MICROSTRUCTURAL EFFECTS ON CREEP AND FRACTURE MECHANICS OF SN-AG-CU SOLDERSMICROSTRUCTURAL EFFECTS ON CREEP AND FRACTURE OF Sn-Ag-Cu SOLDERS
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During service and/or storage, Sn-Ag-Cu (SAC) solder alloys are subjected to temperatures ranging from 0.4 to 0.8 Tm (where Tm is the melting temperature), while also experiencing cyclic strains. These conditions lead to significant microstructural changes, and thereby influence the creep response of the solder, hence the long-term reliability of microelectronic packages. Accurate prediction of long term reliability of SAC solders necessitates quantitative study of microstructure coarsening as well as development of creep models that adapt to ongoing microstructural changes. In this work, microstructure dependence of the creep behavior of two solder alloys (Sn-3.5Ag-0.5Cu and Sn-1.0Ag-0.5Cu) has been studied both experimentally and analytically; a previously developed microstructurally-adpative creep model has been modified to account for the significant contribution of the proeutectic phase to the overall creep behavior of solders with a small amount of eutectic. Additonally, handheld electronic devices are also subjected to drops, which makes the solder joints susceptible to fracture at high strain rates. This is particularly important for solder micro-bumps which connect devices in three-dimensional (3D) electronics, since these micro-bumps typically contain a large proportion of brittle intermetallic compounds (IMC), making these micro-joints highly susceptible to fracture. In this work, the fracture mechanics of thin micro-joints has been studied and a fracture mechanism map has been plotted.The main objectives of the proposed work are to (i) characterize microstructural evolution and model the key microstructural parameters that evolve during different thermal and thermo-mechanical excursions, (ii) study the creep behavior of solders with different thermal/thermo-mechanical histories, (iii) develop a microstructurally adaptive composite model capable of predicting the creep behavior of the solders in situ during microstructural evolution, (iv) study the mixed-mode fracture behavior (fracture toughness Gc and fracture mechanisms) of the thin solder joints under dynamic loading condition (v) develop a fracture mechanism map which may be used for material selection and joint design to enhance the reliability of 3D microelectronic packages. As such, this work provides fundamental insight into the relationship between the solder / joint microstructure and the steady state creep behavior as well as high strain rate fracture mechanics of Sn-Ag-Cu lead-free solders.