Mechanical Characterization of Lead-Free Sn-Ag-Cu Solder Joints by High-Temperature Nanoindentation

The reliability of Pb-free solder joints is controlled by their microstructural constituents. Therefore, knowledge of the solder microconstituents’ mechanical properties as a function of temperature is required. Sn-Ag-Cu lead-free solder alloy contains three phases: a Sn-rich phase, and the intermetallic compounds (IMCs) Cu₆Sn₅ and Ag₃Sn. Typically, the Sn-rich phase is surrounded by a eutectic mixture of β-Sn, Cu₆Sn₅, and Ag₃Sn. In this paper, we report on the Young’s modulus and hardness of the Cu₆Sn₅ and Cu₃Sn IMCs, the β-Sn phase, and the eutectic compound, as measured by nanoindentation at elevated temperatures. For both the β-Sn phase and the eutectic compound, the hardness and Young’s modulus exhibited strong temperature dependence. In the case of the intermetallics, this temperature dependence is observed for Cu₆Sn₅, but the mechanical properties of Cu₃Sn are more stable up to 200°C.     

Mechanisms of Sn Hillock Growth in Vacuum by In Situ Nanoindentation in a Scanning Electron Microscope (SEM)

Nanoindentation is an excellent technique to quantitatively probe the Sn film surface and to introduce controlled compressive stresses. In this work, we have conducted a long-term study of whisker growth in Sn films plated on Cu. In situ indentations were conducted in a scanning electron microscope under vacuum, to elucidate the effect of whiskering without significant oxidation. The evolution of whisker growth up to 1500 h was studied. Measurements of whisker height and width were used to determine the relationship between nodule volume and time. Extensive nodule growth was observed at indentations. Competing mass flow between indentations was observed, with some indentations exhibiting extensive growth, while the growth of others arrested within 100 h. It can be postulated that, when stresses are relieved slowly, hillock heights grow nearly linearly over time. When stress is relieved quickly, a sigmoidal-type curve (arresting growth) is predicted.     

Comparison of 3D reaction rate distributions measured in an Optima2 BWR assembly with MCNPX predictions

As part of a joint research programme between the Paul Scherrer Institute (PSI) and swissnuclear, with the co-operation of the Leibstadt nuclear power plant in Switzerland and fuel suppliers Westinghouse Sweden, measurements and calculations have been made of the axial and radial distributions of fission and ²³⁸U capture rates in the fuel rods of a Westinghouse SVEA-96 Optima2 boiling water reactor assembly. The measurements, made in the zero-energy research reactor PROTEUS at PSI, have been compared with calculations carried out using the Monte Carlo code MCNPX. The results reported are for the regions near the ends of the part-length fuel rods, which are a feature of SVEA-96 Optima2 assemblies. The sudden increase in moderation above the ends of the part-length rods leads to power peaking in the adjacent rods. Careful attention needs to be given to this phenomenon in the deployment of such fuel, the present paper providing experimental evidence for the ability of a stochastic code to predict such effects.     

Porous hierarchical TiO2 nanostructures: Processing and microstructure relationships

A dual porous hierarchical coating of TiO₂ nanotubes (～50 nm diameter) on the nanoscale and large (～1 to 20 μm) pores on the micro-scale can be fabricated on the surface of Ti by anodic oxidation. This unique coating may have potential applications as bioactive coatings for Ti bone implants. This paper details several important aspects of the coating microstructure. TiO₂ coatings were fabricated by anodic oxidation in 1 M H₂SO₄ + 0.1 M NaF solution. Microstructure characterization was carried out using scanning electron microscopy. We also report on the observation of precipitates which form as both a continuous surface layer and of a conical geometry. The mechanism for nanotube formation, precipitate layer formation, and microscopic pitting was discussed. The effect of processing variables (i.e. time, temperature, pH) on the TiO₂ microstructure was studied. Anodization time was found to affect nanotube length and also pit size and density. Lowering the electrolyte pH decreased the nanotube length and microscopic pit density. Increasing electrolyte temperature decreased nanotube length and increased pit/pore and precipitate density. Microscopic pitting, in the nanotube coating was found to occur above grain boundaries in the Ti substrate and above Ti grains with (0 0 0 1) orientation.     

Effect of solvent-induced structural modifications on optical properties of CdS nanoparticles

We have investigated the effects of solvent used during synthesis on structural and optical properties of CdS quantum dots. Different methods of synthesis for the production of CdS quantum dots are presented. These are: (a) wet chemical co-precipitation in non-aqueous medium (i.e. methanol); (b) wet chemical co-precipitation in aqueous medium (deionized water) and (c) solid state reaction. It is demonstrated that the use of methanol as solvent leads to a strong enhancement of PL intensity of CdS quantum dots for use in optoelectronic devices. These products were characterized by X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The change in bandgap with size-quantization was investigated by UV-VIS absorption spectroscopy. CdS nanocrystals prepared in non-aqueous medium have narrow size distribution than those prepared in aqueous medium and solid state reaction. Phase transformation of CdS nanocrystals from a cubic to hexagonal structure was observed in methanol solution. The formation of CdS/Cd(OH)₂ nanostructure was also confirmed using X-ray diffraction pattern. This suggests that the strong enhancement of the PL intensity may have originated from the remarkable reduction of non-radiative recombination process, due to surface defects of quantum dots. The red shift of the Raman peaks compared to that for bulk CdS may be attributed to optical phonon confinement.     

Finite element analysis of thermal stress in magnetron sputtered Ti coating

The thermal, shear and radial stresses generated in the Ti coating deposited on glass and Si substrates were investigated by finite element analysis (ANSYS). The four-node structural and quadratic element PLANE 42 with axisymmetric option were used to model the Ti coating on glass and Si substrates. The influence of deposition temperature, substrate and coating properties on the generation of thermal stress in Ti is analyzed. It is found that the thermal stress of Ti coating exhibits a linear relationship with deposition temperature and Young's modulus of the coating, but it exhibit an inverse relationship with the coating thickness. The results of simulated thermal stress are in accordance with the analytical method. The radial stress and shear stress distribution of the coating–substrate combination are calculated. It is observed that high tensile shear stress of Ti coating on glass substrate reduces its adhesive strength but high-compressive shear stress of Ti on Si substrate improves its adhesive strength.     

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Effect of BN Coating on the Strength of a Mullite Type Fiber

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al₂O₃+28 wt.-% SiO₂+2 wt.-% B₂O₃). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.     

Axisymmetric free convection induced by periodic variation in the temperature of a horizontal plane surface

Axisymmetric free convection flow of a viscous incompressible fluid above a horizontal plane surface is analysed when the temperature of the plane surface oscillates about a non-zero mean. The solution of the multilayered model is obtained by matching the inner and the outer asymptotic expansions. It is found that the order of the mean induced flow is larger than that of the order of the applied mean temperature. Specific series expansions are obtained for the skin-friction and the heat-transfer at the horizontal surface. Estimates of the physical parameters yielding stable fluid motion are derived.     

ULTRAPURIFICATION AND RECYCLING OF HYDROFLUORIC ACID ETCHING SOLUTIONS BY REVERSE OSMOSIS: MEMBRANE PERFORMANCE AND MULTICOMPONENT REJECTION

Complex multicomponent hydrofluoric acid (HF∥ etching solutions are studied here to determine the degree of ultrapurification that can be achieved by membrane separation. Interactions among the components in the solution and the rates of permeation and rejection of the various components were measured for membranes which have the ability to resist attack by corrosive etching solutions. Because hydrofluoric acid has a low ionization constant, it can be purified by membrane systems. Several commercially available membranes were found to perform satisfactorily, even after exposure for extended periods of time to concentrated HF etching solutions

With ternary solutions (HF-fluosilicic acid- water) membranes also can significantly reject fluosilicic acid (FSA), the principal impurity in HF etching solutions, while HF permeates almost 100% through the membrane

Furthermore, membranes significantly reject the trace metallic impurities present in HF etching solutions. Thus, ultrapure HF can be generated by recycling, and our method is expected to have potential for commercial development with applications to a variety of industries.