Relieving the current crowding effect in flip-chip solder joints during current stressing
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Everett C.C. Yeh FrontAnD Technology, Hsinchu 30050, Taiwan, Republic of China
K.N. Tu Department of Materials Science and Engineering, University of California—Los Angeles, Los Angeles, California 90095 (Received 31 January 2005; accepted 19 September 2005)
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-m-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-m UBM of 14770 ⍀ 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 ⍀ cm were applied by 0.2 A.
I. INTRODUCTION
The flip-chip solder joint has become the most important technology of high-density packaging in the microelectronics industry.1 Thousands of solder bumps can be fabricated into one chip. To meet performance requirements, the input/output (I/O) numbers keep increasing, and the size of the joints progressively shrinks. Their diameter is about 100 m or less.2 The design rule of packaging requires that each bump is to carry 0.2–0.4 A, resulting in a current density of approximately 2 × 103 to 2 × 104 A/cm2. Therefore, electromigration has become an important reliability issue for flip-chip solder joints.3–5 In this work, current density distribution in a solder joint was thoroughly studied by a three-dimensional finite element simulation. It was found that the maximum
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0004 J. Mater. Res., Vol. 21, No. 1, Jan 2006
http://journals.cambridge.org
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current density in a solder bump can be much higher than the average one that was previously projected. It locates itself near the solder/underbump metallization (UBM) interface, which serves as a vacancy flux divergence plane and favors electromigration occurring at that location. Consequently, the solder joint is more prone to electromigration. The cause of such locally high current density is a result of the current crowding effect. Current crowding occurring in the solder joints is due to the current flow experienci
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