Topological design of channels for squeeze flow optimization of thermal interface materials

Performance of thermal interface materials (TIMs) used between a microelectronic device and its associated heat spreader is largely dependent on the bulk thermal conductivity of the TIM, but the bond-line thickness (BLT) of the applied material as well as the interfacial contact resistances are also significant contributors to overall performance. Hierarchically Nested Channels (HNCs), created by modifying the surface topology of the chip or the heatsink with hierarchical arrangements of microchannels in order to improve flow, have been proposed to reduce both the required squeezing force and the final BLT at the interfaces. In the present work, a topological optimization framework that enables the design of channel arrangements is developed. The framework is based on a resistance network approximation to Newtonian squeeze flow. The approximation, validated against finite element (FE) solutions, allows efficient, design-oriented solutions for squeeze flow in complex geometries. A comprehensive design sensitivity analysis exploiting the resistance network approximation is also developed and implemented. The resistance approximation and the sensitivity analysis is used to build an automated optimal channel design framework. A Pareto optimal problem formulation for the design of channels is posed and the optimal solution is demonstrated using the framework.     

A dynamic model for predicting the motion of solder droplets during assembly

The dynamic motion of a solder droplet during assembly is a complex, unsteady, free surface problem involving surface tension and viscous effects. The motion of the droplet is coupled with the motion of the component or chip to be assembled and involves dynamic contact lines. A methodology based on a non-uniform rational b-spline (NURBS) discretization has been developed for the dynamic analysis of the droplet motion. A surface energy based formulation has been developed to incorporate the surface tension effects. The developed methodology leads to an updated Lagrangian scheme with a Galerkin in space and Least square in time formulation. The NURBS representation used for the spatial discretization enables the method to handle problems involving complex droplet geometries. The ability of the NURBS representation to provide both global and local control, along with the least square method used in this methodology, enables us to develop an unconditionally stable time integration scheme which can be optimized to achieve desired accuracy and numerical dissipation efficiently. A sample problem of droplet shape evolution has been solved to demonstrate the path prediction capability of the proposed methodology. In future, the method can be applied to solve various real world dynamic motion problems associated with droplets.     

An assessment of change in stress due to cross-sectioning in moire/spl acute/ interferometric characterization of electronic packages

Cross-sectional moire/spl acute/ interferometry technique is a very commonly applied one for the characterization of electronic packages. While the technique is popular, its use has not been accompanied by rigorous evaluation of its measurement accuracy. Such an evaluation, necessary because the cross-sectioning destructively modifies the original geometry, is the goal of the current paper. In the present study, a five-layer specimen with intact axi-symmetry as well as one in which the symmetry is destroyed through cross-sectioning are chosen as the vehicles for developing an understanding of the effect of cross-sectioning. We present a rigorous validation of an analytical elasticity model of axi-symmetric package-like structures in which the model prediction is shown to be accurate to within 1% of experimental measurement. The validated analytical model is used to estimate a reduction in radial stress of approximately 40% due to the cross-sectioning of the circular specimens, while the radial strains were virtually unaffected by cross-sectioning. These results suggest that cross-sectional moire/spl acute/ interferometry is likely to yield accurate strains, but not stresses. Since damage under cyclic loading is a function of both stress and strain, the use of moire/spl acute/ interferometry for studying the evolution of strains in packages under cyclic loading is likely to be grossly in error.     

Constitutive and Aging Behavior of Sn3.0Ag0.5Cu Solder Alloy

We describe double-lap shear experiments on Sn3.0Ag0.5Cu solder alloy, from which fits to Anand's viscoplastic constitutive model, power-law creep model, and to time-hardening primary-secondary creep model are derived. Results of monotonic tests for strain rates ranging from 4.02E-6 to 2.40E-3 s^-1, and creep response at stress levels ranging from 19.5 to 45.6 MPa are reported. Both types of tests were conducted at temperatures of 25degC, 75degC , and 125degC. Following an earlier study where Anand model and time hardening creep parameters for Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu solder alloys were reported, here we report power law model parameters so as to enable a comparison between all three alloys. Primary creep in Sn3.0Ag0.5Cu solder alloy is shown to be significant and are considered in addition to secondary creep and monotonic behavior. Aging influence on behavior is also shown to be significant. On the basis of experimental data, the following four aspects are discussed: 1) difference between testing on bulk versus joint specimen; 2) consistency between the creep and monotonic behaviors; 3) comparison against behaviors of Sn1.0Ag0.5Cu and Sn3.8Ag0.7Cu alloys as well as aganist Sn40Pb, 62Sn36Pb2Ag and 96.5Sn3.5Ag alloys; and 4) comparison of Sn3.0Ag0.5Cu and Sn3.8Ag0.7Cu relative to their aging response.     

EM-based recursive estimation of channel parameters

Recursive (online) expectation-maximization (EM) algorithm along with stochastic approximation is employed in this paper to estimate unknown time-invariant/variant parameters. The impulse response of a linear system (channel) is modeled as an unknown deterministic vector/process and as a Gaussian vector/process with unknown stochastic characteristics. Using these models which are embedded in white or colored Gaussian noise, different types of recursive least squares (RLS), Kalman filtering and smoothing and combined RLS and Kalman-type algorithms are derived directly from the recursive EM algorithm. The estimation of unknown parameters also generates new recursive algorithms for situations, such as additive colored noise modeled by an autoregressive process. The recursive EM algorithm is shown as a powerful tool which unifies the derivations of many adaptive estimation methods     

An experimental investigation of performance and emission characteristics of diesel engine with blends of cottonseed oil methyl ester with cold and hot EGR

An experimental investigation of diesel engine using cottonseed oil biodiesel and its blends with exhaust gas recirculation (EGR) techniques has been carried out. An optimum nozzle opening pressure of 250 bar and lower static injection timing of 20° before top dead centre (bTDC) are considered because it has been observed that these conditions only give minimum emissions. From the test results, it could be noted that there is an increasing trend of emission characteristics of HC, smoke density and NO_x for both cold and hot EGR for all blends of fuel with respect to brake power. As compared with cold EGR, the hot EGR gives lower emissions at all loads. In hot EGR, among the blends, at no-load and full-load conditions, the B100 gives the highest reduction in NO_x of 14.23% and 7.91%, respectively. However, the use of EGR leads to a rise in soot emission because of soot–NO_x trade-off for both the cases.     

Fabrication of Biogenic Silica Nanostructures from Sorghum bicolor Leaves for Food Industry Applications

Silicon - Due to the large production of sorghum, the generation of associated agricultural residues, which contain high contents of silica, is inevitable. Also, these agricultural residues are not...     

Maximum-Entropy Principle for Modeling Damage and Fracture in Solder Joints

A maximum-entropy fracture model (MEFM) is derived from concepts of information theory and statistical thermodynamics. Exploiting the maximum-entropy principle enables life predictions for a structure in the presence of microstructural uncertainty. This single-parameter model relates the probability of fracture to accumulated entropic dissipation at a given material point. Using J ₂ plasticity and equilibrium thermodynamics, entropic dissipation is related to inelastic dissipation. We demonstrate the MEFM by extracting the single damage accumulation parameter for Sn-3.8Ag-0.7Cu solder through cyclical fatigue testing. We then apply the model with the single parameter to numerically predict, in three dimensions, crack initiation and growth in Sn-3.8Ag-0.7Cu solder joints of a wafer-level chip-scale package (WLCSP). The simulated crack fronts are validated against experimentally observed crack fronts obtained by testing 64 packages under conditions identical to those used in the simulations. The model is shown to accurately predict the geometrical profile of the observed crack fronts, and the number of cycles corresponding to the observed crack profile to within 10% of the measured number of cycles.