電流應力下熱電模組銲點界面反應之研究

Effect of electric current stressing on interfacial reactions at soldered junctions of thermoelectric modules

陳文泰(Wen-Tai Chen)

指導教授 ： 廖建能

國立清華大學/工學院/材料科學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00326

摘要

隨著綠色能源的發展，強調潔淨、安全、高效能的熱電材料因應而生，電源產生器以及熱電致冷器為其應用產品。將成對的P型與N型熱電材料以銲料與金屬導線Cu進行連接，並在元件上下兩端外加陶瓷基板以保護元件結構，當中銲料選用Sn-3Ag-05Cu，組裝成熱電致冷器。
本研究探討在熱電材料與銲料間之反應，因元件在運作過程中，由於Peltier effect會在元件兩端形成不對稱的溫度分佈，即冷端與熱端，且當中又有高電流的通過，熱電材料與金屬接點的完整性就顯得非常重要，它將影響元件的效能及壽命。當元件在通入700A/cm2的電流密度下，發現P-type熱電材料與銲料交接熱端處，大量生成SnTe化合物，且因銲料的反應而在接點處產生大尺寸的孔洞(voids)，進而使元件毀壞。
在熱電材料與金屬接點界面上，Ni為其擴散阻障層，可防止Te快速與Sn反應，生成SnTe化合物。在通電測試後可發現在冷端處的Ni保持相當完整，而在熱端處，因電流方向在N-type、P-type熱電材料處不同，在N-type端電子流方向由Ni擴散阻障層往銲錫流，P-type端電子流方向由銲錫往Ni擴散阻障層流，造成P-type熱端處的Sn易穿透Ni層，而與P-type (Bi,Sb)2Te3生成SnTe化合物。
通電後之試片，在SnTe化合物上會有SbSn化合物凸起物生成，因Sn/Te反應偶中是由SnTe及Sn相混和，而Sb、Bi元素亦會溶進Sn相形成固溶體，電流使SbSn化合物析出在SnTe化合物上。由文獻中可知Te跟Sb元素由 “hole wind”朝向銲料接點，而Sn元素由 “electron wind”驅動力。實驗結果顯示：SnTe化合物的成長，主要來自於電流誘導的原子擴散，而不是濃度梯度所造成，在200°C估算的DZ*為10-8–10-9 cm2/s，和文獻上201°C電流下，Sn擴散進SnTe-Te化合物層的數值1.5×10-8 cm2/s相近。
另一反應偶實驗，在Sn/P-type/Sn試片上，發現通電流清況下介金屬化合物(IMC)的厚度比經熱處理的試片厚，主要是因為Sn與Te的反應非常快速，且通電方向更促進它們之間的反應。由界面標記(Marker)實驗，我們可以得知Sn在SnTe內，不管通電流或是熱處理，都是主要擴散元素，在Sn與P-type界面處，有薄薄的一層Sb2Sn3化合物生成，在之後才是SnTe化合物，而當通電流後，陰極處會有非常多的SbSn化合物生成，熱處理也會生成，只是數量少得多，而陽極處幾乎沒有Sb2Sn3化合物生成。其原因主要為Te跟Sb元素係由 “hole wind”朝向銲料接點，而Sn元素則由 “electron wind”驅動，造成不同的IMC厚度所致。
熱處理所生成的SnTe化合物，其活化能為138 KJ/mole，在熱處理140oC情況下的flux為 9*1014 (atoms/cm2s)，而通電流情況下的flux則為9.8*1014 (atoms/cm2s)，依此所計算得之DZ*值為4.27*10-8 cm2/s，比文獻上142°C電流下，Sn擴散進SnTe-Te化合物的數值3.5×10-9 cm2/s大一點。

關鍵字

熱電 ； 銲料反應 ； 介金屬化合物

並列摘要

As the quest for green energy continues, thermoelectric materials that are clean, safe, and highly-efficient become available for, as are the subsequent thermoelectric generators and coolers on the market. N- and P-type pellets are fixed by soldering to copper conductor and sandwiched between two ceramic plates for structural protection. Sn-3Ag-05Cu is often the solder used for commercial thermoelectric coolers.
This research discusses in depth the reaction between solder and thermoelectric material. Since thermoelectric devices are subjected to high electric current, the presence of uneven temperature or in other words the hot and cold regions caused by Peltier effect will make the integrity of thermoelectrics/metal junctions extremely important as the lifetime and performance of the device rely heavily on it. When the device is subjected to a current density of 700A/cm2, the creation of SnTe and the sequential formation of extensive voids at the interface of solder and P-type thermoelectric elements could lead to device failure.
At such interface, nickel diffusion barrier is deposited to prevent the formation of SnTe by reaction between Sn and Te. The structural integrity of this nickel barrier remains intact at the cold regions after electrical current stressing. However, at the hot regions, whether it is at N-type junction where the electron current passes from nickel barrier to solder, or it is at P-type junction where the electron current passes from the solder to nickel barrier, the nickel barrier is depleted as Sn reacts with (Bi,Sb)2Te3 to form SnTe intermetallic compounds (IMCs).
After the reaction couples were subjected to electric sintering, extrusion of SbSn was found on SnTe. Since the reaction layer was a mixture of SnTe and Sn, Sb an Bi elements were dissolved in Sn by forming a solid solution. Other researchers have shown that Te and Sb elements are susceptible to the influence of a hole wind effect and Sn element is influenced by the electron wind force. Experimental results have shown that the formation of SnTe was not caused by concentration gradient, but rather by EM-induced atomic diffusion. The calculated DZ* at 200 oC was 10-8–10-9 cm2/s, which was very closed to the value 1.5×10-8 cm2/s found for the reaction between Sn and Te under electrical stressing.
Electrical stressing of Sn/P-type/Sn reaction couple resulted in much thicker IMC layer compared to the thickness obtained with thermal annealing. The rapid reaction between Sn and Te was facilitated by electrical current. From the mark-line experiments, it was found that Sn was the dominant diffusing species in SnTe for both electrical stressing and thermal annealing. At the interface of Sn and P-type thermoelectric element, a thin SbSn layer was found, followed by SnTe IMCs. After electrical stressing, large quantity of Sb2Sn3 IMCs was found at cathode; such quantity was drastically reduced with thermal annealing. On the contrary, Sb2Sn3 was not observed at anode for both electrical and thermal treatments. The difference in IMC thickness was that Te and Sb were pushed toward solder by the hole wind effect, which coincided with Sn element being pushed by the electron wind effect.
The calculated activation energy for the thermal formation of SnTe was 138 KJ/mole and the flux was 9*1014 (atoms/cm2s) at 140 oC. The flux under electrical stressing was 9.8*1014 (atoms/cm2s), and the resulting calculated DZ* obtained was 4.27*10-8 cm2/s. This value was slightly higher than the literature reported value of 3.5×10-9 cm2/s, which was obtained for the diffusion of Sn into SnTe-Te compound under electrical stressing at 142°C.

並列關鍵字

Thermoelectrics ； Soldering ； Intermetallic compounds

參考文獻

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