應用結構函數於功率放大器元件之PCB分層最佳熱通孔設計

Abstract

本研究建構一暫態熱特性測試平台，透過電晶體的順向電壓與溫度呈線性關係的特性，估測功率放大器在關掉電源後的暫態溫度響應，以獲得結構函數，準確表徵元件封裝內不同分層之熱阻與熱容。此外，我們亦以數值模擬搭配結構函數，探討功率放大器的PCB基板中熱通孔的設計對整體散熱之影響。 實驗結果顯示，PCB基板含有13個熱通孔的功率放大器樣品其熱阻值與僅有4個熱通孔時相比大幅降低65.9%，由9.55 ℃ W-1降低至3.25 ℃ W-1。13個熱通孔的樣品所造成之最高結溫為41.5℃，與4個熱通孔的最高結溫48.5℃相比降低了14.4%，並可減少39.1%熱量由晶片結接面傳遞至PCB底部的熱傳時間。熱通孔的增加有效增加熱傳效率，減少了元件在高溫損壞的可能。 此外，我們發現PCB層的熱阻受到熱通孔填充物材料的熱傳導係數影響甚劇。使用熱傳導係數較大的焊料時，熱通孔數量以及尺寸的增加都有助於降低PCB基板的熱阻，在9個熱通孔、尺寸0.3 mm的組合下有最低的熱阻值3.63 ℃ W-1。若使用熱傳導係數較小的空氣或防焊漆進行填充時，則分別在9個熱通孔、尺寸0.2 mm及0.15 mm有最小的PCB分層熱阻14.07 ℃ W-1與14.05 ℃ W-1。填充物的熱傳導係數下降，則最佳熱通孔尺寸降低，因為熱通孔的尺寸太大，反而會受到填充物的熱傳導係數影響，熱通孔內填充物所占的面積增加，熱阻不降反升。 PCB基板熱容的變化主要可以填充物的比熱分為兩種情形，當熱通孔使用的填充材料比熱為小於FR4材料的焊料時，若熱通孔數量大於6個，因為熱通孔占整體PCB面積較大，則填充物的比熱主導PCB層之熱容，使得PCB層熱容相較6個熱通孔以下的樣品皆明顯降低。當熱通孔數量少於6個時，PCB分層的熱容反而受到熱阻的影響，填充物為焊料時熱通孔尺寸增加會降低PCB熱阻，在特徵時間改變不大的情況下，PCB整體熱容有較大值。我們也發現填充物為焊料時，PCB分層熱容的增加能夠使PCB基板中溫度的分布較為平均，較不會出現局部熱點。若熱通孔填充材料的比熱大於FR4材料，即填充物為空氣與防焊漆時，不論熱通孔的數量，PCB層的熱容僅受PCB層的熱阻主宰，熱通孔小於尺寸約0.2 mm的大小時，PCB層的熱阻隨尺寸增加而下降，則PCB分層的熱容會隨尺寸增加而上升。

This study uses the structure function to investigate the impacts of filler materials and design of thermal vias of the PCB substrate on the thermal networks of a power amplifier. A test platform is constructed to measure the transient temperature response, from which the structure function is derived. We find that the thermal conductivity of the filler material significantly affects the thermal resistance of PCB substrates. When thermal vias are filled with high thermal conductivity materials such as solder, increasing its number and size helps to reduce the thermal resistance of the PCB layer. However, when the thermal conductivity of the filler material is lower (air or solder mask), there exists an optimal via size. Thermal vias that are too large can lead to a higher thermal resistance of the PCB layer. On the other hand, the variation of the thermal capacitance of the PCB layer is more complicated. When the specific heat of the filler material is lower than that of FR4, the thermal capacitance of the PCB layer is controlled by its thermal resistance for a number of thermal vias less than 6. Since the characteristic time remains on the same order for the PCB layer, by reduction of the thermal resistance filling with solder, leads to the increase in the thermal capacitance for the PCB layer. Higher thermal capacitance can promote a more uniform temperature distribution within the substrate and reduce local hotspots. However, when the number of thermal vias exceeds 6, the thermal capacitance of the PCB layer is dominated by the specific heat of the filler material; the thermal capacitance decreases accordingly with the increase in the size of the thermal vias. For a filler material with higher specific heat, such as air or solder mask, the thermal capacitance of the PCB layer is solely determined by its thermal resistance. When the thermal vias are larger than about 0.2 mm, the lower thermal conductivities of air or solder mask increase the thermal resistance of the PCB and decrease the thermal capacitance of the PCB layer.