為了達成提升晶片組的效能和更進一步降低模組的體積，電子封裝技術目前正面臨一轉型的階段，由傳統的平面封裝技術邁向三維立體封裝技術。由於大量電晶體將同時做更高功率的運作，內部的工作溫度相較於傳統平面結構將更加提高，當我們試圖從三維模組表面散去這些電晶體所產生之廢熱時，巨大的溫度梯度可能因此而產生，由溫度梯度所引發之材料擴散，也就是熱遷移，將成為三維立體封裝技術中潛在的可靠度問題。而在三維立體封裝中所使用的銲錫接點尺寸為了因應體積縮減的目標，將會大幅下降至高度剩下數微米高，在如此小的尺度下，一個無鉛銲錫接點即可能僅由單一一個β-錫晶粒所構成。β-錫的晶體結構為體心四方結構(Body-centered tetragonal)，文獻指出銅在錫中之間隙型擴散展現出非等向性，延著錫晶粒的不同軸向的擴散係數亦有所不同，因此在僅存一個錫晶粒的情況中，錫晶體結構之非等向性在可靠度的評估中是一不容忽略的考量。
此論文試圖結合上述兩項在三維立體封裝中可能面臨的可靠度問題，施加一溫度梯度於銅-Sn3.5Ag銲錫-銅之結構的試片，探討錫的非等向性將如何影響銅之熱遷移行為。實驗結果和理論皆證實銅的熱遷移行為強烈受到錫非等向性的影響。當溫度梯度與錫晶粒的c軸平行時，熱端的介金屬化合物層劇烈的溶解並引發嚴重的銅基板消耗現象，銅原子遷移至冷端導致冷端界面的介金屬化合物異常堆積和成長；當溫度梯度與錫晶粒的c軸垂直時，冷熱端界面的介金屬化合物相對維持不變，顯示出熱遷移的效應在此種晶粒取向中大幅被減弱。除了特定晶粒取向對熱遷移通量的抑制之外，我們同時發現存在於熱端附近的Ag3Sn介金屬化合物亦有效地抑制了銅的熱遷移效應。

As the packaging technology transitions toward the three-dimensional integrated circuit (3D IC), a larger temperature gradient is expected to be established when the heat is dissipated from the surface of the stacking module. Furthermore, one solder bump consists of only few grains when the size of the solder bump shrinks into few micrometers. The crystal structure of β-Sn is known to be body-centered tetragonal (BCT) structure and it is anisotropic. Therefore, the effect of anisotropy of Sn is an important issue in the reliability of next-generation packaging technology.
We study the samples with a structure of Cu/Sn3.5Ag/Cu at a presence of temperature gradient. The experimental results and theoretical analysis indicated that thermomigration of Cu was strongly affected by anisotropic structure of Sn. When the c-axis of Sn grain was parallel to the direction of temperature gradient, a large thermomigration flux was induced and Cu atoms were driven to migrate from hot end to cold end. The migration resulted in a prominent asymmetrical feature in microstructure; serious dissolution of intermetallic compounds (IMC) and severe consumption of Cu substrate occurred at hot end whereas accumulation and growth of IMC were observed at cold end. On the other hand, when c-axis of Sn was perpendicular to temperature gradient, thermomigration was strongly mitigated due to a small induced flux. A near symmetrical feature in microstructure was found. Neither dissolution of hot-end IMC, nor abnormal growth of cold-end IMC was observed.
The morphology of hot-end interface was discussed in more details. Different from the serrated dissolution often resulted from electromigration, a relative flat interface remained after thermomigration. The Gibbs-Thomson effect and thermomigration were taken into account to explain the morphological change and IMC dissolution at hot end. Furthermore, the immobile Ag3Sn at the presence of the temperature gradient was found to suppress the thermomigration of Cu.

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