三維立體封裝結構中微型銲錫接點熱遷移之研究

Study of thermomigration on lead-free micro bump in three-dimensional integrated circuits packaging

朱偉誠(Wei-Cheng Jhu)

指導教授 ： 歐陽汎怡

國立清華大學/原子科學院/工程與系統科學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00112

摘要

為了因應消費者對於電子產品的需求，封裝技術已由覆晶封裝轉變為三維立體封裝。相對於覆晶封裝，三維立體封裝中所使用的銲錫高度已經縮減了一個數量級。而在此篇論文將會討論覆晶封裝與三維立體封裝中無鉛銲錫的熱遷移效應。在覆晶封裝中，銲錫高度為100 μm，我們可以發現Sn 原子擴散至熱端，導致熱端有銲錫的質量突出而冷端會有質量消耗甚至有孔洞生成，然而當銲錫高度縮減至5 μm 時，便無觀察到銲錫在熱端有明顯的微結構變化。造成在覆晶封裝與三維立體封裝中會有如此差異之情形，我們假設這有可能是因為銲錫高度縮減使得Sn 原子在三維立體封裝中會比在覆晶封裝中擁有更大的背向應力，導致Sn 原子所受的熱遷移驅動力會被背向應力所抵消掉，以至於在三維立體封裝微型銲錫接點中Sn 原子不會有熱遷移發生。而於本篇論文也會討論其臨界溫度梯度與銲錫高度之關係並且利用critical product 計算得到當工作溫度為150 oC時銲錫高度與驅動熱遷移的臨界溫度梯度曲線圖，當銲錫高度縮減時，驅動Sn 原子產生熱遷移所需的臨界溫度梯度會隨之增大；反之當銲錫高度較高時，Sn 原子產生熱遷移時所需的臨界溫度梯度也將會減少。除此之外，在熱遷移實驗後我們也發現三維立體封裝銲錫接點的熱端Ni UBM有嚴重的消耗以及冷端的Ni3Sn4介金屬化合物有劇烈的成長，這非對稱性的Ni UBM 消耗與Ni3Sn4 介金屬化合物成長是由於Ni 原子受溫度梯度影響導致熱遷移到冷端與Sn 反應所導致。同時，我們也藉由熱端Ni UBM 的消耗計算出Ni 原子在Sn 內的熱傳送值(heat of transport, Q*)為+ 0.578 kJ/mole。

關鍵字

三維立體封裝 ； 熱遷移 ； 無鉛銲錫

並列摘要

In order to deal with the requirements of consumer electronic products, the package technology is currently transition from flip chip technology to three dimensional integrated circuits (3D ICs). Compared to flip chip technology, the bump height of microbumps in 3D ICs is shrunk by an order of magnitude smaller than flip chip package. In this thesis, we studied the microstructural evolution of Pb-free solders in 3D ICs packaged under the
influence of a temperature gradient. The behaviors of thermomigration of Pb-free solder between flip chip and 3D ICs packaged were compared and discussed. When the bump height is about 100 μm in the flip chip samples, Sn atoms moved to the hot end under the temperature gradient, and it induced the Sn mass protrusion at the hot end and Sn mass
depletion at cold end. Additionally, we found voids form at the cold end. However, when the bump height is about 5 μm in the 3D IC samples, no significant microstructural evolution of Sn was found; instead, the serious dissolution of Ni under-bump metallization (UBM) at hot end was observed. We suggest that this discrepancy between flip chip solder joints and 3D IC microbumps is mainly attributed to the effect of stronger back stress in 3D IC microbumps and the presence of thicker Ni UBM in the 3D IC package. In the flip chip solder joints, the thermomigration driving force of Sn is larger than back stress, resulting in the Sn mass protrusion on the hot end and Sn mass depletion on the cold end. On the other hand, for 3D IC samples, the thermomigration driving force is counteracted by back stress. Moreover, the critical temperature gradient in terms of different bump heights is also discussed, showing below which there will be no net effect of thermomigration of Sn. It is noted that the critical temperature gradient to trigger thermomigration of the Sn atom decreases with increasing the solder bump heights. Furthermore, we also found the asymmetrical growth of Ni3Sn4 intermetallic compounds at both interfaces along with serious consumption of Ni UBM on hot end. We proposed this phenomenon mainly attributed to the thermal gradient driven Ni moving toward the cold end. The molar heat of transport of Ni in Sn was calculated to be + 0.578 kJ/mole.

並列關鍵字

3D IC ； thermomigration ； Pb-free solder bump

參考文獻

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