arXiv subjects

Tianhong Gu

Publications and source records attributed to Tianhong Gu.

5 recordsLinked to original sources

arXiv2023

Accessing slip activity in high purity tin with electron backscatter diffraction and measurement of slip strength

Beta-tin has been used widely as an interconnect in modern electronics. To improve the understanding of the reliability of these components, we directly measure the critical resolved shear stress of individual slip systems in beta-tin using micropillar compression tests at room temperature with crystal orientations near-[100] and [001] in the loading direction within a large grain high purity tin (99.99%) sample. This activates the (110)[1-11]/2, (110)[1-1-1]/2, (010)[001] and (110)[001] slip systems. Analysis of the slip traces and load-displacement curves enables measurement of the critical resolved shear stress for epsilon=10^(-4) of tau_(CRSS)^({110}<1-11>/2)=10.4+/-0.4 and tau_(CRSS)^({010}<001>)=3.9+/-0.3 MPa.

cond-mat.mtrl-sci↗

arXiv2022

Multi-scale plasticity homogenization of Sn-3Ag-0.5Cu: from β-Sn micropillars to polycrystals with intermetallics

The mechanical properties of $β$-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within $β$-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal $β$-Sn-rich alloy SAC305 (96.5Sn-3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.

cond-mat.mtrl-sci↗

arXiv2020

The role of lengthscale in the creep of Sn-3Ag-0.5Cu solder microstructures

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 as 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 (T/T_M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction (EBSD) and strain heterogeneity is identified between b/-Sn in dendrites and b/-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.

physics.app-ph↗

arXiv2020

In-situ study of creep in Sn-3Ag-0.5Cu solder

The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt.% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at 298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolutions confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal one grain (loading direction, LD, 10.4 ° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 1, (1-10)[001] slip system are observed and in grain 2, (1-10)[-1-11]/2 and (110)[-111]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.

cond-mat.mtrl-sci↗

arXiv2019

In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint

The heterogeneous evolution of microstructure in a single Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling combined with electron backscatter diffraction (EBSD). Local deformation due to thermal expansion mismatch results in heterogeneous lattice rotation, localised towards the corners of the joint and decreases towards the centre of the joint. This deformation is induced by the constraint from the CTE between the \b{eta}-Sn, Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous increase in misorientation is revealed during deformation, implying the accumulation of plastic slip at the strain-localised regions and the activation of slip systems (1-10)[111]/2 and (11-0)[111]/2.

cond-mat.mtrl-sci↗