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Uni-Directional Growth of CU SN in Microbumps
Uni-Directional Growth of CU SN in Microbumps
The growth of CuSn has been proven as a preferential growth behavior on single crystal copper. However, a layer of single crystal copper is not possible.
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Authored By:
Han-wen Lin, Jia-ling Lu, Chien-min Liu, Chih Chen, King-ning Tu, Delphic Chen, and Jui-Chao Kuo
Department of Materials Science & Engineering, National Chiao Tung University
Hsinchu, Taiwan, R.O.C.
Department of Materials Science and Engineering, University of California Los Angeles
Los Angeles, CA, USA
Department of Materials Science & Engineering, National Cheng Kung University
Tainan, Taiwan, R.O.C.

Summary
The growth of CuSn has been proven as a preferential growth behavior on single crystal copper. However, a layer of single crystal copper is not possible to be electroplated. It can not be utilized in the electronic industry.

In this paper, we electroplated an array of (111) uni-directional Cu pad followed by electroplating SnAg2.3. After being reflowed at 260 degrees C for 1 minute, the CuSn showed a referential growth to (0001) plane. As reflow time extended, the preferential growth behavior would change. It means the preferential growth of CuSn would change during reflow. Eventually, the preferred orientation of CuSn changed after 5 minutes of reflowing. It is also found that this preferential growth behavior of CuSn would be affected by the quality of (111) uni-directional Cu.


Conclusions
The uni-directional Cu with surface covered by (111) plane can be made by electroplating. The shape of Cu grain was columnar. The diameters of these columnar grains were 2 - 5 im. After electroplating SnAg2.3 on the Cu pad and then reflowed at 260 degrees C, the CuSn have shown a preferential growth relationship on the unidirectional Cu.

At the early stage of reflowing, the orientations of CuSn were preferred at (0001). As the time of reflow extended. Since the unidirectional Cu was still poly-crystal metal, the coherence must be achieved by Cu-Sn bonding at the interface between Cu pads and the intermetallics. Electroplating parameters would affect the quality of uni-directional Cu and therefore affecting the preferential behavior of CuSn. With the technique of electroplating (111) unidirectional Cu, it is possible to control the orientations of intermetallics in the solder joints.



Initially Published in the SMTA Proceedings

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