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.     

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.     

Walking pattern generation employing DAE integration method

A stable walking pattern generation method for a biped robot is presented in this paper In general, the ZMP (zero moment point) equations, which are expressed as differential equations, are solved to obtain a stable walking pattern However, the number of differential equations is less than that of unknown coordinates in the ZMP equations It is impossible to integrate the ZMP equations directly since one or more constraint equations are involved in the ZMP equations To overcome this difficulty, DAE (differential and algebraic equation) solution method is employed The proposed method has enough flexibility for various kinematic structures Walking simulation for a virtual biped robot is performed to demonstrate the effectiveness and validity of the proposed method The method can be applied to the biped robot for stable walking pattern generation     

Production of Branched Cyclodextrins by Reverse Reaction of Microbial Debranching Enzymes

Formation of maltosyl cyclodextrins from mixtures of maltose and cyclodextrins by reverse reactions of Flavobacterium isoamylase and Klebsiella pullulanase was experimented and it was found that Klebsiella pullulanase produced 50.4mg maltosyl α-cyclodextrin, 35.0mg maltosyl β-cyclodextrin and 55.4mg maltosyl γ-cyclodextrin per ml of reaction mixture, whereas Flavobacterium isoamylase did not form maltosyl cyclodextrins. Optimum conditions for formation of maltosyl β-cyclodextrin by Klebsiella pullulanase were pH 4, 50°C reaction temperature and proportion of substrate 100mg β-cyclodextrin/600mg maltose per ml.     

Predictive end-point trajectory control of elastic manipulators

This article presents an approach to end-point trajectory control of elastic manipulators based on the nonlinear predictive control theory. Although this approach is applicable to manipulators of general configuration, only planar flexible multi-link manipulators are considered. A predictive control law is derived by minimizing a quadratic function of the trajectory error of the end-points of each link, elastic modes, and control torques. This approach avoids the instability of the zero dynamics encountered in the controller design using feedback linearization and variable structure control techniques for end-point control. Furthermore, the derived predictive controller is robust to uncertainty in the system parameters. Simulation results are presented for a one-link flexible manipulator to show that in the closed-loop system accurate end-point trajectory control and vibration damping can be accomplished. © 1996 John Wiley & Sons, Inc.     

Scheduling cluster tools in wafer fabrication using candidate list and simulated annealing

This paper presents a new method for scheduling cluster tools in semiconductor fabrication. A cluster tool consists of a group of single-wafer chambers organized around a wafer transport device, or robot. Cluster fabrication system considered in this paper consists of serial cluster tools. Due to constraints imposed by multiple routes of each wafer type and machines with no buffer, it is difficult to find an optimal or near-optimal schedule. In order to determine the sequence of the operations to be released and the assignment of the machine to each operation, the proposed method uses a job requirement table with random keys as a solution representation. Simulated annealing seeks the optimal or near-optimal sequence and machine assignment of the operations. In this paper, the scheduling objective is to find a schedule with minimum makespan. A Gantt chart is obtained as the final schedule. To handle the constraints, the proposed method uses a candidate list. To determine which operation can be scheduled in considering the constraints, a negotiation procedure between the operations in the candidate list and a current state of the system is introduced. To show the effectiveness of the proposed method, scheduling example of a real cluster fabrication system is presented. Scheduling results are compared with those obtained by using several dispatching rules. From the experimental results, it is shown that the proposed method is promising.