Abstract
Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near-<110> orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size, indicating a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature ( frac{T}{{T_{M} }} > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction and strain heterogeneity is identified between β-Sn in dendrites and β-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings; however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest λ2 and λe) is only localised next to IMCs without growth in size.
Highlights
Sn-3Ag-0.5Cu wt.% (SAC305) is one of the most commonly used solder alloys in electronic interconnections.[1]
SAC305 consists of primary b-Sn dendrites and eutectic regions with Cu6Sn5 and Ag3Sn embedded within the b-Sn matrix. b-Sn occupies $ 96% of the SAC305 solder and exhibits significant anisotropy in physical properties, such as elastic stiffness and thermal expansion resulting from its body-centred tetragonal (BCT) structure.[2,3]
We explore the effect of secondary dendrite arm spacing (k2), eutectic IMC spacing and IMC size on the creep behaviour and microstructural evolution for constant stress creep testing at a range of temperatures
Summary
Sn-3Ag-0.5Cu wt.% (SAC305) is one of the most commonly used solder alloys in electronic interconnections.[1]. At the end of tertiary stage creep (Fig. 5d, g, k, n, r and u), within the mapped regions the lattice rotation[32,51] is more obvious in the sample with a finer microstructure (Fig. 5g, n and u) because the evenly distributed fine-scaled obstacles (e.g. IMCs and dendrite-eutectic boundaries) result in increase in total stored energy of the sample and shown as polygonisation, k and r). The ‘rainbow’ recrystallised grains are formed in dendritic b-Sn (Fig. 7f), which deforms by gradual lattice rotation with continuous development of polygonisation and causes recrystallisation in the highly strained region.[42] The constrained stored energy is released at the fracture surface showing decrease in misorientation (Fig. 7g, h and i)
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