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Effect of under-bump-metallization structure on electromigration of Sn-Ag solder joints

  • Chen, Hsiao-Yun (Department of Materials Science and Engineering, National Chiao Tung University) ;
  • Ku, Min-Feng (Department of Materials Science and Engineering, National Chiao Tung University) ;
  • Chen, Chih (Department of Materials Science and Engineering, National Chiao Tung University)
  • Received : 2011.08.31
  • Accepted : 2012.03.09
  • Published : 2012.03.25
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Abstract

The effect of under-bump-metallization (UBM) on electromigration was investigated at temperatures ranging from $135^{\circ}C$ to $165^{\circ}C$. The UBM structures were examined: 5-${\mu}m$-Cu/3-${\mu}m$-Ni and $5{\mu}m$ Cu. Experimental results show that the solder joint with the Cu/Ni UBM has a longer electromigration lifetime than the solder joint with the Cu UBM. Three important parameters were analyzed to explain the difference in failure time, including maximum current density, hot-spot temperature, and electromigration activation energy. The simulation and experimental results illustrate that the addition 3-${\mu}m$-Ni layer is able to reduce the maximum current density and hot-spot temperature in solder, resulting in a longer electromigration lifetime. In addition, the Ni layer changes the electromigration failure mode. With the $5{\mu}m$ Cu UBM, dissolution of Cu layer and formation of $Cu_6Sn_5$ intermetallic compounds are responsible for the electromigration failure in the joint. Yet, the failure mode changes to void formation in the interface of $Ni_3Sn_4$ and the solder for the joint with the Cu/Ni UBM. The measured activation energy is 0.85 eV and 1.06 eV for the joint with the Cu/Ni and the Cu UBM, respectively.

Keywords

Acknowledgement

Supported by : National Science Council of R.O.C.

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