雙邊式功率模組銅凸塊電訊接點之可靠度分析與評估

Reliability Analysis of Cu Bump Interconnection used in Double-sided Power Module

蔡佳圻(Tsai, Chia Chi)

指導教授 ： 江國寧

國立清華大學/工學院/動力機械工程學系/碩士(2015年)

摘要

由於科技的發展與使用者的需求，作為提升能源轉換效率之功率模組被廣泛的運用，而高功率與高整合性成為現今功率模組發展的趨勢。傳統功率模組使用打線進行電訊導通，並以強制水冷系統作為主要散熱途徑。但隨著使用功率的提升，傳統功率模組只依靠單邊進行散熱，將無法適用高功率下產生之熱能，因此必須發展出能夠解決高接面溫度與高承載電流的功率模組。雙邊式功率模組因可以有效地減少散熱熱阻以及提供較多的散熱途徑，作為實現高功率模組的結構而被廣泛研究。現有之雙邊式功率模組皆以整面式焊料進行電訊接合，但參考實驗室參與之計畫的研究成果，採用凸塊電訊接點的結構可以有效控制電流密度以及設計電訊接點之DNP位置，本研究將針對使用凸塊電訊接點的雙邊式功率模組之可靠度進行探討。
功率模組在承受往復的功率負載時，造成IGBT晶片接面溫度大幅變化，材料組成間的熱膨脹係數不匹配，將導致電訊接點發生熱應力與應變，熱應力與應變將是接點發生失效之主因。本研究將利用有限元素分析軟體ANSYS，針對上述結構進行電熱耦合與熱固分析，以模擬模組在功率循環下電訊接點之機械行為。接著利用該模擬方法分析此結構於不同散熱條件下進行數個功率循環之後電訊接點的可靠度，針對易破壞之區域進行探討，並根據電子封裝的概念提出改進之方法。最後亦針對用於凸塊接點接合之IMC結構進行參數化分析，探討設計參數對於可靠度之影響。由模擬結果顯示，雙邊式功率模組採用雙邊水冷之方式可以得到較低之接面溫度與凸塊可靠度，但是在IMC之可靠度卻有較差之表現，此問題可以藉由放大角落接點尺寸增加其強度之方式得到有效地改善，另外，IMC之結構參數，若能在可以保持接點有完整接合之情況下，盡量減少IMC之厚度可以有效增加凸塊的可靠度。
本研究探討之結構將使用Cu/Sn IMC進行電訊接點的接合，但是關於該材料之破壞強度未能有一可靠之參考數據，而且於功率模組運作時，其接面溫度會達到200˚C之高溫，常溫下的破壞強度將不適用於此處之可靠度分析。故本研究最後提出建議之探討Cu/Sn IMC破壞強度的實驗設計，藉由設計符合實驗需求之試片，並以熱處理與剪力測試來掌握IMC之成長以及其破壞強度，最後再搭配加熱模組進行剪力測試，藉此找出IMC於高溫下之破壞強度。

關鍵字

雙邊式功率模組 ； 功率循環 ； 凸塊接點 ； 接點可靠度 ； 介金屬化合物

並列摘要

Due to development of technology and the users demand, power modules are widely used with the characteristic that it can promote energy transfer efficiency. Nowadays, high power and high integrated are the modern trends in development of power modules. Conventional power modules utilize bonding wire for interconnection and dissipate heat mainly based on forced cooling system. However, with the rising of working power, it is necessary to create power modules which can solve high junction temperature and effort high working current. Double-sided power modules are one of the structures which can realize high power module application with reducing junction temperature efficiently and providing more heat dissipation path. According to the research results of Tsing Hwau-Delta project, using bump interconnection can control the current density and design the DNP position of bumps freely, so that it is more flexible in the view of design than face-to-face solder interconnection which is utilized in existing double-sided power module. This research is focus on double-sided power module with bump interconnection and investigating reliability of the bump.
When the power modules are subjected to the cyclic power load, the junction temperature varies significantly; the thermal stress and strain resulting from the mismatch between the coefficients of thermal expansion of materials causes the bumps to failure. This research will construct finite element models to conduct electro-thermal and thermal-mechanical analysis in aforementioned structure by commercial software ANSYS, and to simulate the mechanical behaviors of bump interconnection under power cycling. By using this simulation method, an analysis of the reliability of bump interconnection when it is in different cooling conditions and subjected to several power cycles is achieved. The area there the bumps will easily failure is discussed, and proposing methods to enhance its reliability with concepts used in advanced packaging. Finally, parameter analysis of IMC structure in bump interconnection is also executed to understand the effect of design parameters of IMC. From results of simulation, double-sided power module with double-sided cooling can get lower junction temperature and better reliability of copper bump. However, the structure in this condition possesses worse IMC reliability and this disadvantage can be improved by enlarging the corner bumps which have the largest DNP. In addition, under the premise that the bumps can keep complete joint, the thickness of IMC should be reduced as possible as it can, and the reliability of IMC will be increased.
The bump interconnection used in this research is joined by Cu/Sn IMC, and it is needed that to get a credible value of the failure strength of the IMC material. Moreover, when the power module works, the junction temperature will reach 200 °C. Hence, the failure strength of IMC measured at room temperature is not suitable for the reliability analysis in this condition. This research proposes a recommended experiment design to investigate the failure strength of Cu/Sn IMC. Using thermal aging treatment to control the growth of IMC and utilizing shear test with heating module to measure its failure strength at high temperature.

並列關鍵字

double-sided power module ； power cycle ； bump interconnection ； reliability of interconnection ； intermetallic compounds

參考文獻

[45] J. H. Hwang, Y. M. Kim, T. J. Kim, Y. H. Kim, and W. J. Lee, “Intermetallic compound formation in the interface between SAC305 solder and Cu-xZn-yNi substrates, ” International Conference on Electronic Packaging Technology & High Density Packaging, Aug 13-16, 2012, Guilin, China.

[34] W. H. Chen, H. C. Cheng, H. A. Shen, “An Effective Methodology for Thermal Characterization of Electronic Packaging, ” IEEE Transactions on Components and Packaging Technologies, Vol.26, pp.222-232, March 2003.

[18] B. H. Kwak, J. M. Kim, M. H. Jeong, K. W. Lee, J. D. Kim, and Y. B. Park, “Interfacial Microstructure and Mechanical Reliability of Cu Pillar/Sn-Ag Bump for 3D packages, ” Electronics Packaging Technology Conference, pp.49-52, Dec 7-9, 2011, Singapore.

[3] C. M.Johnson, C. Buttay, S. J. Rashid, F. Udrea, G. A. J. Amaratunga, P. Ireland and R. K. Malhan, “Compact Double-Side Liquid-Impingement-Cooled Integrated Power Electronic Module, ”Power Semiconductor Devices and IC's, pp.53-56, 27-31 May 2007.

[1] T. Y. Hung, S. Y. Chiang, C. J. Huang, C. C. Lee and K. N. Chiang, “Thermal–mechanical behavior of the bonding wire for a power module subjected to the power cycling test, ” Journal of Microelectronics Reliability, Vol. 51, pp.1819-1823, 2011.