How to choose PCB materials for microwave amplifiers/millimeter wave bartender

5G represents the latest and greatest technologies in wireless technology. Design and manufacturing will face challenges. Of course, the circuit board materials are also facing challenges, because it wants to run at many different frequencies, such as 6 GHz and below, and millimeter wave frequencies ( Usually 30 GHz and above). It will also combine network access from the ground **** and rail satellite. However, by careful consideration of mechanical and electrical requirements, high -frequency circuit materials can be specified. No matter what the frequency, the design and development of 5G power amplifiers can be achieved.

       Ideally, a single circuit material is a proper starting point for all frequency power amplifiers. However, amplifiers of different frequencies have different design requirements and have the best support for circuit materials with different characteristics with different frequencies. For example, according to the type of circuit material, insert losses or losses are large or small. Each circuit material will suffer a certain amount of loss, and the loss usually increases as the frequency increases. The loss performance of the given circuit material may be acceptable within the microwave frequency used in the 5G network, but it is unacceptable within the range of the millimeter wave frequency, because the signal power will be reduced as the frequency increases. Providing low -loss circuit materials required for high PA gain and output power at microwave frequency may not be the best material choice of PA at the PA frequency.

For microwave frequencies, the design requirements of key circuit material parameters (dielectric constant DK) are very different. With the above millimeter wave frequencies, the design requirements are very different. To select the best circuit material for each band, you need to understand which DK value. It is best to support 2 different frequencies. Then find circuit materials with these DK values, and make it as much as possible with other circuit material attributes to create high -quality, high -performance, high -frequency power amplifiers.

        Regardless of the microwave frequency or millimeter wave frequency, the circuit material of the high -frequency PA must be able to support the matching of the circuit to match the impedance of the power transistor in those PAs. This impedance matching for active devices in low -power amplifiers, such as the drive amplifier, and even a low noise amplifier (LNA).

       The circuit materials suitable for this impedance matching network must be able to maintain changes in the circuit impedance to the lowest range. Usually it is achieved by strict control of the thickness of the substrate, that is, there is no change in the thickness of the substrate. Maintain the same impedance; strictly control the copper thickness of the circuit layer pressure plate; and strictly control the DK of the circuit material, especially the DK when the temperature changes, to achieve this goal. Although the use of strict control of DK circuit materials (such as 3.50 ± 0.05) can help maintain the impedance range of high -frequency transmission lines within a small range, which may be exactly what the impedance matching in the PA circuit is required, but the thickness of the substrate's thickness board is thick Changes may have a greater impact on maintaining the impedance of high -frequency transmission lines. The DK tolerance is ± 0.05 or lower circuit materials, which are considered to be strictly controlled DK values.

        As the frequency increases, the signal wavelength will continue to decrease, and the smaller and smaller circuit features are required. Many PA circuit structures for microwave and millimeter wave frequencies, such as Doherty amplifiers, depend on a quarter -long transmission line circuit structure. The size of these structures is a function of the thickness of the substrate. If there is no strict control of the thickness of the circuit substrate, it is easy to understand how the impedance of the polarized transmission line and the circuit structure changes with the thickness of the substrate. Generally, changes in the thickness of ± 10%or smaller substrates are a sign that strictly control the thickness of the circuit materials.

Feel heat

       Whether in the microwave frequency or in the millimeter -wave frequency band, whether the temperature change is caused by the active device from the operating environment or the PA itself, such as the power transistor or IC, the PA circuit is vulnerable to the performance change caused by temperature changes. Influence. When looking for circuit materials for microwave and millimeter -wave power amplifiers suitable for 5G applications, finding circuit materials that can perform effective thermal management are important for reducing the performance changes of the power amplifier Essence When evaluating the thermal properties of the material, the parameters of the two circuit materials are particularly useful -thermal conductivity and dielectric constant temperature coefficient (TCDK).

       The high thermal conductivity can effectively take the heat from any heating -like device (such as the power transistor of the PA) installed on the PCB. Continuous heat flow can not only eliminate the heat of threatening the reliability of the transistor, but also help to minimize the PA performance changes caused by the heat. People think that 0.5 W / MK or higher thermal guidance is beneficial to PCB materials.

        TCDK is a PCB circuit material parameter that is used to indicate how DK the material is affected by temperature changes. Ideally, the TCDK of the material is 0 ppm /℃, that is, DK will not change with the temperature changes. However, the actual circuit material DK value will change with changes in temperature. For circuit materials, we think that the TCDK of 50 ppm /℃ is very good, and DK is small with temperature changes. For the amplifier and other circuits in the 5G system, it depends on the fine circuit structure of a quarter of the wavelength. At this time, the circuit material with a low TCDK value will help reduce performance changes to the maximum.

        Compared with low -frequency microwave power amplifiers and circuits, the short wavelength and smaller circuit features required for millimeter wave power amplifier and circuit usually require thinner substrate materials, and need to maintain strict thickness tolerance, which is similar to the thick materials. Compared to these requirements are equally important. Thin -thin circuit materials may also have a thicker circuit material. The impact of other circuit materials (such as copper surface roughness) is more sensitive. Copper surface roughness can cause circuit effects such as transmission line loss and phase change. Therefore, in 5G microwave and millimeter wave power amplifier, for any circuit material specified in circuits with short wavelength and high frequency, the copper surface roughness should be as small as possible as possible as possible as possible as possible. Essence

       For example, Rogers provides various materials required for two different frequency range and other characteristics. For 5G power amplifiers with 6 GHz and below, the thickness of the Ceramic base RO4385 circuit layer of 20 dense ears and 30 dense ears is a low -cost circuit material, which can be consistent with the performance within a large temperature range. At 10GHz, the Z -axis DK is 3.48, and the tolerance is strictly controlled within ± 0.05. They are very suitable for competitive applications and can use standard epoxy resin/glass (FR-4) process.