当高频/微波射频信号馈入 PCB 电路，电路本身和电路材料造成的损耗，难免会产生一定的热量。 损耗越大，通过PCB材料的功率就越大，产生的热量也越大。 当电路的工作温度超过额定值时，电路可能会出现一些问题。 例如，PCB 中众所周知的典型工作参数 MOT 是最高工作温度。 当工作温度超过 MOT 时，PCB 电路的性能和可靠性将受到威胁。 通过电磁建模和实验测量相结合，了解射频微波PCB的热特性，有助于避免高温导致的电路性能下降和可靠性下降。

了解电路材料中的插入损耗是如何发生的，有助于更好地描述与高频 PCB 电路热性能相关的重要因素。 本文将以微带传输线电路为例，讨论与电路热性能相关的权衡。 在双面PCB结构的微带电路中，损耗包括介质损耗、导体损耗、辐射损耗和泄漏损耗。 不同损耗分量之间的差异很大。 除了少数例外，高频 PCB 电路的泄漏损耗通常非常低。 在本文中，由于泄漏损失值很低，暂时忽略。

辐射损失

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 electromagnetic waves in the circuit. The difference in transmission. 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.

与辐射损耗相关的电路材料参数主要是介电常数和PCB材料厚度。 电路基板越厚，造成辐射损耗的可能性就越大； PCB材料的εr越低，电路的辐射损耗越大。 综合权衡材料特性，采用薄电路基板可以作为抵消低εr电路材料造成的辐射损耗的一种方式。 电路基板厚度和 εr 对电路辐射损耗的影响是因为它是一个频率相关的函数。 当电路基板的厚度不超过20mil，工作频率低于20GHz时，电路的辐射损耗很低。 由于本文大部分电路建模和测量频率低于20GHz，本文讨论将忽略辐射损耗对电路发热的影响。

忽略20GHz以下的辐射损耗后，微带传输线电路的插入损耗主要包括介质损耗和导体损耗两部分。 两者的比例主要取决于电路基板的厚度。 对于较薄的基板，导体损耗是主要组成部分。 由于许多原因，通常很难准确预测导体损耗。 例如，导体的表面粗糙度对电磁波的传输特性有很大的影响。 铜箔的表面粗糙度不仅会改变微带电路的电磁波传播常数，还会增加电路的导体损耗。 由于趋肤效应，铜箔粗糙度对导体损耗的影响也与频率有关。 图 1 比较了基于不同 PCB 厚度的 50 ohm 微带传输线电路的插入损耗，分别为 6.6 mils 和 10 mils。

The simulation results are obtained 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 circuit material parameters used in the simulation model and 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.

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 due to the use of smooth copper foil at 20GHz; 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.

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.

为了解决电路发热问题，理想的薄电路应具有以下特性：电路材料的损耗因数低，铜薄表面光滑，低εr和高热导率。 与高εr的电路材料相比，低εr条件下得到的相同阻抗的导体宽度可以更大，有利于降低电路的导体损耗。 从电路散热的角度来看，虽然大多数高频PCB电路基板相对于导体的导热性非常差，但电路材料的导热性仍然是一个非常重要的参数。

关于电路基板的热导率的讨论在之前的文章中有很多阐述，本文将引用之前文章中的一些结果和信息。 例如，下面的公式和图 3 有助于理解与 PCB 电路材料的热性能相关的因素。 式中，k为热导率（W/m/K），A为面积，TH为热源温度，TC为冷源温度，L为热源与热源的距离冷源。