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  • 學位論文

在先進覆晶銲點尺度下觀察beta-Sn於電流應力中的微結構變化

Observation of Electromigration-Induced beta-Sn Microstructure Evolution in Real Solder Joints with Various Joint Sizes

指導教授 : 何政恩

摘要


固態錫在13 度C以上呈body-center tetragonal (BCT) (a = b = 5.83 Å; c = 3.18 Å)之晶體結構。由於Sn具有異向性(anisotropy),故使其在不同軸向具不同的性質。本研究透過一Cu/Sn(50 μm)/Cu擴散偶與傳統line-bump覆晶銲點,進一步探討Sn微結構隨時間之逐步變化情形。在Sn reorientation方面,當Cu/Sn/Cu擴散偶被導入一電流時,電流驅使之Sn reorientation係有階段性的差異。在通電初期,相鄰晶粒彼此會先合併成長而無優選方位(preferred orientation)。隨著時間增長,合併後之較大晶粒的c-axis才逐漸轉趨垂直電子流。本研究發現此一Sn reorientation現象常由晶界區域開始發生,再逐步擴及至整體晶粒。此一微結構變化類似grain boundary migration (GBM)現象,而非文獻所述之整顆“晶粒旋轉”機構。有趣的是,Cu6Sn5介金屬相的出現將箝制Sn的晶格變動。此外,研究發現,由於Sn在不同軸向具有相異的擴散系數。因此,Cu擴散路徑通過不同晶粒取向的組合時,會影響Cu6Sn5的生長位置。主要取決於陰極端晶粒與陽極端晶粒的擴散系數的大小關係。本研究結果顯示,當陰極端晶粒大於陽極端晶粒時,Cu會累積於晶界中。隨著通電時間增加,晶界中累積的Cu與Sn反應形成Cu6Sn5。Cu6Sn5生長會隨著厚度增加進而受到back stress的影響,逐漸轉趨於parabolic生長模式。最後,本研究藉由Cu/Sn/Cu擴散偶建立的beta-Sn晶粒變動理論基礎下,深入了解line-bump覆晶銲點的晶粒變化。研究結果發現,在電流密度不均的條件下,Sn晶粒的變化仍相似於Cu/Sn/Cu擴散偶中的Sn reorientation。有趣的是,決定Sn reorientation的關鍵因素,將取決於晶粒尺寸的大小。

並列摘要


White Tin (beta-Sn) has a body-center tetragonal (BCT) (a = b = 5.83 Å; c = 3.18 Å) structure. Due to the anisotropy, the axis has different properties. In this study, we observation of electromigration-induced beta-Sn microstructure evolution in Cu/Sn/Cu diffusion couple and line-bump flip chip solder joints, respectively. First, Sn grains reorientation has significantly different in Cu/Sn/Cu diffusion couple under the current stressing with time. In early stage, with an increase in grain size, the initially unsteady stage behavior of grain growth Voronoi microstructure approaches the steady stage behavior an equiaxed microstructure. With current stressing time, reorientation of Sn grains to realign themselves so that their c-axis is perpendicular to the current direction. It has been found the Sn grains reorientation starts promptly at the beginning on grain boundary, and expanded moved toward the grain. The Sn grains reorientation mechanism is different from literature, because these phenomenon show a resemblance development of the microstructure to those observed in grain boundary migration (GBM). Interestingly, Sn grains reorientation can be inhibited due to the pinning force resulting from the coherency Cu6Sn5 appeared in the grain boundaries. Second, due to Sn grains that exhibit highly anisotropic behaviors in diffusion properties. Thus, the diffusion of Cu through in different grains orientation combination, resulting in effect of a main perturbation in Cu6Sn5 position on the reaction. The results of this study revealed that the diffusivity cathode bigger then anode when the Cu accumulation in grain boundary and a large Cu6Sn5 reaction region are found. Additionally, the growth of the Cu6Sn5 follows parabolic growth rule, and we propose that the back stress induced in the Cu6Sn5 plays is a significant role. Finally, we observation of electromigration-induced beta-Sn microstructure evolution in line-bump flip chip solder joints. From the theory of Sn grains reorientation in Cu/Sn/Cu diffusion couple, the line-bump flip chip solder joint follow that. Interestingly, it is obvious that the Sn grains orientation is dominated by the grain size.

參考文獻


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被引用紀錄


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