When the high frequency/microwave radio frequency signal is fed into the பிசிபி circuit, 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. When the operating temperature of the circuit exceeds the rated value, the circuit may cause some problems. For example, the typical operating parameter MOT, which is well known in PCBs, is the 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 measurements, understanding the thermal characteristics of RF microwave PCBs can help avoid circuit performance degradation and reliability degradation caused by high temperatures.

Understanding how insertion loss occurs in circuit materials helps to better describe the important factors related to the thermal performance of high-frequency PCB circuits. This article will take the microstrip transmission line circuit as an example to discuss the trade-offs related to the thermal performance of the circuit. In a microstrip circuit with a double-sided PCB structure, losses include dielectric loss, conductor loss, radiation loss, and leakage loss. The difference between the different loss components is large. With a few exceptions, the leakage loss of high-frequency PCB circuits is generally very low. In this article, since the leakage loss value is very low, it will be ignored for the time being.

Radiation loss

Radiation loss depends on many circuit parameters such as operating frequency, circuit substrate thickness, PCB dielectric constant (relative dielectric constant or εr) and design plan. As far as design schemes are concerned, radiation loss often stems from poor impedance transformation in the circuit or differences in electromagnetic wave transmission in the circuit. Circuit impedance transformation area usually includes signal feed-in area, step impedance point, stub and matching network. Reasonable circuit design can realize smooth impedance transformation, thereby reducing the radiation loss of the circuit. Of course, it should be realized that there is the possibility of impedance mismatch leading to radiation loss at any interface of the circuit. From the point of view of operating frequency, usually the higher the frequency, the greater the radiation loss of the circuit.

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 causing radiation loss; the lower the εr of the PCB material, the greater the radiation loss of the circuit. Comprehensively weighing material characteristics, the use of thin circuit substrates can be used as a way to offset the radiation loss caused by low εr circuit materials. The influence of circuit substrate thickness and εr on circuit radiation loss is because it is a frequency-dependent function. When the thickness of the circuit substrate does not exceed 20mil and the operating frequency is lower than 20GHz, the radiation loss of the circuit is very low. Since most of the circuit modeling and measurement frequencies in this article are lower than 20GHz, the discussion in this article will ignore the influence of radiation loss on circuit heating.

After ignoring the radiation loss below 20GHz, the insertion loss of a microstrip transmission line circuit mainly includes two parts: dielectric loss and conductor loss. The proportion of the two mainly depends on the thickness of the circuit substrate. For thinner substrates, conductor loss is the main component. For many reasons, it is generally difficult to accurately predict conductor loss. For example, the surface roughness of a conductor has a huge influence on the transmission characteristics of electromagnetic waves. The surface roughness of copper foil will not only change the electromagnetic wave propagation constant of the microstrip circuit, but also increase the conductor loss of the circuit. Due to the skin effect, the influence of copper foil roughness on conductor loss is also frequency-dependent. Figure 1 compares the insertion loss of 50 ohm microstrip transmission line circuits based on different PCB thicknesses, which are 6.6 mils and 10 mils, respectively.

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Figure 1. Comparison of 50 ohm microstrip transmission line circuits based on PCB materials of different thicknesses

Measured and simulated results

The curve in Figure 1 contains the measured results and simulation results. The simulation results are obtained by using Rogers Corporation’s MWI-2010 microwave impedance calculation software. The MWI-2010 software quotes the analytical equations in the classic papers in the field of microstrip line modeling. The test data in Figure 1 is obtained by the differential length measurement method of a vector network analyzer. It can be seen from Fig. 1 that the simulation results of the total loss curve are basically consistent with the measured results. It can be seen from the figure that the conductor loss of the thinner circuit (the curve on the left corresponds to a thickness of 6.6 mil) is the main component of the total insertion loss. As the circuit thickness increases (the thickness corresponding to the curve on the right is 10mil), the dielectric loss and the conductor loss tend to approach, and the two together constitute the total insertion loss.

The simulation model in Figure 1 and the circuit material parameters used in the actual circuit are: dielectric constant 3.66, loss factor 0.0037, and copper conductor surface roughness 2.8 um RMS. When the surface roughness of the copper foil under the same circuit material is reduced, the conductor loss of the 6.6 mil and 10 mil circuits in Figure 1 will be significantly reduced; however, the effect is not obvious for the 20 mil circuit. Figure 2 shows the test results of two circuit materials with different roughness, namely Rogers RO4350B™ standard circuit material with high roughness and Rogers RO4350B LoPro™ circuit material with low roughness.

Figure 2 shows the advantages of using a smooth copper foil surface substrate to process microstrip circuits. For thinner substrates, the use of smooth copper foil can significantly reduce the insertion loss. For the 6.6mil substrate, the insertion loss is reduced by 0.3 dB at 20GHz due to the use of smooth copper foil; the 10mil substrate is reduced by 0.22 dB at 20GHz; and the 20mil substrate, the insertion loss is only reduced by 0.11 dB.

As shown in Figure 1 and Figure 2, the thinner the circuit substrate, the higher the insertion loss of the circuit. This means that when the circuit is fed with a certain amount of RF microwave power, the thinner the circuit will generate more heat. When comprehensively weighing the issue of circuit heating, on the one hand, a thinner circuit generates more heat than a thick circuit at high power levels, but on the other hand, a thinner circuit can obtain more effective heat flow through the heat sink. Keep the temperature relatively low.

In order to solve the heating problem of the circuit, the ideal thin circuit should have the following characteristics: low loss factor of the circuit material, smooth copper thin surface, low εr and high thermal conductivity. Compared with the circuit material of high εr, the conductor width of the same impedance obtained under the condition of low εr can be larger, which is beneficial to reduce the conductor loss of the circuit. From the perspective of circuit heat dissipation, although most high-frequency PCB circuit substrates have very poor thermal conductivity relative to conductors, the thermal conductivity of circuit materials is still a very important parameter.

A lot of discussions about the thermal conductivity of circuit substrates have been elaborated in earlier articles, and this article will quote some results and information from earlier articles. For example, the following equation and Figure 3 are helpful to understand the factors related to the thermal performance of PCB circuit materials. In the equation, k is the thermal conductivity (W/m/K), A is the area, TH is the temperature of the heat source, TC is the temperature of the cold source, and L is the distance between the heat source and the cold source.