Interfacial reaction studies on lead (Pb)-free solder alloys

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.     

Nature of Interatomic Bonding in Controlling the Mechanical Properties of Calcium Silicate Hydrates

Calcium silicate hydrate (C–S–H) is the most important phase of hydrated cement gel which is the key material in construction industry. It is well accepted that hardened cement paste consists of either poorly crystalline or completely disordered phases. Although a myriad of speculative atomistic models of disordered C–S–H have been proposed, the fundamental basis of structure–property relationships remain elusive. This study focuses upon the correlations between mechanical properties and electronic structure based on well‐defined quantum mechanical parameters. We use 20 CSH minerals with known structure to gain fundamental understanding of structure–property relationship. The results indicate Si–O bond order density, which represents the cumulative bond strength of SiO bonds, has no direct correlation with bulk mechanical properties which is counterintuitive and against conventional wisdom. The variations are determined more precisely by the overall atomic and electronic structure dictated by bond order density of the Ca–O and hydrogen bonds (HB). Most importantly, there is a multifaceted balance between different types of interatomic bonds including the HBs in controlling mechanical properties. HBs categorized in relation to next nearest neighbor (NNN) enable us to identify specific types of HBs that are prevalent in CSH. In certain crystals such as suolunite, the HB network is organized in such a unique way that enhances its mechanical properties. The approach and findings presented in this paper points to a broad roadmap for the developing next‐generation cements.     

Vibration and dynamic stability of pipes conveying fluid on elastic foundations

The paper deals with the vibration and dynamic stability of cantilevered pipes conveying fluid on elastic foundations. The relationship between the eigenvalue branches and corresponding unstable modes associated with the flutter of the pipe is thoroughly investigated. Governing equations of motion are derived from the extended Hamilton’s principle, and a numerical scheme using finite element methods is applied to obtain the discretized equations. The critical flow velocity and stability maps of the pipe are obtained for various elastic foundation para-meters, mass ratios of the pipe, and structural damping coefficients. Especially critical mass ratios, at which the transference of the eigenvalue branches related to flutter takes place, are precisely determined. Finally, the flutter configuration of the pipe at the critical flow velocities is drawn graphically at every twelfth period to define the order of the quasi-mode of flutter configuration.     

Damping in partially delaminated composites

A simple model was developed to predict the material damping in partially delaminated composites. First, we evaluated the damping loss factors experimentally in three kinds of specimens corresponding to various partial delamination areas. Second, the stiffness loss with delamination growth was assumed to result directly in the loss of energy from the oscillatory system because the delamination due to interlaminar stress is accompanied with stiffness loss in numerous laminated composites. By correlating the laminate stiffness reduction and the corresponding delamination area, a model for their basic material damping properties was formulated using the elastic-viscoelastic principle, the rule-of-mixtures law and modified Hashin’s Model. We predicted the damping of any partially delaminated composites with different stacking sequences based on Adams and Ni’s work and their basic damping loss factors. Numerical and experimental results demonstrate that damping is significantly influenced by the size of delamination area in laminated composites. In addition, experimental improvements in making accurate damping measurements are discussed as well.     

Structure and properties of hydrogrossular mineral series

The hydrogrossular (HG) mineral series (Ca₃Al₂(SiO₄)_3?x(OH)_4x; 0≤x≤3) are water‐bearing minerals found in the upper part of the Earth's mantle and as well as on its surface. They are vital to the planet's hydrosphere under different hydrothermal conditions. The composition and structure of this mineral series are important in geoscience and share many commonalities with cement and clay materials. Other than the end‐members of the series grossular (x=0) and katoite (x=3) which have a cubic garnet structure, the structures of the intermediate‐members are totally unknown. We used large‐scale ab initio modeling to investigate the structures and properties for HG series for x=0, 0.5, 1, 1.5, 2, 2.5, 3. Results indicate that for x>0 and x<3, the structures are tetragonal or nearly tetragonal, which partially justify that this garnet family is known to be cubic or pseudocubic. We also show that there are structural changes related to the disorder introduced with the composition mixing of SiO₄ tetrahedra and AlO₆ octahedra in the series. Based on the models constructed, the mechanical and interband optical properties of the HG series are calculated. For grossular and katoite, results are in good agreement with the available experimental data. The calculated electronic structure, total bond order explains the atomistic origin of the change in the compressibility of the series. Careful analysis of patterns of these results indicates that the series can be roughly divided into three regions: region I (x=0‐0.75), region II (x=0.75‐2.25), and region III (x=2.25‐3) with high, intermediate, and low Si/H ratio respectively. We also point out the inappropriateness of using the ill‐defined concept of treating O₄H₄ as a compressible structural unit in interpreting the structure and properties of HG series. Furthermore, the phonon spectra of grossular and katoite are calculated.     

A study on numerical solution method for efficient dynamic analysis of constrained multibody systems

A new and efficient computational method for constrained multibody systems is proposed. In the proposed method, local parametrization method is employed to apply the same solution method for position, velocity, and acceleration analyses since the coefficient matrices for each analysis have an identical matrix pattern. The skyline solution method is used to overcome numerical inefficiency when solving large scaled equations. Also, subsystem mpartitioning method is derived systematically to perform parallel processing for real time simulation. To show the numerical accuracy and efficiency of the proposed method, three numerical problems are solved.     

Encapsulation of herbal aqueous extract through absorption with ca-alginate hydrogel beads

Encapsulation of herbal aqueous extract through absorption with ca-alginate hydrogel beads was studied. A model herbal aqueous extract, Piper sarmentosum, was used in this study. The effect of process variables (i.e. alginate M/G ratio, alginate concentration, extract concentration, bead size and bead water content) on encapsulation efficiency and biochemical compounds stability were studied. The stability of biochemical compounds was evaluated by using mass balance analysis and FT-IR spectroscopy. The results show that the encapsulation efficiency was mainly affected by alginate M/G ratio and bead water content. In general, ca-alginate beads made of higher alginate M/G ratio or dried to a lower water content were found to absorb significantly more aqueous extract. However, the beads made of higher M/G ratio were less rigid after the absorption process. Besides, the mass balance analysis reveals that the encapsulation process and material did not degrade the bioactive compounds, as the total antioxidant content remained unchanged. This is well supported by the FT-IR analysis where the characteristic bands of chemical groups remained unaltered. Interestingly, the beads made of lower alginate M/G ratio were found to have higher antioxidant affinity. In conclusion, this study demonstrates the potential of using absorption process and hydrogel material for encapsulation of herbal aqueous extract.     

An Abstraction Hierarchy based mobile PC display design in NPP maintenance considering the level of expertise

Recently, the importance of effective maintenance in nuclear power plants (NPPs) has been emphasized and research into effective maintenance by adopting mobile maintenance aids (MMAs) have been attempted. For improved and effective use of an MMA display design method based on the hierarchy is proposed and its design considerations are discussed in this study. Six levels of hierarchy are proposed in this paper to classify the maintenance information. By classifying and organizing maintenance information using the hierarchy, maintenance information can be used effectively by users with either high or low levels of expertise. When information classification has been finished, the information for MMA design is selected and designed. With the considerations of MMA design analysis and guidelines, a hierarchy-based MMA is designed for the maintenance tasks.An experiment is conducted using the hierarchy-based MMA in order to estimate the effectiveness of the proposed method for the maintenance tasks and to identify design considerations to enhance the proposed MMAs. The result indicated that a hierarchy-based manual was more effective than a conventional manual in terms of task completion time and number of errors. The workload for the hierarchy-based manual was estimated less than the conventional manual for subjects with low level of expertise. As the level of expertise increases, subjects tended to follow more abstract information while the number of navigations decreased. It is believed that when mobile devices become pervasive in NPP maintenance fields, the hierarchy model applied MMAs can be used as an effective maintenance supporting tool.     

Design of a robust controller for rollover prevention with active suspension and differential braking

Journal of Mechanical Science and Technology - This paper presents a method to design a robust controller for rollover prevention. Several types of controllers for rollover prevention have been...     

Two FMS scheduling methods based on Petri nets: A global and a local approach

We are interested in the Flexible Manufacturing System (FMS) scheduling problem. Different methods have been explored to solve this problem and to master its combinatorial complexity, which is NP-hard in the general case. In this paper we will give two different scheduling methods based on Petri nets. The first one tends to solve the general scheduling problem (acyclic schedule) using the Constraint Programming method to avoid exhaustive search. The second method is a dedicated cyclic scheduling method. The aim is not to compare the methods' performances (computation time, results' quality) because they do not solve exactly the same problem, but to compare their application domains in terms of parts number.