Localized recrystallization and cracking of lead-free solder interconnections under thermal cycling
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Maik Mueller Electronics Packaging Laboratory (IAVT), Technische Universität Dresden, 01062 Dresden, Germany
Tonu Tuomas Mattilaa) and Jue Li Department of Electronic, Aalto University School of Science and Technology, FIN-00076 Aalto, Finland
Xuwen Liu Laboratory of Materials Science, Department of Materials Science and Engineering, Aalto University School of Chemical Technology, FIN-00076 Aalto, Finland
Klaus-Juergen Wolter Electronics Packaging Laboratory (IAVT), Technische Universität Dresden, 01062 Dresden, Germany
Mervi Paulasto-Kröckel Department of Electronic, Aalto University School of Science and Technology, FIN-00076 Aalto, Finland (Received 21 January 2011; accepted 18 May 2011)
The failure mechanism of lead-free solder interconnections of chip scale package–sized Ball Grid Array (BGA) component boards under thermal cycling was studied by employing cross-polarized light microscopy, scanning electronic microscopy, electron backscatter diffraction, and nanoindentation. It was determined that the critical solder interconnections were located underneath the chip corners, instead of the corner most interconnections of the package, and the highest strains and stresses were concentrated at the outer neck regions on the component side of the interconnections. Observations of the failure modes were in good agreement with the finite element results. The failure of the interconnections was associated with changes of microstructures by recrystallization in the strain concentration regions of the solder interconnections. Coarsening of intermetallic particles and the disappearance of the boundaries between the primary Sn cells were observed in both cases. The nanoindentation results showed lower hardness of the recrystallized grains compared with the nonrecrystallized regions of the same interconnection. The results show that failure modes are dependent on the localized microstructural changes in the strain concentration regions of the interconnections and the crack paths follow the networks of grain boundaries produced by recrystallization.
I. INTRODUCTION
Solder interconnections still remain the predominant method to form electrical and mechanical connections between electronic components. Solder interconnection reliability has received an increasing attention during the recent years due to the miniaturization trend of electronic devices as well as the adoption of the lead-free solder alloys.1–6 Numerous studies have been carried out to identify the root causes of failures in electronic devices under different operation conditions. Most often electrical failures are caused by the mismatch in the coefficient of thermal expansion (CTE) between adjoining materials as a consequence of changes of the device temperature due to either internally a)
Address all correspondence to this author. e-mail: Toni.Mattila@hut.fi DOI: 10.1557/jmr.2011.197 J. Mater. Res., Vol. 26, No. 16, Aug 28, 2011
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