Modeling the structural breakdown of solder paste using the structural kinetic model

Solder paste is the most important strategic bonding material used in the assembly of surface mount devices in electronic industries. It is known to exhibit a thixotropic behavior, which is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterization of this time-dependent rheological behavior of solder pastes is crucial for establishing the relationships between the pastes structure and flow behavior; and for correlating the physical parameters with paste printing performance. In this article, we present a novel method which has been developed for characterizing the time-dependent and non-Newtonian rheological behavior of solder pastes and flux mediums as a function of shear rates. We also present results of the study of the rheology of the solder pastes and flux mediums using the structural kinetic modeling approach, which postulates that the network structure of solder pastes breaks down irreversibly under shear, leading to time and shear-dependent changes in the flow properties. Our results show that for the solder pastes used in the study, the rate and extent of thixotropy was generally found to increase with increasing shear rate. The technique demonstrated in this study has wide utility for R&D personnel involved in new paste formulation, for implementing quality control procedures used in solder-paste manufacture and packaging; and for qualifying new flip-chip assembly lines.

Modelling the time-dependent behaviour of solder pastes used in the electronics assembly applications

Solder paste is the most widely used bonding material in the assembly of surface mount devices in electronic industries. It generally has a flocculated structure (show aggregation of solder particles), and hence are known to exhibit a thixotropic behavior. This is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterisation of this timedependent rheological behaviour of solder pastes is crucial for establishing the relationships between the pastes’ structure and flow behaviour; and for correlating the physical parameters with paste printing performance. In this paper, we present a novel method which has been developed for characterising the timedependent and non-Newtonian rheological behaviour of solder pastes as a function of shear rates. The objective of the study reported in this paper is to investigate the thixotropic build-up behaviour of solder pastes. The stretched exponential model(SEM) has been used to model the structural changes during the build-up process and to correlate model parameters with the paste printing process.

A study on the cost sharing model for supply lead time reduction with component commonality

This paper studies a two-level supply chain consisting of components supplier and product assembly manufacturer, while the manufacturer shares the investment on shortening supply lead time. The objective of this research is to investigate the benefits of cost sharing strategy and adopting component commonality. The result of numerical analysis demonstrates that using component commonality can help reduce the total cost, especially when the manufacture shares a higher fraction of the cost of investment in shortening supply lead time.

Exploring the Red Sea seasonal ecosystem functioning using a three-dimensional biophysical model

The Red Sea exhibits complex hydrodynamic and biogeochemical dynamics, which vary both in time and space. These dynamics have been explored through the development and application of a 3-D ecosystem model. The simulation system comprises two off-line coupled submodels: the MIT General Circulation Model (MITgcm) and the European Regional Seas Ecosystem Model (ERSEM), both adapted for the Red Sea. The results from an annual simulation under climatological forcing are presented. Simulation results are in good agreement with satellite and in situ data illustrating the role of the physical processes in determining the evolution and variability of the Red Sea ecosystem. The model was able to reproduce the main features of the Red Sea ecosystem functioning, including the exchange with the Gulf of Aden, which is a major driving mechanism for the whole Red Sea ecosystem and the winter overturning taking place in the north. Some model limitations, mainly related to the dynamics of the extended reef system located in the southern part of the Red Sea, which is not currently represented in the model, still need to be addressed.

Fundamental shift in vitamin B12 eco-physiology of a model alga demonstrated by experimental evolution.

A widespread and complex distribution of vitamin requirements exists over the entire tree of life, with many species having evolved vitamin dependence, both within and between different lineages. Vitamin availability has been proposed to drive selection for vitamin dependence, in a process that links an organism's metabolism to the environment, but this has never been demonstrated directly. Moreover, understanding the physiological processes and evolutionary dynamics that influence metabolic demand for these important micronutrients has significant implications in terms of nutrient acquisition and, in microbial organisms, can affect community composition and metabolic exchange between coexisting species. Here we investigate the origins of vitamin dependence, using an experimental evolution approach with the vitamin B(12)-independent model green alga Chlamydomonas reinhardtii. In fewer than 500 generations of growth in the presence of vitamin B(12), we observe the evolution of a B(12)-dependent clone that rapidly displaces its ancestor. Genetic characterization of this line reveals a type-II Gulliver-related transposable element integrated into the B(12)-independent methionine synthase gene (METE), knocking out gene function and fundamentally altering the physiology of the alga.

COVALENT LIPOPROTEIN(A) ASSEMBLY: Characterization of Oxidase Activity Responsible for Catalyzing Covalent Lipoprotein(a) Assembly

Lipoprotein(a) (Lp(a)) has been identified as an emerging risk factor for the development of vascular diseases. The Lp(a) particle is assembled in a 2-step process upon secretion of the LDL and apo(a) components from hepatocytes. Work done by the Koschinsky group has identified an oxidase-like activity present in the conditioned medium (CM) harvested from human hepatoma (HepG2), as well as HEK 293 (human endothelian kidney) cells that catalyzes the rate of covalent Lp(a) formation. We have taken a candidate enzyme approach to identifying this oxidase activity. Specifically, we have proposed that the QSOX (Quiescin/sulfhydryl oxidase) is responsible for catalysis of covalent Lp(a) assembly. An oxidase activity assay developed by Dr. Thorpe (University of Delaware) was used to detect QSOX1 in CM harvested from cultured cell lines that catalyze covalent Lp(a) assembly. In addition, the QSOX1 transcript was identified in each cell line and quantified with the use of Real-Time RT-PCR. Quantitative assays of covalent Lp(a) assembly were performed to study some characteristics of the unkwown oxidase activity. First, conditioned medium was dialyzed through a 5 kDa cutoff, as this has previously been shown to reduce the aforementioned oxidase activity. Purified QSOX was then added back to the reaction and the rate of catalysis was observed. The addition of QSOX appeared to enhance the rate of covalent Lp(a) assembly in a dose-dependent manner. Additional covalent Lp(a) assembly assays were performed where various chemicals were added to determine whether Lp(a) assembly was affected. The addition of EDTA did not affect covalent assembly, suggesting that the oxidase activity may not be metallo-dependent. Moreover, dose-dependent addition of Calcium, DTT, Copper and glutathione to dialyzed medium also did not affect the rate of Lp(a) assembly. Taken together, these studies will aid in identifying the nature of the oxidase activity that catalyzes covalent Lp(a) assembly. This will provide us with valuable information on how Lp(a) particles are assembled, and may lead to the development of drugs inhibiting Lp(a) formation.

B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models

High-resolution UCLES/AAT spectra are presented for nine B-type supergiants in the SMC, chosen on the basis that they may show varying amounts of nuclear-synthetically processed material mixed to their surface. These spectra have been analysed using a new grid of approximately 12 000 non-LTE line blanketed tlusty model atmospheres to estimate atmospheric parameters and chemical composition. The abundance estimates for O, Mg and Si are in excellent agreement with those deduced from other studies, whilst the low estimate for C may reflect the use of the C II doublet at 4267 Å. The N estimates are approximately an order of magnitude greater than those found in unevolved B-type stars or H II regions but are consistent with the other estimates in AB-type supergiants. These results have been combined with results from a unified model atmosphere analysis of UVES/VLT spectra of B-type supergiants (Trundle et al. 2004, A&A, 417, 217) to discuss the evolutionary status of these objects. For two stars that are in common with those discussed by Trundle et al., we have undertaken a careful comparison in order to try to understand the relative importance of the different uncertainties present in such analyses, including observational errors and the use of static or unified models. We find that even for these relatively luminous supergiants, tlusty models yield atmospheric parameters and chemical compositions similar to those deduced from the unified code fastwind.

An analytic geometry-variant approach to line ratio enhancement above the optically thin limit

We describe a simple theoretical model to investigate the anomalous effects of opacity on spectral line ratios, as previously studied in elements such as Fe XV and Fe XVII. The model developed is general: it is not specific to a particular atomic system, thus giving applicability to a number of coronal and chromospheric plasmas; furthermore, it may be applied to a variety of astrophysically relevant geometries. The analysis is underpinned by geometrical arguments, and we outline a technique for it to be used as a tool for the explicit diagnosis of plasma geometry in distant astrophysical objects.

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

He-like x-ray line emission from laser irradiated sources

Photon yields for the 1s(2)-1s2p (He-alpha) transition of He- like ions have been measured for laser irradiated, thin foils of Ti, V and Fe. The laser pulses were of 0.527 mum wavelength and of either 80 or 300 ps duration. The data shows significant shot-to-shot variation but the Ti data is broadly consistent with previous results. In this work, we extend the previous results to include, new elements, longer pulse lengths and yields measured for emission from both surfaces of the foils. We compare our data to simulations using a hydrodynamic code and a collisional radiative model.

Field configured assembly: Programmed manipulation and self-assembly at the mesoscale

Field configured assembly is a programmable force field method that permits rapid, "hands-free" manipulation, assembly, and integration of mesoscale objects and devices. In this method, electric fields, configured by specific addressing of receptor and counter electrode sites pre-patterned at a silicon chip substrate, drive the field assisted transport, positioning, and localization of mesoscale devices at selected receptor locations. Using this approach, we demonstrate field configured deterministic and stochastic self-assembly of model mesoscale devices, i.e., 50 mum diameter, 670 nm emitting GaAs-based light emitting diodes, at targeted receptor sites on a silicon chip. The versatility of the field configured assembly method suggests that it is applicable to self-assembly of a wide variety of functionally integrated nanoscale and mesoscale systems.

Willingness to Pay for Rural Landscape Improvements: Combining Mixed Logit and Random-Effects Models

This paper reports the findings from a discrete-choice experiment designed to estimate the economic benefits associated with rural landscape improvements in Ireland. Using a mixed logit model, the panel nature of the dataset is exploited to retrieve willingness-to-pay values for every individual in the sample. This departs from customary approaches in which the willingness-to-pay estimates are normally expressed as measures of central tendency of an a priori distribution. Random-effects models for panel data are subsequently used to identify the determinants of the individual-specific willingness-to-pay estimates. In comparison with the standard methods used to incorporate individual-specific variables into the analysis of discrete-choice experiments, the analytical approach outlined in this paper is shown to add considerable explanatory power to the welfare estimates.

Analytical description of stochastic field-Line wandering in magnetic turbulence

A nonperturbative nonlinear statistical approach is presented to describe turbulent magnetic systems embedded in a uniform mean magnetic field. A general formula in the form of an ordinary differential equation for magnetic field-line wandering (random walk) is derived. By considering the solution of this equation for different limits several new results are obtained. As an example, it is demonstrated that the stochastic wandering of magnetic field-lines in a two-component turbulence model leads to superdiffusive transport, contrary to an existing diffusive picture. The validity of quasilinear theory for field-line wandering is discussed, with respect to different turbulence geometry models, and previous diffusive results are shown to be deduced in appropriate limits.

Model selection approaches for non-linear system identification: a review

The identification of non-linear systems using only observed finite datasets has become a mature research area over the last two decades. A class of linear-in-the-parameter models with universal approximation capabilities have been intensively studied and widely used due to the availability of many linear-learning algorithms and their inherent convergence conditions. This article presents a systematic overview of basic research on model selection approaches for linear-in-the-parameter models. One of the fundamental problems in non-linear system identification is to find the minimal model with the best model generalisation performance from observational data only. The important concepts in achieving good model generalisation used in various non-linear system-identification algorithms are first reviewed, including Bayesian parameter regularisation and models selective criteria based on the cross validation and experimental design. A significant advance in machine learning has been the development of the support vector machine as a means for identifying kernel models based on the structural risk minimisation principle. The developments on the convex optimisation-based model construction algorithms including the support vector regression algorithms are outlined. Input selection algorithms and on-line system identification algorithms are also included in this review. Finally, some industrial applications of non-linear models are discussed.

Increased anti-tumour efficacy of doxorubicin when combined with sulindac in a xenograft model of an MRP-1-positive human lung cancer

Background: A number of cellular proteins, including P-glycoprotein (P-gp) and Multiple drug Resistance Protein (MRP-1), act as drug efflux pumps and are important in the resistance of many cancers to chemotherapy. We previously reported that a small number of NSAIDs could inhibit the activity of MRP-1. Materials and Methods: We chose sulindac as a candidate agent for further investigation as it has the most favourable efficacy and toxicity profile of the agents available for a potential specific MRP-1 inhibitor. NCI H460 cells expressed MRP-1 protein (by Western blot) and also the toxicity, of doxorubicin (a substrate of MRP-1) could be potentiated in this line using non-toxic concentrations of the MRP-1 substrate/inhibitor sulindac. These cells were implanted in nude mice and the animals divided into various groups which were administered doxorubicin and/or sulindac. Results: Sulindac was shown to significantly potentiate the tumour growth inhibitor activity of doxorubicin in this MRP-1-overexpressing human tumour xenograft model. Conclusion: Sulindac may be clinically useful as an inhibitor of the MRP-1 cancer resistance mechanism.