Search

Abstract: It is well known that some discontinuous Bi particles and voids often appear at the interfaces for the SnBi/Cu couples, leading to the interfacial embrittlement during the thermo-aging procedure. In the current study, the experimental results confirmed that Bi segregation at the SnBi/Cu interface mainly originated from the Bi diffusion during reflowing and aging procedure. Small amounts of Ag, Al, Sn or Zn elements were deliberately added into the Cu substrate to restrain the interfacial Bi embrittlement of the SnBi/Cu couples. It does not only restrain the Bi segregation, but also eliminate the formation of voids at the interfaces. Besides, it is interesting to find that the addition of these elements above can significantly improve the mechanical properties of SnBi/Cu alloy joints even after long-term aging. Based on the experimental evidence, it is suggested that alloying Cu substrate is beneficial to the wide application of SnBi and other lead-free solders, and has the potential of being a common substrate in the electronic packaging field in the future. [Copyright &y& Elsevier]

Abstract: During the reflowing procedure, the Cu concentration in the solder affects the coarsening mechanisms of intermetallic compound (IMC) grains. For the Sn3Cu solder, the mean radius of the IMC grains was proportional to the cube root of the reflowing time; while it follows the square root relation with the reflowing time for the SnAgCu and Sn solders. It is proposed that the flux from the substrate was only consumed to coarsen the IMC grains for Sn3Cu solder, while it was separated into two fluxes for the SnAgCu and Sn solders at the reflowing initial procedure. For the Sn3.8Ag0.7Cu/Cu and Sn/Cu couples, the size distribution of the IMC grains well agrees with the modified flux driven ripening (FDR) model when the value of r/〈r〉 is above 1; while it would closely match with Marqusee and Ross analysis when the value of r/〈r〉 is below 1. For Sn3Cu/Cu couple, the size distribution of IMC grains shows good agreement with the FDR model. However, for SnPb/poly-Cu, during the solid-state aging procedure, the mean radius of the IMC grains was proportional to the cube root of the aging time. And the size distribution of IMC grains is well consistent with the FDR model. [Copyright &y& Elsevier]

Abstract: The effects of aging time, strain rate and solder thickness on fracture behaviors of Sn–3Cu/Cu single crystal joints were investigated. Experimental results showed that short aging time (less than 10days) did not obviously affect the tensile strength and fracture behaviors of the solder joints. However, strain rate and solder thickness played significant roles in the strength and fracture behaviors of the solder joints. It was observed that these joints broke in a ductile manner at low strain rates with a rapid increase in the tensile strength, but displayed a brittle manner at high strain rates with a slow increase in the tensile strength. A model was proposed to explain such ductile-to-brittle transition between solder and intermetallic compounds. In addition, the solder joints broke in a ductile manner and presented a low tensile strength for the joints with thicker solder, but displayed a brittle manner and a higher tensile strength for the joints with thinner solder. [Copyright &y& Elsevier]

The segregation of Bi at the Cu/Cu3Sn interfaces of the SnBi/Cu couple dramatically decreases this couple’s mechanical properties. Here, we deliberately add Ag, Al, Sn and Zn elements into the Cu substrate to eliminate the interfacial segregation and embrittlement of the SnBi/Cu couple. Experimental results confirmed that there is always a perfect Cu3Sn/Cu alloy interface without Bi segregation, and excellent mechanical properties are thus maintained. The most important finding is that the interfacial embrittlement of SnBi/Cu alloy joints was successfully eliminated even after prolonged aging. [Copyright &y& Elsevier]

Abstract: Growth kinetics and interfacial morphologies of the intermetallic compounds (IMCs) between single crystal Ag and Sn–4Ag, Sn–3Cu and Sn–37Pb solders were investigated by solid-state aging at 160°C and liquid-state aging at 260°C. Isothermal equation of chemical reaction and phase diagrams were used to explain the effects of Ag, Cu and Pb on the growth kinetics of IMCs under solid-state and liquid-state aging conditions. The diffusion coefficients for the three solder joints of Sn–4Ag/Ag, Sn–3Cu/Ag and Sn–37Pb/Ag were calculated after solid-state and liquid-state aging. It is found that Pb can effectively retard the growth of IMCs during liquid-state aging but has little influence on the growth rate of IMCs during the solid-state aging. Some local small cracks were frequently observed in the Cu6Sn5 particles near interfaces of the Sn–3Cu/Ag solder joints after solid-state aging for several days. However, there were no such local small cracks when solders or interfaces did not contain the Cu6Sn5 particles after the same aging time. [Copyright &y& Elsevier]

Abstract: The morphologies and orientation relationships of Cu6Sn5 grains formed between Sn and (001), (011), (111) and (123) Cu single crystals under liquid- and solid-state aging conditions were systematically investigated. The regular prism-type Cu6Sn5 grains formed on (001) and (111) Cu single crystals are elongated either along two perpendicular directions or along three preferential directions with an angle of 60° between each pair of directions. The orientation relationships between Cu and Cu6Sn5 lattice structures were determined by electron backscatter diffraction and were explained in terms of their minimum misfit. However, on (011) and (123) Cu single crystal surfaces, the Cu6Sn5 grains were mainly scallop-type, with only a few regular prism-type grains. Furthermore, the regular prism-type Cu6Sn5 grains will change into scallop-type after long reflow or aging times. Meanwhile it is considered that the growth of the scallop-type grains is supplied by two fluxes: the flux of the interfacial reaction and the flux of ripening. However, the growth of the prism-type grains is only supplied by the flux of the interfacial reaction. The kinetics of IMCs growth between Sn and Cu single crystals was also investigated. [Copyright &y& Elsevier]

Abstract: This paper compares the predictive performance of ARIMA, artificial neural network and the linear combination models for forecasting wheat price in Chinese market. Empirical results show that the combined model can improve the forecasting performance significantly in contrast with its counterparts in terms of the error evaluation measurements. However, as far as turning points and profit criterions are concerned, the ANN model is best as well as at capturing a significant number of turning points. The results are conflicting when implementing dissimilar forecasting criteria (the quantitative and the turning points measurements) to evaluate the performance of three models. The ANN model is overall the best model, and can be used as an alternative method to model Chinese future food grain price. [Copyright &y& Elsevier]

Abstract: In this study the authors present and discuss the results of the investigation on the fatigue fracture behaviors in a series of as-soldered and thermal-aged copper/lead-free solder joints deformed under both monotonic and cyclic loadings. The observation results showed that fatigue cracks generally initiate around the IMC/solder interface when the loading axis is vertical to the interface. The intrinsic deformation behaviors are little different for different solder joints resulting from strain localization induced by the stain mismatch. Fracture surface observations revealed the crack propagation path and fatigue resistance of the solder joints to be affected by the yield strength and mechanical property of the solder. When the copper/solder interface is parallel to the loading axis, the interfacial IMC layer failed approximately perpendicular to the interface when the cumulative strain exceeded the fracture strain, then the cracks propagated to the IMC/solder interface, leading to the fracture along the interface. The failure mechanisms and factors influencing interfacial fatigue are discussed. [Copyright &y& Elsevier]

Several piezocone penetration test (CPTu)-based soil behaviour classification systems (SBCs) have been developed for standard sites, where clays, silt, and sand dominate. However, problems can occur when applying the SBCs to offshore sites, where the marine soils may be decomposed from rocks or mixed with artificial fills. This study evaluates the accuracy of six CPTu-based SBCs for marine soils at a site offshore Hong Kong based on 16 CPTu soundings with 25 367 data points by comparing them with composition-based SBCs from borehole records in the vicinity of each sounding. The soil types are determined from six common CPTu-based SBCs. The interpretation of CPTu data is first performed to generate soil type variables comparable to borehole data, followed by a cross-validation study. The soil classification performance of each SBC is quantified by the weighted kappa coefficient and the Kendall correlation coefficient between the soil types generated by the CPTu-based and composition-based SBCs. The classification accuracy for each soil type is also evaluated via the root mean squared error and the mean absolute error. The classified soil types from the CPTu data are associated with a median degree of consistency, indicating the need for combining CPTu-based and composition-based SBCs for marine soil classification. [ABSTRACT FROM AUTHOR]

Deep cement mixing (DCM) is an effective technique for ground improvement by injecting dry cement or cement slurry into the ground with continuous mixing and finally forming cemented soils with improved mechanical properties. A large-scale spatial distribution model is established in this study to investigate the characteristics of spatial variation in the unconfined compressive strength (UCS) of DCM clusters in a marine clay area. Variograms were used to determine the spatial variability and scale of fluctuations in the UCS data of DCM coring samples and the UCS of the natural ground from cone penetration tests. Referring to the layering characteristics of the study site, the auto-correlation structure was determined using random field modeling. Based on the generated spatial distribution and probabilistic assessment of UCS for the DCM construction site, this study compares the variations in spatial magnitude, uniformity, and variability of site-wide UCS before and after the construction of DCM clusters. [ABSTRACT FROM AUTHOR]