Recrystallization and Precipitate Coarsening in Pb-Free Solder Joints During Thermomechanical Fatigue |
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Authors: | Liang Yin Luke Wentlent Linlin Yang Babak Arfaei Awni Oasaimeh Peter Borgesen |
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Institution: | 1. Universal Instruments Corporation, Conklin, NY, 13748, USA 2. Department of Systems Science & Industrial Engineering, Binghamton University, Binghamton, NY, 13902, USA
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Abstract: | 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. |
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