HKUST Institutional Repository
(5.016 recursos)
Repository of Hong Kong University of Science and Technology. Managed by the HKUST Library.
Mostrando recursos 1 - 20 de 115
1.
Efficient spatial-temporal chaotic mixing in microchannels - Lee, Yi-Kuen; Niu, Xize
A chaotic micro mixer with multiple side channels is designed and investigated, in which fluid can be stirred by pumps through the side channels. By stretching and folding fluid in the main and side channels, chaotic mixing can be achieved. A simple mathematic model is derived to understand the movement of particles in the microchannel. Spatial trajectories of fluid particles are projected to Poincaré sections by mapping. The route from the quasi-period to chaos is revealed to be destruction of KAM curves and shrinkage of the quasi-periodic areas. Lyapunov exponents (LE) are used as the mixing index and the criteria...
2.
Lyapunov exponents of a micro chaotic mixer - Lee, Yi-Kuen
In this paper, we study the Lyapunov exponents of a micro chaotic mixer to understand the mixing effectiveness under different operating parameters. A simple kinematic micro mixer model is used for the simulated flow visualization and then for the computation of Lyapunov exponents using two different numerical methods. These two methods generate consistent result to corroborate the existence of positive Lyapunov exponent which is the key signature of a chaotic system.
3.
Assessment of board level solder joint reliability for PBGA assemblies with lead-free solders - Lee, Ricky Shi-Wei; Lui, Ben Hoi Wai; Kong, Y. H.; Baylon, Bernard; Leung, Timothy; Umali, Pompeo; Agtarap, Hector
Evaluates the board level reliability of plastic ball grid array (PBGA) assemblies under thermal and mechanical loading, with the objective of characterizing the reliability of lead-free solder joints for various assembly conditions. A five-leg experiment was designed which included various combinations of solder materials and peak reflow temperatures. It was found that the lead-free solder joints have a much longer thermal fatigue life than the 63Sn-37Pb solder. The 63Sn-37Pb solder joints seem to perform slightly better than the lead-free solder under mechanical loading.
4.
Mechanical properties of fabric materials for draping simulation - Wu, Z.; Au, C. K.; Yuen, Matthew Ming-Fai
Most of the cloth simulation and modelling techniques rely on the energy function of the system. The geometric deformation is related to the energy function by the fabric material characteristics, which are usually difficult to measure directly. This paper discusses how the fabric material properties are related to the measurable mechanical properties of the fabric such as tensile modulus, Poisson's ratio etc. These properties are incorporated into a cloth simulator to produce draping results. The simulated image and real object are then compared to show the realism.
5.
Electrokinetic generation of microvortex patterns in a microchannel liquid flow - Ng, Alex Sin Wai; Hau, Winky Lap Wing; Lee, Yi-Kuen; Zohar, Yitshak
The technology developed for micropatterning the electric surface charge to be negative, positive or neutral enables the realization of complex liquid flows in simple microchannels. A commercial CFD code is utilized to numerically simulate a variety of electrokinetically-generated liquid flows in a straight and uniform microchannel due to non-uniform surface charge distribution under an externally applied, steady electric field. We present design methodologies to electrokinetically drive vortical flows in any desired direction. In particular, we investigate surface charge patterns required to generate single or multi, co-rotating or counter-rotating, in-plane or out-of-plane vortices. Finally, in view of its potential application to...
6.
Solder joint reliability of cavity-down plastic ball grid array assemblies - Lee, Ricky Shi-Wei; Lau, John H.
A computational model was established in this study to simulate cavity-down plastic ball grid array (PBGA) assemblies. Stress analysis was performed to investigate the solder joint reliability of a PBGA-PCB (printed circuit board) assembly. The packages under investigation had two different body sizes and two kinds of ball population. The diagonal cross-section of the assembly was modeled by plane-strain elements and was subjected to a uniform thermal loading. The solder joints were stressed due to the mismatch of the assembly's coefficient of thermal expansion (CTE). The accumulated effective plastic strain was evaluated as an index for the reliability of solder...
7.
Solder joint reliability of plastic ball grid array with solder bumped flip chip - Lee, Ricky Shi-Wei; Lau, John H.
A computational parametric study on the solder joint reliability of a plastic ball grid array (PBGA) with solder bumped flip chip (FC) is presented. The basic configuration of the PBGA is 27mm package-size and 1.27mm ball-pitch. There were three kinds of ball population: four-row perimeter grid array with/without thermal balls, and full grid array. A total number of 24 cases, involving various chip sizes, chip thicknesses and substrate thicknesses, were studied. The diagonal cross-section of the PBGA-printed circuit board (PCB) assembly was modeled by plane-strain elements and was subjected to uniform thermal loading. Through mismatch of coefficient of thermal expansion...
8.
Analysis on solder ball shear testing conditions with a simple computational model - Lee, Ricky Shi-Wei; Huang, Xingjia
This paper introduces a simple computational model for the analysis on the solder ball shear testing conditions. Both two-dimensional (2-D) and three-dimensional (3-D) finite element models are used to investigate the effect of shear ram speed on the solder ball shear strength of plastic ball grid array (PBGA) packages. An effective thickness is identified for the 2-D finite element analysis. By using this effective thickness as a scale factor, it is shown that the 2D model is feasible for the study of 3-D problems. The computational model is validated by experimental data in terms of load-displacement curves. The results from...
9.
Sensitivity study on material properties for the fatigue life prediction of solder joints under cyclic thermal loading - Lee, Ricky Shi-Wei; Zhang, Xiaowu
A computational study is presented in this paper to investigate the effect of variation in material properties on the fatigue life prediction of solder joints subjected to cyclic thermal loading. The package under investigation was a plastic quad flat pack (PQFP) with gull-wing leads. A commercial finite element code, ABAQUS, was employed to perform a two-dimensional plane stress analysis. While all other constituents of the PQFP assembly were assumed to be linear elastic, the solder joint was considered to be elastic-viscoplastic. The creep model was adopted from Norton's equation and was implemented in the finite element analysis via a user-defined...
10.
Effect of chip dimension and substrate thickness on the solder joint reliability of plastic ball grid array packages - Lee, Ricky Shi-Wei; Lau, J. H.
This paper presents a non-linear numerical study to investigate the effect of chip dimension and substrate thickness on the solder joint reliability of plastic ball grid array (PBGA) packages. The package under investigation was a 225-pin full-grid PBGA assembly. The diagonal cross-section of the PBGA together with the printed circuit board (PCB) was modelled by plane-strain elements. A uniform thermal loading was applied and the solder joints were stressed due to the mismatch of coefficient of thermal expansion (CTE) and constructions of the PCB assembly. The effective stress and accumulated plastic strain of solder balls against various chip dimensions and...
11.
Design for plastic ball grid array solder joint reliability - Lee, Ricky Shi-Wei; Lau, J. H.
Computational stress analysis was performed in this study to investigate the solder joint reliability of plastic ball grid array (PBGA) packages with various configurations. The packages under investigation were 27 mm
body-size, 1.27 mm ball-pitch, perimeter PBGAs with and without thermal balls at the centre. The diagonal cross-section of the PBGA-printed circuit board (PCB) assembly was modelled by plane-strain elements. The model was subjected to a uniform thermal loading and the solder joints were stressed due to the mismatch of coefficient of thermal expansion (CTE). A total number of 24 cases, involving different solder ball populations, chip sizes, and substrate thicknesses,...
12.
Deformation of DNA molecules by hydrodynamic focusing - Lee, Yi-Kuen
The motion of a DNA molecule in a solvent flow reflects the deformation of a nano/micro scale flexible mass-spring structure by the forces exerted by the fluid molecules. The dynamics of individual molecules can reveal both fundamental properties of the DNA and basic understandings of the complex rheological properties of long chain molecules. In this study, we report the dynamic of isolated DNA molecules under homogeneous extensional flow. Hydrodynamic focusing is able to generate homogeneous extensional flow with uniform velocity in the transverse direction. The deformation of individual DNA molecules in the flow was visualized with video fluorescence microscopy. A...
13.
Micro bubble dynamics in DNA solutions - Deng, Peigang; Lee, Yi-Kuen; Cheng, Ping
Micro bubble generation and its subsequent dynamic behavior in single-stranded DNA (ssDNA) solutions are presented in this paper. A micro vapor bubble was generated in ssDNA using a micro bubble actuator, which is capable of producing periodic and stable single vapor bubbles under pulse heating. The growth and collapse of the micro vapor bubble were visualized by a high-speed CCD camera, and the bubble dynamics was investigated at different ssDNA concentrations and under various pulse widths. It was observed that an increase in the ssDNA concentration led to an increase of the electric power required for incipient bubble nucleation. Based...
14.
Development and characterization of a rotary motor driven by anisotropic piezoelectric composite laminate - Lee, Ricky Shi-Wei; Li, H. L.
A new actuation principle is introduced in this paper to drive a rotary motor by an anisotropic piezoelectric composite laminate. The driving element is a three layer laminated beam with piezoceramics sandwiched between two anti-symmetric composite laminae. By taking advantage of material anisotropy, torsional motion can be induced from in-plane strain actuation. With this structural coupling, a rotary motor can be implemented. In addition to analytical formulation and conceptual design, a prototype has been fabricated. Actual motion was observed in the laboratory to verify the proposed actuation principle. The prototype was characterized for rotating speed, torque, power output, efficiency and...
15.
Effects of electric fields on the bending behavior of PZT-5H piezoelectric laminates - Cheng, Jinquan; Qian, Caifu; Zhao, Minghao; Lee, Ricky Shi-Wei; Tong, Pin; Zhang, Tong-Yi
This paper investigates the bending behavior of piezoelectric laminates under combined mechanical and electrical loads. The laminates have a PZT-5H ceramic core sandwiched between two graphite/epoxy plates. Three-point bending tests and in situ acoustic emission measurements were conducted on the PZT-5H sandwich laminates with a sustained applied electric field. The results show that the PZT-5H core fractures first and then delaminaton occurs along the tensile stressed interface between the PZT ceramic and a graphite/epoxy layer. Finite element analysis was performed to analyze the stresses in the sandwich structure and the energy release rate for interface delamination under combined mechanical and...
16.
Modeling of torsional vibration induced by extension-twisting coupling of anisotropic composite laminates with piezoelectric actuators - Zhu, Mei-Ling; Lee, Ricky Shi-Wei; Li, Hing Leung; Zhang, Tong-Yi; Tong, Pin
In this paper we present a dynamic analytical model for the torsional vibration of an anisotropic piezoelectric laminate induced by the extension-twisting coupling effect. In the present approach, we use the Hamilton principle and a reduced bending stiffness method for the derivation of equations of motion. As a result, the in-plane displacements are not involved and the out-of-plane displacement of the laminate is the only quantity to be calculated. Therefore, the proposed method turns the twisting of a laminate with structural coupling into a simplified problem without losing its features. We give analytical solutions of the present model with harmonic...
17.
Buckling of polysilicon microbeams during sacrificial layer removal - Zhang, Tong-Yi; Zhang, Xin; Zohar, Yitshak
In situ observations of buckling evolution of polysilicon microbeams during etch of the underneath sacrificial layer were carried out under an optical microscope. The surface geometry was obtained by AFM measurements. As the etching progressed, three buckling patterns were identified. Closed formulas were derived from theoretical analysis based on both boundary conditions: simply supported and clamped. The theory predicts either the buckling pattern for a given residual stress or the compressive stress level for a given buckling pattern. The residual stress evaluated from the buckling pattern agrees with that measured by the curvature method.
18.
High accuracy optical particle sizing in phase-Doppler anemometry - Qiu, Hui-He; Jia, W.; Hsu, Chin-Tsau; Sommerfeld, M.
A novel structure for phase-Doppler anemometry (PDA) is proposed in order to eliminate the measurement-volume effect (MVE). This new PDA system consists of four receiving detectors in an optimized optical orientation. The method for the determination of the optimized optical orientation angle is introduced. In this method the vector sum of refractive and reflective rays is taken into consideration in describing a dual-mechanism-scattering model caused by a nonuniformly illuminated PDA measurement volume. The constraint of the single-mechanism-scattering model in the conventional PDA is removed. As a result the error caused by MVE, which consists of a Gaussian beam defect and...
19.
The variant selection criteria in single-crystal CuAlNi shape memory alloys - Zhang, X. Y.; Brinson, L. C.; Sun, Qing-Ping
Two variant selection criteria are often used in shape memory alloys (SMAs). One is the maximum resolved shear stress (RSS) criterion and the other is the maximum transformation work criterion. In this paper these two measures are calculated using the crystallographic theory of martensite (CTM) for the shape memory effect (SME) case and the crystallographic theory of martensite + detwinning, distortion and slip (CTM + DDS) for the
superelasticity (SE) case. The results are compared with the experimental measurements of a CuAlNi single-crystal SMA. The comparisons show that work is better as a variant selection criterion than RSS. In the SME...
20.
Modelling and test of a thermally-driven phase-change nonmechanical micropump - Song, Y. J.; Zhao, Tian-Shou
A thermally driven phase-change nonmechanical micropump has been investigated theoretically and experimentally. This micropump consisted of a microchannel and a number of uniformly-spaced heating elements along the channel. The pumping of fluids in such a pump was realized by using the actuation of a moving vapour slug (bubble) generated by suitably phased heating elements. The pumping mechanism was studied theoretically by considering a liquid-filled tube heated locally by a moving heating source. To verify the theoretical analysis, a pumping device consisting of a microchannel with twelve embedded heaters along the channel was fabricated and tested using deionized water as the...
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