軟性電子的表面處理與界面反應

Surface Finish and Interfacial Reaction in Flexible Electronics

楊青峰(Ching-feng Yang)

指導教授 ： 陳信文

國立清華大學/工學院/化學工程學系/博士(2010年)

摘要

電子產品不但持續朝向輕薄短小的目標邁進，更進一步要求在使用上符合人因工程，「軟性電子」由於能滿足這些需求而開始廣泛的研究發展。軟性基板具有輕薄、可撓性等優點，為軟性電子之重要核心。軟性電路板與傳統印刷電路板相同之處，在於其同樣會與晶片、IC載板或電子元件連接，其上將會有許多的接點。基板之表面處理與接點之界面反應對於接點性質與可靠度之評估非常的重要，因此本研究探討軟性電子之表面處理與接點之界面反應。
Au凸塊熱壓合與銲料迴銲是軟性電子常用之連結方式。Au凸塊熱壓接合是透過熱壓的方式，將Au凸塊與經過Sn表面處理之銅箔作連結，因此Au/Sn/Cu界面反應為重要議題。本研究使用電鍍Sn之表面處理以及反應偶的方式進行探討，藉著電鍍法製備不同Sn層厚度進行Au/Sn/Cu界面反應，以瞭解Sn耗盡後之反應行為，進而得到Au/Sn/Cu界面反應之完整演變流程，並且觀察到文獻較少探討的Au24.5Sn50.2Cu25.3三元相。
使用銲料作迴銲連接的部分，本研究選擇適用較低溫製程之低熔點Sn-Zn與Sn-In二元系統銲料進行探討。Sn-Zn銲料的部分，針對常見之商用合金Sn-8wt.%Zn-3wt.%Bi銲料作試驗，利用實驗方法獲得此三元系統於160oC下之等溫橫截面圖，並透過計算方法建立Sn-Bi-Zn三元系統之熱力學模型與參數。除此之外並研究Sn-8wt.%Zn-3wt.%Bi銲料對常用Cu、Ni及Ag片材基板以及電鍍表面處理之界面反應。反應結果顯示電鍍薄層之基板亦有類似片材基板的反應情形，會生成相同的反應相，但其成長速率有所差異。
Sn-In銲料部分，本研究結果顯示添加Zn可以有效降低Sn- 20wt.%In銲料之過冷的現象，減緩基材溶解於銲湯的速率。在Sn-In-(Zn)/Ag界面反應部分，隨著Zn添加量的增加，可觀察到介金屬生成相由Ag-In為主之化合物轉變成Ag-Zn為主之化合物。Sn-In-(Zn)/Ni界面反應的部分，則觀察到反應生成相由Ni-Sn為主之化合物轉變成Ni-Zn為主之化合物。此部分有良好之成果並已提出新型Sn-In-Zn無鉛銲料合金之專利申請，可以提供作為適合軟性基板連接之無鉛銲料選擇。

關鍵字

軟性電子 ； 界面反應 ； 表面處理 ； 相平衡 ； 金/錫/銅界面反應 ； 錫-鋅-鉍 ； 錫-銦-鋅 ； 過冷現象 ； 溶解度

並列摘要

Flexible electronics have recently attracted very intensive studies because they are thinner, lighter, and more flexible. Similar to other electronic products, flexible electronics products comprise numerous devices and modules. The interconnection technologies of these devices and modules to the flexible substrates are crucial for manufacturing. Two metallic interconnection technologies, Au-Sn bonding and soldering, are frequently used in flexible electronic packaging. Thus, interfacial reactions in the two interconnections with different surface finishes are investigated in this study.
Au bumps are formed on the chip side, and the Cu tracks on flexible substrates protected by the Sn surface finish are attached to the Au bumps. Consequently, a three-layer Au/Sn/Cu structure is frequently encountered in flexible electronic products. The reaction progression of the Au/Sn/Cu interfacial reactions was determined. Initially, the reaction path in the Au/Sn/Cu specimen is Au/AuSn/ AuSn2/AuSn4/Sn/Cu6Sn5/Cu3Sn/Cu. At longer reaction time, the (Cu,Au)6Sn5 phase was formed on the Au/Sn side interface as well. The Sn phase is completely consumed with even longer reaction time, the (Cu,Au)6Sn5 phases on the two sides would merge together and the reaction path then becomes Au/AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Cu3Sn/Cu. The AuSn4 and AuSn2 phases disappeared step by step. The reaction path is then Au/Au5Sn/AuSn/(Cu,Au)6Sn5/Cu3Sn/Cu. The reaction path would continuously evolve until the specimen reaches thermodynamic equilibrium, and the final phases can be predicted from the phase diagrams.
Low melting-point Pb-free solders, Sn-Zn and Sn-In based alloys, are selected to investigate in this study. Interfacial reactions between the Sn-8wt.%Zn-3wt.%Bi alloy and the Cu, Ag, and Ni substrates are examined. Two different kinds of substrates, bulk plate and electroplating layer, are used, and the reactions are carried out at 250 and 220oC. Although the Zn content is only 8wt.%, gamma-Cu5Zn8 and epsilon-CuZn5 phases are formed in the Sn-Zn-Bi/Cu couples. In the Sn-Zn-Bi/Ag couples, three Zn-Ag compounds are observed. The gamma-Ni5Zn21 phase is formed in the Sn-Zn-Bi/Ni couples. Similar results are found in the couples prepared with an electroplating layer: the reaction phases are the same, but the growth rates are different. Phase equilibria of Sn-Bi-Zn ternary system are determined in this study as well.
Addition of Zn in Sn-20wt.%In alloys is effective on reducing undercooling and dissolution rate of Ag and Ni substrates in molten solders. For Sn-In-(Zn)/Ag and Sn-In-(Zn)/Ni interfacial reactions, the Zn dominating reaction products are formed when the Zn concentration is higher. A new kind of Sn-In based Pb-free solders with Zn addition is proposed in this study.

並列關鍵字

Flexible electronics ； Interfacial reaction ； Surface finish ； Phase equilibria ； Au/Sn/Cu interfacial reaction ； Sn-Zn-Bi ； Sn-In-Zn ； Undercooling ； Dissolution rate

參考文獻

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