當高頻/微波射頻信號饋入 PCB 電路，電路本身和電路材料造成的損耗，難免會產生一定的熱量。 損耗越大，通過PCB材料的功率就越大，產生的熱量也越大。 當電路的工作溫度超過額定值時，電路可能會出現一些問題。 例如，PCB 中眾所周知的典型工作參數 MOT 是最高工作溫度。 當工作溫度超過 MOT 時，PCB 電路的性能和可靠性將受到威脅。 通過電磁建模和實驗測量相結合，了解射頻微波PCB的熱特性，有助於避免高溫導致的電路性能下降和可靠性下降。

了解電路材料中的插入損耗是如何發生的，有助於更好地描述與高頻 PCB 電路熱性能相關的重要因素。 本文將以微帶傳輸線電路為例，討論與電路熱性能相關的權衡。 在雙面PCB結構的微帶電路中，損耗包括介質損耗、導體損耗、輻射損耗和洩漏損耗。 不同損耗分量之間的差異很大。 除了少數例外，高頻 PCB 電路的洩漏損耗通常非常低。 在本文中，由於洩漏損失值很低，暫時忽略。

輻射損失

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.

與輻射損耗相關的電路材料參數主要是介電常數和PCB材料厚度。 電路基板越厚，造成輻射損耗的可能性就越大； PCB材料的εr越低，電路的輻射損耗越大。 綜合權衡材料特性，採用薄電路基板可以作為抵消低εr電路材料造成的輻射損耗的一種方式。 電路基板厚度和 εr 對電路輻射損耗的影響是因為它是一個頻率相關的函數。 當電路基板的厚度不超過20mil，工作頻率低於20GHz時，電路的輻射損耗很低。 由於本文大部分電路建模和測量頻率低於20GHz，本文討論將忽略輻射損耗對電路發熱的影響。

忽略20GHz以下的輻射損耗後，微帶傳輸線電路的插入損耗主要包括介質損耗和導體損耗兩部分。 兩者的比例主要取決於電路基板的厚度。 對於較薄的基板，導體損耗是主要組成部分。 由於許多原因，通常很難準確預測導體損耗。 例如，導體的表面粗糙度對電磁波的傳輸特性有很大的影響。 銅箔的表面粗糙度不僅會改變微帶電路的電磁波傳播常數，還會增加電路的導體損耗。 由於趨膚效應，銅箔粗糙度對導體損耗的影響也與頻率有關。 圖 1 比較了基於不同 PCB 厚度的 50 ohm 微帶傳輸線電路的插入損耗，分別為 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.

為解決電路發熱問題，理想的薄電路應具有以下特點：電路材料的損耗因數低，銅薄表面光滑，低εr和高熱導率。 與高εr的電路材料相比，低εr條件下得到的相同阻抗的導體寬度可以更大，有利於降低電路的導體損耗。 從電路散熱的角度來看，雖然大多數高頻PCB電路基板相對於導體的導熱性非常差，但電路材料的導熱性仍然是一個非常重要的參數。

關於電路基板的熱導率的討論在之前的文章中有很多闡述，本文將引用之前文章中的一些結果和信息。 例如，下面的公式和圖 3 有助於理解與 PCB 電路材料的熱性能相關的因素。 式中，k為熱導率（W/m/K），A為面積，TH為熱源溫度，TC為冷源溫度，L為熱源與熱源的距離冷源。