Effect of electromigration on mechanical shear behavior of flip chip solder joints
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Kyung-Wook Paik Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
K.N. Tu Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095-1595 (Received 14 October 2005; accepted 2 December 2005)
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 bulk of 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.
I. INTRODUCTION
Reliability testing and failure analysis are important for estimating and predicting the life time of electronic products.1 Electromigration has become a serious reliability concern in flip chip solder joints because the dimensions of solder joints are expected to decrease and current density to increase.2 Many papers have been published about the electromigration behavior of flip chip solder joints.3–14 The general electromigration induced failure mode in flip chip solder joints is the loss of under bump metallurgy (UBM) and the interfacial void formation at the contact interface between the interconnect line
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0086 698
J. Mater. Res., Vol. 21, No. 3, Mar 2006 http://journals.cambridge.org Downloaded: 03 Apr 2015
and the solder bump. Furthermore, the effect of electromigration on mechanical behavior of flip chip solder joints has attracted much attention lately because mechanical reliability of solder joints can be affected by electromigration induced intermetallic compound (IMC) formation at the solder joint interfaces.15 However, the combination of electromigration and me
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