Thermomigration of Pb in eutectic SnPb flip chip solder joints

論文發表人：歐陽汎怡(加州大學洛杉磯校區材料科學與工程博士班)

http://www.tms.org/Meetings/Annual-07/AnnMtg07Home.html

隨著半導體工業持續地追求高密度的輸入/輸出點和消費電子產品持續的小型化，覆晶封裝中的銲錫球的電遷移和熱遷移效用獲得越來越多的重視。當電流密度高到 10⁴ A/cm²時，覆晶封裝中的銲錫球的熱遷移效用是不容忽略的。熱遷移是一種由於溫度梯度所造成的質量移動。此現象是個不可逆的過程。一個小小1度C的溫度梯度跨過一個10微米的焊錫球就會產生一個每1公分1000度C的溫度差，這個溫度差非常的大。所以我們不可以忽略熱遷移的效用。此外，當電遷移和熱遷移一起發生在覆晶封裝銲錫球時，由於大的孔洞形成在銲錫球的陰極和熱端，快速的破壞機制發生 。在這個研究中，我門調查覆晶封裝中的共晶錫鉛銲錫球的熱遷移效用，從我們的結果發現，隨著鉛是主要的控制元素和擴散到銲錫球的冷端，我們觀察到鉛和錫的重新分布。另一個有趣的發現是，相對於傳統的Soret效用，我們發現沒有鉛或錫線性濃度差存在。此外，銲錫球的微結構分佈變的更細小，這個現在指出自由能和介面能的增加。最後，鉛的莫耳傳播值被計算為每莫耳25千焦。

Thermomigration is defined as mass flow driven by a temperature gradient. It is an irreversible process. A temperature difference of 1 ℃ across a solder joint with 10 μm in height will produce a temperature gradient of 1000 ℃/cm, which cannot be ignored. When electromigration is combined with thermomigration in flip chip solder joints, fast failure due to large void formation at the cathode and the hot end occurs. In this talk, thermomigration in flip chip solder joints of eutectic SnPb will be presented. Redistribution of Sn and Pb has been observed, with Pb being the dominant diffusing species and moving to the cold end. No linear concentration gradient in either Pb or Sn is observed, contrary to the classical Soret effect. The lamellar structure becomes finer after thermomigration, indicating an increase in lamellar interfaces and in free energy. The molar heat of transport of Pb has been calculated to be - 25 kJ/mole.