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 non-recrystallized 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.
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The authors would like to thank Prof. Emer. Jorma Kivilahti for his inextinguishable enthusiasm for the topics discussed in this article. The author would like to thank Dr. V. Vuorinen for his valuable discussion and help in the SEM studies. The authors would also like to thank Mr. Jussi Hokka for his help in sample preparation and managing the thermal cycling tests. The Academy of Finland is acknowledged for funding this work.
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Chen, H., Mueller, M., Mattila, T.T. et al. Localized recrystallization and cracking of lead-free solder interconnections under thermal cycling. Journal of Materials Research 26, 2103–2116 (2011). https://doi.org/10.1557/jmr.2011.197