含稀土銲錫合金接點之界面反應、電遷移與錫鬚成長研究

Studies on the Interfacial Reactions, Electromigration and Whisker Growth in rare-earth doped solder joints

林修任(Hsiu-Jen Lin)

指導教授 ： 莊東漢

國立臺灣大學/工學院/材料科學與工程學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.01320

摘要

銲錫添加稀土元素已被證實可改善其潤濕性、機械強度、疲勞壽命及抗潛變性，但由於稀土元素的高活性，銲錫接點將在短時間內產生足以造成短路的鬚晶(whisker)，因此本研究的第一部份為含稀土銲錫鬚晶成長之研究，並嘗試尋找抑制錫鬚成長之方法；另外在高I/O腳數、低封裝面積的發展趨勢下，「高溫、高電流密度」的操作環境已無法被避免，故本研究的第二部份將針對數種含有稀土元素的Sn-Ag-Cu與Sn-Zn合金，搭配兩種不同的銲墊化層(Au/Ni/Cu、Ag/Cu)，研究添加稀土元素對Sn-Ag-Cu與Sn-Zn銲錫微結構、接點機械性質、接點電遷移行為、錫鬚成長速率、界面介金屬成份與成長速率等性質之影響。
研究結果顯示: 若Sn6.6RE合金之Sn-RE介金屬越細小，則錫鬚越難以生成，而由Sn9Zn0.5Ce與Sn9Zn0.5La合金之錫鬚研究可知:銲錫中若有Zn原子的存在，則可細化Sn-RE介金屬之尺寸，因此在Sn3Ag0.5Cu0.5Ce銲錫中添加Zn元素將可使Sn-RE介金屬細小化，進而達到抑制錫鬚成長之效果，但過高的Zn添加量將使銲錫難與基材潤濕，因此理想之Zn添加量為0.2 wt. %。
於Sn3Ag0.5Cu0.5Ce合金內添加0.2wt.%的Zn元素不僅可解決錫鬚異常生長之問題，亦可明顯提升材料之抗拉強度與接點強度，接點界面介金屬之成長也將因Zn元素的添加而被抑制；如同SnAgCu銲錫，於Sn9Zn銲錫內添加稀土元素亦可有效提升Sn9Zn合金之性能，但相較於Sn9Zn0.5La合金，Sn9Zn0.5Ce明顯具有較佳的抗拉強度、接點強度、延展性與硬度且無鬚晶成長的問題，因此於Sn9Zn銲錫內添加Ce元素應要比添加La元素來的恰當；不論是SnAgCu系統或是SnZn系統，銲錫內添加稀土元素均將加速接點電遷移反應的進行(接點溫度為80℃的情況下)，由於電遷移反應亦會嚴重影響銲錫接點之可靠度，因此含稀土銲錫接點電遷移行為，是需要持續被關注與研究的。

關鍵字

無鉛銲錫 ； 稀土元素 ； 界面反應 ； 電遷移 ； 錫鬚成長

並列摘要

Alloying with rare earth (RE) elements in Pb-free solders has been reported to have beneficial effects on their physical and mechanical properties. Since rare earth elements have a special characteristic of high chemical activity, they can much more easily react with the oxygen in the air and accelerate tin whisker growth in a rare earth element-containing solder alloy. It is known that such whiskers can cause short circuits in solder joints and lead to the failure of electronic devices in application. Addressing the tin whisker problem, first part of this thesis is the research of whisker growth in rare-earth doped solder joints and tries to find out the methods for the inhibition of whisker growth. Furthermore, second part of this dissertation verify the physical and mechanical performances of Sn-Zn and Sn-Ag-Cu solders added with rare earth elements, such as melting points, microstructure, tensile properties, hardness, interfacial reactions, bonding strength, and the behaviors of electromigration.
The smaller size of the Sn-RE clusters leads to the lower amount of tin atoms released and lower compressive stress accumulated after oxidation, which result in the slower whisker and hillock growth in Sn-6.6RE alloy. The prevention of the lengthening and coarsening of tin sprouts in Sn9Zn0.5Ce and Sn9Zn0.5La solder is also attributed to the small size of its rare-earth containing precipitation clusters. According to the studies of whisker growth in Sn6.6RE and Sn9Zn0.5RE alloy, the problem of whisker growth in Sn3Ag0.5Cu0.5Ce solder can be settled by the addition of Zn. However, the wettability of Sn-Ag-Cu solder might be degraded because of adding an excessive amount of Zn into the alloy; hence the addition of 0.2 wt. % Zn into Sn3Ag0.5Cu0.5Ce solder is appropriate.
Adding 0.2 wt. % Zn into Sn3Ag0.5Cu0.5Ce alloy can not only inhibit the growth of whisker but also improve the mechanical properties and bonding strength of solder joints. The addition of RE into Sn9Zn alloy also can improve the behaviors of Sn-Zn alloy, but adding Ce into Sn9Zn is more comportable than the addition of La into Sn9Zn solder. Besides, addition of RE into SnAgCu and SnZn alloy might deteriorate electromigration-induced failure of solder joints due to higher grain boundary diffusivity of RE-doped solder joints.

並列關鍵字

Pb-free solders ； rare earth elements ； interfacial reactions ； electromigration ； whisker growth

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