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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]

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: 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]

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]