Diffusion behavior of Sn atoms in Sn58Bi solder joints under the coupling effect of thermomigration and electromigration
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Thermomigration (TM) and electromigration (EM) are two persistent reliability issues and they generally appear concurrently in solder joints. Many previous studies have attempted to understand the fundamental principles behind these phenomena with the majority of which focusing their interest into the faster migration elements in solders like Bi, Ni, or Cu. However, Sn as the slower migration element has not received that much attention. In the present study, a special linearly symmetrical structure was used. An unusual TM phenomenon of Sn atoms in the Sn58Bi solder joint was observed. The unusual TM of Sn atoms along the vertical edges was attributed to the coupled effect of the EM in the horizontal direction and the TM in vertical direction. The relationships between the microstructural characteristics and the temperature distribution were established. The results also indicated that elevated temperature and sufficient thermal gradient were the two major factors that caused TM.
I. INTRODUCTION
Miniaturizing and high-density packaging of microelectronic devices are responsible for making electromigration (EM)1,2 and thermomigration (TM)1,3,4 two of the most persistent reliability issues in interconnections. TM usually occurs with EM when a sufficient thermal gradient is created, caused by the inhomogeneous distribution of Joule heating.1,5–7 In a real solder joint, the nonuniform Joule heating mainly stems from the geometrical configurations and the chip side produces much more heat than the substrate side.8 As a result, a thermal gradient can be formed across the solder joint, resulting to TM. A nonuniform Joule heating can also be formed in a heterogeneous microstructure, due to the resistivity divergence of the different phases involved.9 The critical thermal gradient value, that is sufficient to trigger TM, is generally considered to be the 1000 °C/cm,1 although some previous studies have reported that a value of ;400 °C/cm could also result to TM.10 Various studies suggest that the thermal gradient may be the only necessary condition for the formation of TM. Also, the ambient temperature factor contributes positively to the formation of TM, because the diffusion process is Contributing Editor: C. Robert Kao Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) e-mail: [email protected] d) e-mail: [email protected] DOI: 10.1557/jmr.2016.145
a temperature-dependent process. However, this statement has not been experimentally demonstrated yet. Various studies have previously addressed the issues related to the EM and TM in solder joints.3–8,10–18 Most of them were focused on conventional SnPb solder joints,1,17 or Sn-based solder joints.12,14–16 Huang et al.17 discovered that in a composite SnPb solder joint, where Sn97Pb was observed in the warmer side of the joint and eutectic Sn63Pb on the colder, Pb had migrated to the colder side and Sn to the warmer. For Sn-based solder joints, the major migration chemical constituents that were induce
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