectively transfer heat from the circuit to the radiator.

The frequency-heat relationship indicates that given the same RF power at both frequencies, more heat will be generated as the frequency increases. Taking some thermal management experiments conducted by Rogers as an example, it was found that the heat of microstrip transmission lines loaded with 80W RF power at 3.6 GHz increased by about 50°C. When the same circuit is tested with 80W power applied at the frequency of 6.1 GHz, the heat rise is about 80°C.

There are many reasons why the temperature increases with frequency. One reason is that the Df of the material will increase with the increase in frequency, which will lead to more dielectric loss and, ultimately, an increase in insertion loss and heat.
Another problem is that conductor losses increase with increasing frequency. The increase of conductor loss is almost due to the decrease of skin depth with the increase of frequency. In addition, as the frequency increases, the electric field will become denser and there will be a greater power density in a given area of the circuit, which will also increase the heat.

Finally, the TCDk of materials has been mentioned many times in this paper. It is an inherent property of materials whose Dk changes with temperature, and it is a material property that is often neglected. For power amplifier circuits, 1/4 wavelength lines are designed to match networks that are very sensitive to Dk fluctuations. When the Dk changes significantly, the 1/4 wavelength match will be offset, resulting in a change in the power amplifier's efficiency, which is highly undesirable.

In summary, when selecting high frequency materials for high power RF applications, the materials should have low Df, relatively smooth copper foil, high thermal conductivity, and low TCDk.
There are a number of trade-offs involved when considering the properties of these materials and the end-use requirements.
Therefore, when selecting materials for high-power RF applications, designers are always wise to contact their material suppliers.

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