Kian Meng Lim
Email Address
mpelimkm@nus.edu.sg
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115 results
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Publication Analytical solutions for interconnect stress in board level drop impact(2007-11) Wong, E.H.; Mai, Y.-W.; Seah, S.K.W.; Lim, K.-M.; Lim, T.B.; MECHANICAL ENGINEERINGClosed form analytical solutions for the stresses in the interconnects between the integrated circuit (IC) package and the printed circuit board (PCB) when the PCB assembly is subjected to a mechanical shock have been developed and validated. The solutions offer useful insights into the mechanics of board level interconnects when subjected to mechanical shock, and have been used to establish the following key findings: 1) for the same magnitude of strain measured on the PCB, symmetric bending will result in the highest stress in the interconnect while anti-symmetric bending will result in the least stress; 2) the cross-section area of the interconnect is the single most critical parameter; 3) the eight-layer buildup board specified in JEDEC standard JESD22-B111 can be replaced with an equivalent conventional board that exhibits similar natural frequency as the eight-layer buildup board. © 2007 IEEE.Publication Three-dimensional singular boundary elements for corner and edge singularities in potential problems(2005-02) Ong, E.T.; Lim, K.M.; MECHANICAL ENGINEERINGIt is well known that the spatial derivative of the potential field governed by the Laplace and Poisson equations can become infinite at corners (in two and three dimensions) and edges (in three dimensions). Conventional elements in the finite element and boundary element methods do not give accurate results at these singular locations. This paper describes the formulation and implementation of new singular elements for three-dimensional boundary element analysis of corner and edge singularities in potential problems. Unlike the standard element, the singular element shape functions incorporate the correct singular behavior at the edges and corners, specifically the eigenvalues, in the formulation. The singular elements are used to solve some numerical examples in electrostatics, and it is shown that they can improve the accuracy of the results for capacitance and electrostatic forces quite significantly. The effects of the size of the singular elements are also investigated. © 2004 Elsevier Ltd. All rights reserved.Publication Comparison between the radial point interpolation and the Kriging interpolation used in meshfree methods(2003-09) Dai, K.Y.; Liu, G.R.; Lim, K.M.; Gu, Y.T.; MECHANICAL ENGINEERINGThe radial point interpolation method (RPIM) based on local supported radial basis function (RBF) and the Galerkin weak form has been developed and successfully applied to many engineering problems. Recently, a new meshfree method was proposed based on the universal moving Kriging interpolation. This paper studies the difference between the meshfree shape functions created based on the point interpolation and the Kriging interpolation. It is found that both the two methods yield the same shape function as long as the same radial basis function or semivariogram is adopted for interpolation. Although the two methods lead to the same shape function, the theorem in Kriging formulation may provide an alternative theoretical support for the RPIM. Some common semivariograms used in Kriging may also be incorporated in the RPIM. In addition, in order to satisfy the conformability requirements, a penalty technique is introduced in this paper to form a conforming Kriging, which can pass the standard patch test exactly.Publication A fast boundary element method for underwater acoustics(2005-12-20) Lim, K.M.; Ong, E.T.; Lee, H.P.; Lim, S.P.; MECHANICAL ENGINEERINGA fast algorithm for the boundary element method is developed to handle problems in underwater acoustics. The algorithm employs the multipole and local expansions to approximate far-field potentials, and exploits the discrete convolution nature of mapping multipole to local expansions to accelerate the potential evaluation process. The speedup in the solution process is achieved by fast Fourier transform on the multipole and local expansion coefficients on a regular grid. The method is demonstrated by a three-dimensional underwater acoustics scattering problem, and it is found to achieve accurate results with relatively low order of expansion. © World Scientific Publishing Company.Publication Finite element modeling of electronic packages subjected to drop impact(2005-09) Tan, V.B.C.; Tong, M.X.; Lim, K.M.; Lim, C.T.; MECHANICAL ENGINEERINGAs more electronic products become portable, many product manufacturers have started to pay more attention to the robustness of their products. Finite element (FE) simulation has become increasingly popular in the analysis of products subjected to impact loading. The need for details in a FE mesh is always balanced by considerations of simulation time and available computational resources. In this paper, three commonly used approaches to FE modeling of a ball grid array (BGA) package subjected to drop impact are evaluated. The first model comprises a detailed mesh of the printed circuit board (PCB), integrated circuit (IC) package and interconnecting solder balls using solid three-dimensional (3-D) elements. The degrees of freedom is reduced for the second mesh by using shell elements for the PCB and IC package while retaining the detailed mesh of the solder balls using solid 3-D elements. The third mesh is a further simplification of the second mesh whereby the solder balls are replaced by a single beam element each. The stresses within the solder balls are then obtained in a separate FE analysis of a detailed solder ball mesh using the displacement history of nodes around the beam elements from the previous analysis as inputs. Solder ball stresses from all three meshes were found to differ by as much as 40% although PCB deflection compared favorably. © 2005 IEEE.Publication On-chip trapping and characterization of Cryptosporidium using surface coated ITO-PDMS bonded chips(2011) Unni, H.N.; Hartono, D.; Lim, K.-M.; MECHANICAL ENGINEERING; CHEMICAL & BIOMOLECULAR ENGINEERINGDielectrophoresis has been shown to have significant potential for the characterization of cells and could become an efficient tool for rapid identification and assessment of microorganisms. The present work is focused on the trapping and characterization of Cryptosporidium pathogen using a microfluidic chip fabricated through a simple, effective bonding of surface coated ITO glass and PDMS. Lithographically patterned ITO glass plates were coated with an alkoxysilane solution DMOAP (N, N - dimethyl-N-octadecyl-3- aminopropyltrimethoxysilyl chloride) prior to bonding with PDMS microchannels. The silane coating was found to enhance the bonding between ITO and PDMS and reduce cell adhesion on the electrodes. Cryptosporidium oocysts, which are 2-4 microns in size and nearly spherical in shape represent the preliminary stage of cell development. The dielectrophoretic transport of cells is dependent on electrical properties such as permittivity and conductivity of the cells. Computational simulations were performed in order to study the effects of channel height, buffer conductivity and applied voltage on the flow and cell transport inside the DEP chip and facilitate effective cell trapping. Videomicroscopic experiments were performed using the fabricated device and the real part of Clausius-Mossotti factor of the cells was estimated from values of critical voltages of particle trapping (corresponding to various field frequencies) at the electrodes. The dielectric properties of the cell compartments (cytoplasm and membrane) were calculated based on a single shell model of the Cryptosporidium oocyst cell. © (2011) Trans Tech Publications, Switzerland.Publication An adaptive mesh redistribution method for the incompressible mixture flows using phase-field model(2007-07-01) Tan, Z.; Lim, K.M.; Khoo, B.C.; SINGAPORE-MIT ALLIANCE; MECHANICAL ENGINEERINGA phase field model which describes the motion of mixtures of two incompressible fluids is presented by Liu and Shen [C. Liu, J. Shen, A phase field model for the mixture of two incompressible fluids and its approximation by a Fourier-spectral method, Phys. D 179 (2003) 211-228]. The model is based on an energetic variational formulation. In this work, we develop an efficient adaptive mesh method for solving a phase field model for the mixture flow of two incompressible fluids. It is a coupled nonlinear system of Navier-Stokes equations and Allen-Cahn phase equation (phase-field equation) through an extra stress term and the transport term. The numerical strategy is based on the approach proposed by Li et al. [R. Li, T. Tang, P.-W. Zhang, Moving mesh methods in multiple dimensions based on harmonic maps, J. Comput. Phys. 170 (2001) 562-588] to separate the mesh-moving and PDE evolution. In the PDE evolution part, the phase-field equation is numerically solved by a conservative scheme with a Lagrange multiplier, and the coupled incompressible Navier-Stokes equations are solved by the incremental pressure-correction projection scheme based on the semi-staggered grid method. In the mesh-moving part, the mesh points are iteratively redistributed by solving the Euler-Lagrange equations with a parameter-free monitor function. In each iteration, the pressure and the phase are updated on the resulting new grid by a conservative-interpolation formula, while the velocity is re-mapped in a non-conservative approach. A simple method for preserving divergence-free is obtained by projecting the velocity onto the divergence-free space after generating the new mesh at the last iterative step. Numerical experiments are presented to demonstrate the effectiveness of the proposed method for solving the incompressible mixture flows. © 2007 Elsevier Inc. All rights reserved.Publication Response suppression and transient behavior in a nonlinear active cochlear model with feed-forward(2003-09) Lim, K.-M.; Steele, C.R.; MECHANICAL ENGINEERINGA nonlinear active cochlear model is used to simulate the steady-state frequency response and transient response to clicks of the basilar membrane. The model includes the three-dimensional viscous fluid effects, an orthotropic cochlear partition with dimensional and material property variation along its length, and a nonlinear active feed-forward mechanism to represent the activity of the outer hair cells. A hybrid asymptotic and numerical method is used to provide a fast and efficient iterative procedure for modeling and simulation of the nonlinear responses in the active cochlea. The simulation results exhibit some of the characteristic nonlinear behavior of the basilar membrane commonly observed in experimental measurements, such as significant amplification and sustained "ringing" in the transient response at low stimulus level. The simple feed-forward mechanism is able to capture the properties of the noncausal active process in the cochlea without a second filter or resonance. © 2003 Elsevier Ltd. All rights reserved.Publication A three-dimensional nonlinear active cochlear model analyzed by the WKB-numeric method(2002-08) Lim, K.-M.; Steele, C.R.; MECHANICAL ENGINEERINGA physiologically based nonlinear active cochlear model is presented. The model includes the three-dimensional viscous fluid effects, an orthotropic cochlear partition with dimensional and material property variation along its length, and a nonlinear active feed-forward mechanism of the organ of Corti. A hybrid asymptotic and numerical method combined with Fourier series expansions is used to provide a fast and efficient iterative procedure for modeling and simulation of the nonlinear responses in the active cochlea. The simulation results for the chinchilla cochlea compare very well with experimental measurements, capturing several nonlinear features observed in basilar membrane responses. These include compression of response with stimulus level, two-tone suppressions, and generation of harmonic distortion and distortion products. © 2002 Elsevier Science B.V. All rights reserved.Publication NIR-to-visible upconversion nanoparticles for fluorescent labeling and targeted delivery of siRNA(2009) Jiang, S.; Zhang, Y.; Lim, K.M.; Sim, E.K.W.; Ye, L.; BIOENGINEERING; MECHANICAL ENGINEERINGNear-infrared (NIR)-to-visible upconversion fluorescent nanoparticles were synthesized and used for imaging and targeted delivery of small interfering RNA (siRNA) to cancer cells. Silica-coated NaYF4 upconversion nanoparticles (UCNs) co-doped with lanthanide ions (Yb/Er) were synthesized. Folic acid and anti-Her2 antibody conjugated UCNs were used to fluorescently label the folate receptors of HT-29 cells and Her2 receptors of SK-BR-3 cells, respectively. The intracellular uptake of the folic acid and antibody conjugated UCNs was visualized using a confocal fluorescence microscope equipped with an NIR laser. siRNA was attached to anti-Her2 antibody conjugated UCNs and the delivery of these nanoparticles to SK-BR-3 cells was studied. Meanwhile, a luciferase assay was established to confirm the gene silencing effect of siRNA. Upconversion nanoparticles can serve as a fluorescent probe and delivery system for simultaneous imaging and delivery of biological molecules. © 2009 IOP Publishing Ltd.