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Effect of electromigration on mechanical shear behavior of flip chip solder joints

Published online by Cambridge University Press:  01 March 2006

Jae-Woong Nah ,
Fei Ren ,
Kyung-Wook Paik  and
K.N. Tu
Jae-Woong Nah*
Affiliation:
Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095-1595
Fei Ren
Affiliation:
Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095-1595
Kyung-Wook Paik
Affiliation:
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
K.N. Tu
Affiliation:
Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095-1595
*
a) Address all correspondence to this author. e-mail: [email protected]
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Abstract

Effect of electromigration on mechanical shear behavior of flip chip solder joints consisting of 97Pb3Sn and 37Pb63Sn composite solder joints was studied. The under bump metallurgy (UBM) on the chip side was TiW/Cu/electroplated Cu, and the bond pad on the board side was electroless Ni/Au. It was found that the mode of shear failure has changed after electromigration and the mode depends on the direction of electron flow during electromigration. The shear induced fracture occurs in the bulkof 97Pb3Sn solder without current stressing, however, after 10 h current stressing at 2.55 × 104 A/cm2 at 140 °C, it occurs alternately at the cathode interfaces between solder and intermetallic compounds (IMCs). In the downward electron flow, from the chip to substrate, the failure site was at the Cu–Sn IMC/solder interface near the Si chip. However, in the upward electron flow, from the substrate to chip, failure occurred at the Ni–Sn IMC/solder interface near the substrate. The failure mode has a strong correlation to microstructural change in the solder joint. During the electromigration, while Pb atoms moved to the anode side in the same direction as with the electron flow, Sn atoms diffused to the cathode side, opposite the electron flow. In addition, electromigration dissolves and drives Cu or Ni atoms from UBM or bond pad at the cathode side into the solder. These reactions resulted in the large growth of Sn-based IMC at the cathode sides. Therefore, mechanical shear failure occurs predominantly at the cathode interface.

Keywords

ElectromigrationPackaging
Type
Articles
Information
Journal of Materials Research , Volume 21 , Issue 3 , March 2006 , pp. 698 - 702
Copyright
Copyright © Materials Research Society 2006

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