Microwave Tech - Discussion on the thermal effect of high-frequency PCB circuit

When the hf/microwave rf signal is fed into the high-frequency PCB circuit board, the loss caused by the circuit itself and the circuit material will inevitably generate a certain amount of heat. The greater the loss, the higher the power passing through the PCB material, and the greater the heat generated will be. When the operating temperature of the circuit exceeds the rated value, some problems may occur with the circuit. For example, a typical operating parameter known to PCBs is MOT or maximum operating temperature. When the operating temperature exceeds the MOT, the performance and reliability of the PCB circuit will be threatened. Through the combination of electromagnetic modeling and experimental measurement, understanding the thermal characteristics of RF microwave PCB can help to avoid circuit performance degradation and reliability degradation caused by high temperature.

1., Radiation loss

The radiation loss depends on the operating frequency, the thickness of the circuit substrate, the dielectric constant (relative dielectric constant or ε R) of the PCB, and many other circuit parameters such as the design scheme. In terms of design schemes, radiation losses often result from poor impedance conversion in the circuit or differences in electromagnetic wave transmission in the circuit. The circuit impedance transform area usually includes signal input area, step impedance point, stub line, and matching network. Reasonable circuit design can realize smooth impedance transformation and thus reduce the radiation loss of the circuit. Of course, it should be recognized that there is a possibility of impedance mismatch at any interface of the circuit leading to radiation losses. From the perspective of operating frequency, the higher the frequency, the greater the radiation loss of the circuit will be.

The parameters of circuit materials related to radiation loss are mainly dielectric constant and PCB material thickness. The thicker the circuit substrate, the greater the possibility of radiation loss; The lower the ε R of PCB material, the greater the radiation loss of the circuit. Use of a thin circuit substrate can be used as a way to offset the radiation losses caused by low ε R circuit materials in a combination of material characteristics. The influence of substrate thickness and ε R on the radiation loss of the circuit is because it is a frequency-dependent function. When the thickness of the circuit substrate is not more than 20mIL and the operating frequency is less than 20GHz, the radiation loss of the circuit is very low. Since most of the circuit modeling and measurement frequencies in this paper are below 20GHz, the effects of radiation loss on circuit heating will be ignored in this discussion.

After the radiation loss is ignored below 20GHz, the insertion loss of the microstrip transmission line circuit mainly includes two parts: dielectric loss and conductor loss, the proportion of which mainly depends on the thickness of the circuit substrate. For the thinner substrate, conductor loss is the main component. For many reasons, it is difficult to predict conductor loss accurately. For example, the surface roughness of a conductor has a huge effect on the propagation characteristics of electromagnetic waves. The surface roughness of copper foil will not only change the electromagnetic wave propagation constant of the microstrip line circuit but also increase the conductor loss. Due to the skin effect, the influence of copper foil roughness on conductor loss is also related to frequency.

2, Thermal model

In a microstrip line circuit, the top conductor layer acts as the signal plane, the bottom conductor layer acts as the ground plane, and the dielectric layer is filled between the two planes. Assume that the signal plane acts as a heat source and the heat is generated by the signal plane, the grounding plane has a heat sink and acts as a cold source, and the substrate acts as a heat conductor to transfer heat from the signal plane to the grounding plane. Although the actual heat generation process in microstrip circuits is complex, such assumptions are acceptable for simple thermal models. The circuit substrate is a very poor thermal conductor. For example, copper is a good thermal conductor, its thermal conductivity of 400W/m/K; However, the thermal conductivity of most commercial PCB substrates is far less than this value, only 0.2 to 0.3W /m/K. The heat flow equation explains why thin circuits (smaller L) can improve heat flow and achieve better heat dissipation at high power levels. At the same time, under high power conditions, compared with low thermal conductivity substrate, high thermal conductivity substrate can achieve higher heat flow and better heat dissipation.

The rf microwave power of PCB is limited by the MOT of the circuit and the working environment of the circuit. The power level is acceptable if the load power does not cause the circuit to heat more than the MOT of the circuit. Of course, the loaded power will cause the circuit to heat up and make the circuit temperature exceed the external ambient temperature. When the external temperature is +25°C, the heat generated by the loaded RF microwave power does not exceed the MOT. When the same power level is applied to the circuit at an external temperature of +50°C, the heat generated by the circuit may exceed the MOT and cause problems with the circuit. As analyzed above, the power of high-frequency PCB circuit board also depends on the external working environment to some extent.

3, Influence factor

To better understand the factors affecting the thermal performance of the PCB circuit, the 50-ohm microstrip transmission line circuit with the structure of Figure 1 and Figure 2 was used to carry out the research. Circuits with different thicknesses and different copper roughness were machined on the same type of PCB material. In addition, in addition to the tightly coupled grounded coplanar waveguide microstrip circuits machined on the low-loss PCB materials, circuits were machined on the high-loss PCB materials for evaluation. The input RF microwave power ranges from 5W to 85W, and all circuits have return losses greater than 18dB at 3.4GH with a 0.25 inch covered copper fin. The circuit is coated with COOLSPAN® electrothermal conductor film. This thermosetting adhesive material has a thermal conductivity of 6 W/m/K.

An infrared imager was used to record the circuit heating under certain power conditions. To ensure the accuracy of the measurement, the color of the circuit and its surface in the infrared imager's field of view should be consistent. Using black paint as the surface color enables the thermal imager to obtain accurate thermal images. The downside is that using black paint increases insertion losses on transmission lines. An increase in insertion loss will result in an increase in recorded heat, which can be considered the worst-case heat. In addition, the effect of insertion loss (temperature rise) on the ground coplanar waveguide is greater than that of the microstrip circuit because the ground - signal - the ground region of the coplanar waveguide is covered with black paint and the current density in this region is high.

4., Conclusion

From the perspective of heat control, different factors of insertion loss, a simple thermal model, and some main circuit material parameters are analyzed to understand the thermal effect of the PCB circuit under the condition of high power RF and microwave signal. In general, relatively thin circuit materials, high thermal conductivity, smooth copper foil surface, and low loss factor are conducive to reducing the heating effect of high-frequency PCB circuit board under the condition of high power RF and microwave signal.