Damages and microstructural variation of high-lead and eutectic SnPb composite flip chip solder bumps induced by electro
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The electromigration behavior of the high-lead and eutectic SnPb composite solder bumps was investigated at 150 °C with 5 × 103 A/cm2 current stressing for up to 1711 h. The diameter of the bumps was about 125 m. The underbump metallization (UBM) on the chip side was sputtered Al/Ni(V)/Cu thin films, and the Cu pad on the board side was plated with electroless Ni/Au. It was observed that damages occurred in the joints in a downward electron flow (from chip side to the substrate side), while those joints having the opposite current polarity showed only minor changes. In the case of downward electron flow, electromigration damages were observed in the UBM and solder bumps. The vanadium in Ni(V) layer was broken under current stressing of 1711 h while it was still intact after current stressing of 1000 h. The electron probe microanalyzer (EPMA) elemental mapping clearly shows that the Al atoms in the trace migrated through the UBM into the solder bump during current stressing. Voids were found in the solder bump near the UBM/solder interface. The Sn-rich phases of the solder bumps showed gradual streaking and reorientation upon current stressing. This resulted in the formation of uniaxial Sn-rich phases in the middle of the solder bump, while the columnar and fibrous Sn-rich phases were formed in the surrounding regions. The formation mechanism of electromigration-induced damage to the UBM structure and solder bump were discussed.
I. INTRODUCTION
As the solder bump size on flip chip package continues to decrease, the current density which passes through the contact area of a solder bump increases very rapidly. As indicated in the International Technology Roadmap for Semiconductors, the current density per bump for midrange to high-performance ICs is about 2500 A/cm2, i.e., 100 m bump with 0.2 A within 1–2 years.1 However, the dimension of the solder bump is expected to approach 50 m within 1–2 years, in which the current density may be as high as 104 A/cm2. At high current densities, the momentum transfer between electrons and metal atoms of the solder becomes important. It results in a net mass transport along the direction of the electron movement. The mass transfer results in the formation of voids on cathode side and hillocks formed on anode side.2 The voids decrease the cross-sectional area of the metal contact which increases the local current density and the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0271 2184
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 8, Aug 2005 Downloaded: 16 Mar 2015
local resistance. Under this circumstance, electromigration can lead to the electrical failure of bumps in relatively short time instead of failures like bump crack or delamination under thermal-mechanical stress. In recent years, the high-lead and eutectic SnPb composite solder bumps are widely used in flip chip packaging, microprocessor packaging, and flip-chip ball grid array packaging technologies.2–4 In contrast to the use of high-lead sol
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