Recrystallization and Precipitate Coarsening in Pb-Free Solder Joints During Thermomechanical Fatigue

Authors: Yin, Liang1; Wentlent, Luke2; Yang, Linlin2; Arfaei, Babak2; Oasaimeh, Awni2; Borgesen, Peter2

Source: Journal of Electronic Materials, Volume 41, Number 2, February 2012 , pp. 241-252(12)

Publisher: Springer

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The recrystallization of β-Sn profoundly affects deformation and failure of Sn-Ag-Cu solder joints in thermomechanical fatigue (TMF) testing. The numerous grain boundaries of recrystallized β-Sn enable grain boundary sliding, which is absent in as-solidified solder joints. Fatigue cracks initiate at, and propagate along, recrystallized grain boundaries, eventually leading to intergranular fracture. The recrystallization behavior of Sn-Ag-Cu solder joints was examined in three different TMF conditions for five different ball grid array component designs. Based on the experimental observations, a TMF damage accumulation model is proposed: (1) strain-enhanced coarsening of secondary precipitates of Ag3Sn and Cu6Sn5 starts at joint corners, eventually allowing recrystallization of the Sn grain there as well; (2) coarsening and recrystallization continue to develop into the interior of the joints, while fatigue crack growth lags behind; (3) fatigue cracks finally progress through the recrystallized region. Independent of the TMF condition, the recrystallization appeared to be essentially complete after somewhat less than 50% of the characteristic life, while it took another 50% to 75% of the lifetime for a fatigue crack to propagate through the recrystallized region.

Keywords: Pb-free solder; microstructure; precipitate coarsening; recrystallization; thermomechanical fatigue

Document Type: Research Article


Affiliations: 1: Universal Instruments Corporation, Conklin, NY, 13748, USA, Email: 2: Department of Systems Science & Industrial Engineering, Binghamton University, Binghamton, NY, 13902, USA

Publication date: February 1, 2012

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