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Relieving the current crowding effect in flip-chip solder joints during current stressing

Published online by Cambridge University Press:  01 January 2006

S.W. Liang
Affiliation:
National Chiao Tung University, Department of Material Science and Engineering, Hsin-chu 30050, Taiwan, Republic of China
T.L. Shao
Affiliation:
National Chiao Tung University, Department of Material Science and Engineering, Hsin-chu 30050, Taiwan, Republic of China
Chih Chen*
Affiliation:
National Chiao Tung University, Department of Material Science and Engineering, Hsin-chu 30050, Taiwan, Republic of China
Everett C.C. Yeh
Affiliation:
FrontAnD Technology, Hsinchu 30050, Taiwan, Republic of China
K.N. Tu
Affiliation:
Department of Materials Science and Engineering, University of California—Los Angeles, Los Angeles, California 90095
*
a)Address all correspondence to this author. e-mail: chih@cc.nctu.edu.tw
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Abstract

Three-dimensional simulations for relieving the current crowding effect in solder joints under current stressing were carried out using the finite element method. Three possible approaches were examined in this study, including varying the size of the passivation opening, increasing the thickness of Cu underbump metallization (UBM), and adopting or inserting a thin highly resistive UBM layer. It was found that the current crowding effect in the solder bump could be successfully relieved with the thick Cu UBM or with the highly resistive UBM. Compared to the solder joint with Al/Ni(V)/Cu UBM, for instance, the maximum current density in a solder bump decreased dramatically by a factor of fifteen, say from 1.11 × 105 A/cm2 to 7.54 × 103 A/cm2 when a 20-μm-thick Cu UBM was used. It could be lowered by a factor of seven, say to 1.55 × 104 A/cm2, when a 0.7-μm UBM of 14770 μΩ cm was adopted. It is worth noting that although a resistive UBM layer was used, the penalty on overall resistance increase was negligible because the total resistance was dominated by the Al trace instead of the solder bump. Thermal simulation showed that the average temperature increase due to Joule heating effect was only 2.8 °C when the solder joints with UBM of 14770 μΩ cm were applied by 0.2 A.

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Articles
Copyright
Copyright © Materials Research Society 2006

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References

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