絕緣柵雙極電晶體在JEDEC熱循環標準下之疲勞壽命與可靠度評估

Abstract

近年來由於新能源車以及風力發電等產業日益興盛，為了提供更高的功率密度及較大的電流，致使絕緣柵雙極電晶體(Insulated Gate Bipolar Transistor, IGBT)在這些領域成為不可或缺之功率半導體元件，也使IGBT模組可靠度成為炙手可熱的研究議題。IGBT模組在其生命週期中，模組內之焊接層(Solder)元件會遭受往復熱疲勞循環，導致模組失效，因此聯合電子裝置工程委員會(Joint Electron Device Engineering Council, JEDEC)固態技術協會(JEDEC Solid State Technology Association)特別制定熱循環測試標準，以確保IGBT模組的可靠度。本研究前半段以有限元素法模擬一IGBT模組焊接層，包括晶片焊接層(Chip Solder)及基板焊接層(Baseplate Solder)受到JEDEC所規範往復熱循環負載之力學行為，而後透過Coffin-Manson疲勞壽命預測模型來估算該IGBT模組之疲勞壽命，並且找出關鍵失效之焊接層。值得一提的是，以往經由有限元素模擬IGBT模組封裝體受熱循環所分析出之壽命往往為一定值，此情況無法反應實際應用或實驗測試一批同類封裝體所得壽命具有的離散特性，亦無法進一步評估其壽命分布、失效機率、失效率等可靠度指標。為此，本研究後半段利用蒙地卡羅法將基板焊接層幾何尺寸及機械性質不確定性納入有限元素模擬中，並透過疲勞壽命預測模型得到離散之疲勞壽命，接著經由統計檢定獲得相關之可靠度指標。本研究結果發現，所分析探討之IGBT模組的基板焊接層為該模組之關鍵失效元件；而在考量前述提及之不確定性後，該IGBT模組在JEDEC所規範之熱循環環境下，疲勞壽命介於70.98至90.79個熱循環次數；經由統計檢定，本研究發現以三參數韋伯分布描述所得壽命分布最合適，其最小壽命為67.76，尺度參數為13.19，形狀參數則為2.89，以上反映參數不確定性對IGBT模組疲勞壽命之離散性與可靠度確有一定程度的影響。

In recent years, new-energy vehicles and wind power industry have developed rapidly. To provide high power-density and current for those applications, the reliability of insulated gate bipolar transistor (IGBT) modules have also become an important issue. In fact, when IGBT modules are in use, their solder layers including chip solder and baseplate solder are frequently damaged by thermal fatigue and eventually result in failures of modules. The JEDEC Solid State Technology Association has therefore issued standard thermal-cycling test to guarantee the reliability of IGBT modules. This study uses finite element method to simulate the mechanics behavior of a certain type of IGBT module under JEDEC-specified thermal-cyclic load. Special attention is paid to solder layer of the IGBT module. After plastic strain range of the chip solder and baseplate solder are found, Coffin-Manson model is employed for predicting the thermal fatigue life of the IGBT module. It is worth mentioning that most studies of this kind are limited to finding a fixed value of life for a certain type of IGBT module, which may not reflect the fact that, when being tested or in real use, the fatigue lives of IGBT modules have certain degrees of discrepancy. No further reliability information such as life distribution, failure probability and failure rate can be obtained either. To overcome the shortcoming, this study incorporates uncertainties of geometric dimensions and material properties into the aforementioned FEM simulation by Monte-Carlo method, which results in fatigue life distribution for the IGBT module. Statistical tests are then carried out, and the reliability information such as mean time to failure (MTTF), failure probability and failure rate function is obtained. The results show that baseplate solder is the key failure component of the IGBT module and, after considering the uncertainties mentioned above, the fatigue life of the studied IGBT modules under JEDEC-specified thermal cyclic load are between 70.98 and 90.79 cycles. Through statistical tests, it is found that 3-parameter Weibull distribution is suitable to describe the life distribution. For the studied IGBT modules, its minimum life is 67.76, scale parameter is 13.19 and the shape parameter is 2.89. The above result reflects that parameter uncertainty has a certain degree of influence on the life dispersion and reliability of the IGBT module.