Authored By:
Kendra Young
Portland State University, OR, USA
Nilesh Badwe, Raiyo Aspandiar, Satyajit Walwadkar
Intel Corporation, OR, USA
Young-Woo Lee, Hui-Joong Kim, Jung-Tak Moon
MK Electron Co., Ltd., Yongin, Korea
Tae-Kyu Lee
Cisco Systems, CA, USA
Summary
Recent studies on Sn-Bi based low melting temperature solder interconnects with minor elemental tuning presents enhanced interconnect solder joint thermo-mechanical performance compared to eutectic Sn-Bi. The degradation mechanism towards thermal cycling induced failure is related to twin boundary formation, a different mechanism compared to general Sn-Ag-Cu solder joint interconnect thermal cycling induced degradation. An attempt to slow down the degradation rate is implemented by adding Ag and In into the eutectic Sn-Bi based alloy system.
Ag and In contained Sn-Bi solder ball alloy compositions and Sn-Bi based paste with and without Ag content were assembled and the thermal cycling performance compared. In this study, 12x12 mm ball grid array (BGA) components on 62 mil (1.6mm) thick boards were thermally cycled with a -40 to 100oC profile and 10 min dwells. The correlation between crack initiation, crack propagation, sub-grain development and localized recrystallization were compared with cross section analyses using electron–backscattered diffraction (EBSD) imaging. The analysis revealed the potential thermal cycling performance enhancement mechanism in Sn-Bi solder interconnects.
Conclusions
The Sn-Bi eutectic system microstructure has different damage accumulation mechanism due to the Bi crystal lattice with a Rhombohedral A7 unit cell structure, which is less ductile and has temperature dependent Bi solubility level into Sn. The nature of the reduced ductility in the Sn-Bi alloy system reveals itself with a different damage accumulation process during thermal cycling compared to Sn-Ag-Cu solder material. Sn-58Bi solder joints show a development of twin boundaries after a certain number of thermal cycles, which help contribute to the stress concentration shift to localized regions, which eventually initiate a crack. Microalloying with In and Ag can mitigate the Bi diffusion into the Sn-grain structure and can delay the twin boundary development thus enabling a higher characteristic thermal life cycle number.
Initially Published in the SMTA Proceedings
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