Electromigration in flip chip solder joints under extra high current density

Electromigration in flip chip solder joints under extra high current density

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Electromigration in flip chip solder joints under extra high current density Y. W. Lin, 1 J. H. Ke, 1 H. Y. Chuang, 1 Y. S. Lai, 2 and C. R. Kao 1, a 1 Department of Materials Science and Engineering, National Taiwan University, Taipei, 10617 Taiwan 2 Advanced Semiconductor Engineering, Inc., Kaohsiung City, 81170 Taiwan Received 7 December 2009; accepted 23 February 2010; published online 12 April 2010 Electromigration in flip chip solder joints under extra high current density 4.5 10 4 A / cm 2 is studied. At such a high current density level, due to Joule heating, the chip temperature is strongly coupled to the applied current density. Accordingly, it is highly desirable to have the capability to decouple the chip temperature and the current density. Two experimental setups were used in this study, one with a cooling module to keep the chip temperature constant and the other one without a cooling module. Without the cooling module, the temperature increased rapidly with the applied current. When the current density reached 4.5 10 4 A / cm 2 , a rapid failure caused by excessive Joule heating was observed only after 10 min of current stressing. With the cooling module attached, the joint exhibited a much longer life 935 h under 4.5 10 4 A / cm 2 . It was successfully demonstrated that the cooling module was able to decouple the applied current density and the chip temperature. © 2010 American Institute of Physics . doi: 10.1063/1.3371711 I. INTRODUCTION To meet the requirement for decreasing packaging size, the electron current density through flip chip solder joints has to increase with every new generation of devices. The cur- rent design rule requires each solder joint to carry 0.2 A and will reach 0.4 A in the near future. 1 When the applied current is 0.2 A, the current density will be in the order of 10 4 A / cm 2 for solder joints of 50 m in diameter. 1 At this current density, electromigration in flip chip solder joints be- come a serious issue. 1 In the past few years, many studies on electromigration in flip chip solder joints were carried out by several research groups, and several failure mechanisms were identified. 2 18 Yeh et al. 11 reported that, in eutectic SnPb joint under a current density of 2.25 10 4 A / cm 2 , a void first nucleated near the current crowding region, and then extended across the entire joint, forming a gap between under-bump- metallurgy UBM and solder. The formation of the gap pro- duced an open circuit failure. 11 Zhang et al. 12 also observed this type of void formation-and-propagation mechanism in SnAg 4 Cu 0.5 solder joint at 3.67 10 3 A / cm 2 . The flip chip joints could also fail by the by an asym- metrical consumption of the UBM near the current crowding region. 13 17 Both asymmetrical consumption of Cu 13 , 14 and Ni 15 17 had been reported. Lin et al.
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