How would you……describe the overall signifi cance of this paper? Sn-Ag based solders constitute an important part of modern microelectronic packages. During service, these solders undergo microstructural changes, leading to continuous property changes during use. This paper summarizes some of these changes and assesses their impact on mechanical properties. …describe this work to a materials science and engineering professional with no experience in your technical specialty? This paper studies in situ microstructural evolution in solders during isothermal aging and thermomechanical cycling, and provides mechanistic insight into the interaction of microstructure with properties. …describe this work to a layperson? This paper describes how and why the properties of lead-free solders used in microelectronic packages change during service. This can have signifi cant impact on the reliability and performance of all electronics.Solders based on Sn-Ag alloys are susceptible to microstructural coarsening during storage or service, resulting in evolution of joint properties, and hence reliability, over time. Coarsening can occur during static aging, and even faster during thermo-mechanical cycling (TMC). The kinetics of coarsening may also depend on the scale of the joint. These effects lead to evolution of the mechanical properties of the joint over time, as well as spatial variations of property within the joint. Therefore, accurate prediction of joint properties during service or storage requires a quantitative understanding of coarsening under both isothermal and TMC conditions, and incorporating these in constitutive laws. This paper discusses the kinetics of coarsening in Sn-Ag based solders, and presents a rationale for joint-scale dependence of coarsening. The impact of coarsening on creep and fracture properties of joints under drop conditions are also presented.
In several recent applications, including those aimed at developing thermal interface materials, nanoparticulate systems have been proposed to improve the effective behavior of the system. While nanoparticles by themselves may have low conductivities relative to larger particles owing to interfacial resistance, their use along with larger particles is believed to enhance the percolation threshold leading to better effective behavior overall. One critical challenge in using nanoparticulate systems is the lack of knowledge regarding their thermal conductivity. In this paper, the thermal conductivity of silica clusters (or nanoparticles) as well as nanowires is determined using molecular dynamics (MD) simulations. The equilibrium MD simulations of nanoparticles using Green-Kubo relations are demonstrated to be computationally very expensive and unsuitable for such nanoscaled systems. A nonequilibrium MD method adapted from the study of Müller-Plathe is shown to be faster and more accurate. The method is first demonstrated on bulk amorphous silica (using both cubic and orthorhombic simulation cells) and silica nanowires. The thermal conductivity values are compared to those reported in the literature. The mean thermal conductivity values for bulk silica and silica nanowire were estimated to be 1.2 W/mK and 1.435 W/mK, respectively. To model nanoparticles, the Müller-Plathe technique is adapted by dividing the cluster into concentric shells so as to capture the naturally radial mode of heat transfer. The mean thermal conductivity value of a 600-atom silica nanoparticle obtained using this approach was 0.589 W/mK. This value is approximately 50-60% lower than those of bulk silica or silica nanowire.
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