在覆晶封裝及晶圓級封裝中，省略黏晶、打線等製程，改以內連接結構取代，故有較佳之電性、較低之電力消耗、較小之元件尺寸等優勢，但也因此在掉落測試或溫度循環等狀態下，整體材料受應力的影響相對嚴重，近期製程中或使用上也都開始發現介電薄膜有脫層或自身破裂的失效情形發生。為增加封裝體設計上的可靠度並了解介電薄膜本身的機械特性，本研究選擇以目前使用度最廣、具有低介電常數、高機械撓度以及高溫穩定性等優勢的聚醯亞胺作為試件，進行一系列的單調及循環負載實驗，由實驗結果得到該材料的基本機械性值與疲勞特性，藉此使封裝體在壽命評估上有更多的參考依據。
為了使試件更接近實際封裝體中的狀態，製備時，試件由實際製程中的晶圓上取得，厚度約5~6 um。單軸拉伸實驗分別在25 °C、75 °C、125 °C下進行，量測得最大伸長量約19 %、最大應力約100~170 MPa，25 °C下楊氏係數為4.19 GPa到了125 °C時則下降32.2 %，而疲勞實驗分為兩個部分，分別是位移控制疲勞實驗以及應力控制疲勞實驗，在位移控制底下以頻率0.5 Hz、應變量10 %等條件，進行了100000個循環後材料並未發生疲勞破壞，但從中發現材料具有明顯的黏彈特性；應力控制底下以負載速率5 MPa/s、最大應力130 MPa等條件，進行了5000個循環後，材料亦未發生疲勞破壞，但材料有產生明顯的塑性形變。為探討此現象本研究建立了一個疲勞模型，此模型可描述在應力循環負載下，隨循環數及最大應力值改變之黏彈性與塑性反應，且對實驗數據擬合有良好的一致性，同時若得知應力循環負載下可承受之最大應變量，此模型也可預測材料達疲勞破壞所需的負載次數。

Fatigue characteristics of polyimide thin film strips prepared with realistic wafer-level redistribution processes were investigated experimentally. Uniaxial tensile tests were first conducted on the thin film specimens to characterize the stress-strain relationship, and to measure ultimate strength and elongation. Both strain- and stress-controlled fatigue cycling experiments were then performed. Under strain-controlled cyclic fatigue loading an obvious stress relaxation behavior was observed. The stress relaxation characteristic depends only on the applied strain range, but not on the level of the average strain. Under stress-controlled cyclic fatigue loading the polyimide thin film exhibited both viscoelastic and plastic responses, and the peak plastic strain followed a power-law increasing trend as the fatigue cycle increased. A fatigue strain evolution model was developed by statistically fitting the stress-controlled fatigue responses with a physics-based mathematical model. The fatigue response model can be applied for developing design rules for extending the polyimide based redistribution interconnect technology to the regimes of larger chip size and higher input/output densities.

[1]D. Edwards, “Package interconnects can make or break performance,” Electronic Design, 2012.

[2]江國寧, “微電子系統封裝基礎理論與應用技術,” 滄海書局, 2006.

[3]J. A. Sauer, G. C. Richardson, “Fatigue of polymers,” International Journal of Fracture, Vol. 16, pp. 499-532, 1980.

[4]W. Liu, Z. Z. Gao, Z. F. Yue, “Steady ratcheting strains accumulation in varying temperature fatigue tests of PMMA,” Materials Science and Engineering A, Vol. 492, pp. 102-109, 2008.

[5]M. Shariati, H. Hatami, H. Yarahmadi, H. R. Eipakchi, “An experimental study on the ratcheting and fatigue behavior of polyacetal under uniaxial cyclic loading,” Materials and Design, Vol. 34, pp. 302-312, 2012.

[6]ASTM D638-03, Standard Test Method for Tensile Properties of Plastics, ASTM International, West Conshohocken, PA, 2003.

[7]Y. J. Kim, M. G. Allen, “In situ measurement of mechanical properties of polyimide films using micromachined resonant string structures,” IEEE Transactions on Components and Packaging Technology, Vol. 22, pp. 282-290, 1999.

[8]O. Kraft, R. Schwaiger, P. Wellner, “Fatigue in thin films: lifetime and damage formation,” Materials Science and Engineering, pp. 919-923, 2001.

[9]B. E. Alaca, J. C. Selby, M. T. A. Saif, H. Sehitoglu, “Biaxial testing of nanoscale films on compliant substrates: Fatigue and fracture,” Review of Scientific Instruments, Vol. 73, pp. 2963-2970, 2002.

[10]R. S. Li, J. Jiao, “The effects of temperature and aging on Young’s moduli of polymeric based flexible substrates,” The International Journal of Microcircuits and Electronic Packaging, Vol 23, pp. 456-461, 2000.

[11]J. A. Bannantine, J. J. Comer, J. L. Handrock, Fundamentals of Metal Fatigue Analysis, Prentice Hall, 1989.

[12]尚德廣, 王德俊, 多軸疲勞強度, 科學出版社, 2007.

[13]D. Kujawski, F. Ellyin, “A unified approach to mean stress effect on fatigue threshold conditions,” International Journal of Fatigue, Vol. 17, pp. 101-106, 1995.

[14]康国政, 阚前华, 工程材料的棘轮行為和棘轮－疲劳交互作用, 西南交通大學出版社, 2006.

[15]謝立禹, 無鉛銲錫接點於低週疲勞下潛變與棘齒行為之分析, 國立成功大學機械工程學系碩士論文, 2011.

[16]李郁婷, 高分子介電薄膜之黏彈性行為量測與本構模型, 國立成功大學機械工程學系碩士論文, 2013.

[17]Z. Xia, D. Kujawski, F. Ellyin, “Effect of mean stress and ratcheting strain on fatigue life of steel,” International Journal of Fatigue, Vol. 18, pp. 335-341, 1996.

[18]N. Barbosa III, Microtensile testing and cyclic deformation of freestanding Al thin films, Ph.D. Dissertation, Lehigh University, 2005.