Bump Configuration Research Articles

Neural Network with Local Converging Input for Unstructured-Grid Computational Fluid Dynamics

In recent years, surrogate models based on deep neural networks have been widely used to solve partial differential equations for fluid flow physics. This kind of model focuses on global interpolation of the training data and thus requires a large network structure. The process is both time consuming and computationally costly. In the present study, we develop a neural network with local converging input (NNLCI) for high-fidelity prediction using unstructured data. The framework uses the local domain of dependence with converging coarse solutions as input, thereby greatly reducing computational resource and training time. As a validation case, the NNLCI method is applied to study two-dimensional inviscid supersonic flows in channels with bumps. Different bump geometries and locations are examined to benchmark the effectiveness and versatility of this new approach. The NNLCI method can accurately and efficiently capture the structure and dynamics of the entire flowfield, including regions with shock discontinuities. For a new bump configuration, the method can perform prediction with only one neural network, eliminating the need for repeated training of multiple networks for different geometries. A saving of computing wall time is achieved by several orders of magnitude against the high-fidelity simulation with the same level of accuracy. The demand on training data is modest, and the training data can be allocated sparsely. These features are especially advantageous compared with conventional global-to-global deep learning methods and physics-informed methods.

Surrogate-based optimization on bump for shock wave/boundary layer interaction control

Supersonic and hypersonic vehicles have complex shock wave/boundary layer interaction problems during flight, and it adversely affect the aerodynamic performance of the vehicle. In this paper, the bump passive control approach is introduced to control the shock wave/boundary layer interaction, and its configuration is optimized with the minimum separation area and the maximum total pressure recovery coefficient. The Kriging model is established by means of the Latin hypercube experimental design approach to obtain the sample points. The Non-dominated Sorting Genetic Algorithm-II (NSGA-II) algorithm is utilized to optimize the parameters of the bump configuration, and the flow field structures of the uncontrolled condition, the special basic condition and the optimized condition are compared and analyzed. The obtained results show that the surrogate-based optimized model shows better performances than the basic condition. In the parameter setting range considered in this study, the optimized Case 3c with the middle position in the optimization front owns a larger total pressure recovery coefficient and a smaller separation area, with its separation area and total pressure recovery coefficient being 0.80 mm2 and 67.39% respectively.

Design and optimization of bump (compression surface) for diverterless supersonic inlet

In this article design and optimization scheme of a three-dimensional bump surface for a supersonic aircraft is presented. A baseline bump and inlet duct with forward cowl lip is initially modeled in accordance with an existing bump configuration on a supersonic jet aircraft. Various design parameters for bump surface of diverterless supersonic inlet systems are identified, and design space is established using sensitivity analysis to identify the uncertainty associated with each design parameter by the one-factor-at-a-time approach. Subsequently, the designed configurations are selected by performing a three-level design of experiments using the Box–Behnken method and the numerical simulations. Surrogate modeling is carried out by the least square regression method to identify the fitness function, and optimization is performed using genetic algorithm based on pressure recovery as the objective function. The resultant optimized bump configuration demonstrates significant improvement in pressure recovery and flow characteristics as compared to baseline configuration at both supersonic and subsonic flow conditions and at design and off-design conditions. The proposed design and optimization methodology can be applied for optimizing the bump surface design of any diverterless supersonic inlet system for maximizing the intake performance.

Effect of adhesive force on underfill process based on lattice Boltzmann method

Purpose The purpose of this study is to investigate the effect of the adhesive force and density ratio using lattice Boltzmann method (LBM) during underfill process. Design/methodology/approach To deal with complex flow in underfill process, a framework is proposed to improve the lattice Boltzmann equation. The fluid flows with different density ratio and bump arrangement in underfill are simulated by the incorporated Carnahan–Starling (CS) equation of state (EOS). The numerical study conducted by finite volume method (FVM) and experimental results are also presented in each case at the different filling percentage for verification and validation purpose. Findings The numerical result is compared well with those acquired experimentally. Small discrepancy is detected in their flow profile. It was found that the adhesive force between fluid and solid was affected by the density ratio of the fluids and solder bump configuration. LBM has shown better adhesive force effect phenomenon on underfill process compared to FVM. LBM also demonstrated as a better tool to study the fluid flow in the underfill process. Practical implications This study provides a basis and insights into the impact of adhesive force and density ratio to the underfill process that will be advancing the future design of flip chip package. This study also provides superior guidelines, and the knowledge of how adhesive force is affected by flip chip package structure. Originality/value This study proposes the method to predict the adhesive force and density ratio effect for underfill flow in flip chip package. In addition, the proposed method has a good performance in representing the adhesive force during the underfill simulation for its natural physical basic. This study develops understanding of flow problems to attain high reliability for electronic assemblies.

Bacterial detachment from a wall with a bump line.

The interactions of bacteria with surfaces have important implications in numerous areas of research, such as bioenergy, biofilm, biofouling, and infection. Recently, several experimental studies have reported that the adhesion of bacteria can be reduced considerably by microscale wall features. To clarify the effect of wall configurations, we numerically investigated the behavior of swimming bacteria near a flat wall with a bump line. The results showed that the effects of bump configuration are significant; a detachment time larger than several seconds can be achieved in certain parameter sets. These results illustrate that the number density of bacteria near the wall may be reduced by appropriately controlling the parameter sets. When background shear flow was imposed, the near-wall bacterium mainly moved towards the vorticity axis. The detachment time of cells increased significantly by adjusting the bump line to have 45^{∘} relative to the flow direction. The knowledge obtained in this study is fundamental for understanding the interactions between bacteria and surfaces according to more complex geometries, and is useful for reducing the adhesion of cells to walls.

Inverse design and Mach 6 experimental investigation of a pressure controllable bump

The low kinetic flow starting from the airframe leading edge is a neglected aspect in designing hypersonic air-breathing flight vehicles. Compared with other boundary layer treating technologies, the bump concept can obtain a good balance of boundary layer removal, external drag control, shock system simplification, and integration design flexibility capabilities. On the basis of conventional conical-flow theory and the new 3D inverse design method, this study proposes a pressure controllable bump concept that can generate the bump configuration inversely by the prescribed pressure distribution. The bump/inlet integration pattern is analyzed, and the basic design methodology is presented. To validate whether the pressure distribution can be used in diverting the boundary layer, experimental study of the bump and numerical simulation are conducted. Results show that the bump has generated identical pressure distribution to the design. The bump can also divert approximately 50% of the boundary layer from the incoming low kinetic flow in Mach 6. Compared with the conventional cone-derived bump in Mach 6, the new bump is 25.8% shorter in height. The flow structure is adjusted nearly parallel to the x-direction, thereby promoting the flow quality of the inlet entrance. Hence, the new inverse design method of pressure distribution expands the applicable Mach range of the hypersonic airframe forebody.

CPI Parametric Investigation of UBM-Al Interface for Cu Pillar Flip-Chip Application

Under bump metallization (UBM)–Al delamination is a high-value chip-package interaction reliability problem in flip-chip Cu pillar packaging. We devised a test vehicle for quick reliability assessment. We demonstrate that preclean is a critical step to ensure reliable UBM-Al adhesion and achieve bump shear strength >12 g/mil2 for 15- $\mu \text{m}$ polymer opening. Key parameters affecting bump shear strength, such as UBM critical dimension (CD), polyimide (PI) CD, SiN CD, and PI thickness, were reviewed from both simulation and experimental standpoints and good agreement was achieved between the two. Based on the observed failure mode, a 1-D model was also proposed to correctly predict the experimental trend. Optimum design rules for Cu pillar were outlined. Chip-substrate assembly was demonstrated on 80- $\mu \text{m}$ bump pitch with staggered bump configuration using mass reflow. Progressive thermal cycling was performed to evaluate package performance. Failure mode was found to be solder crack, verifying the improvement in UBM-Al adhesion.

Global Instability Analysis of Laminar Boundary Layer Flow Over a Bump at Transonic Conditions

Modal three-dimensional BiGlobal linear instability analysis is performed in steady, spanwise-homogeneous two-dimensional laminar compressible boundary-layer flow past a millimeter-tall hemispherical bump at transonic conditions. Starting with subsonic inlet flow, at the flow conditions considered a stationary shock is formed near the downstream end of the bump. The interplay of shock and adverse-pressure-gradient results in a steady spanwise homogeneous laminar two-dimensional laminar separation bubble being formed at the downstream end of the bump.The objective of the present analysis is to interrogate this basic flow with respect to its potential to sustain low-frequency unsteadiness arising from linear amplification of unstable traveling global flow eigenmodes. Such unsteadiness, coupled to eigen- frequencies of the structure, can lead to resonance phenomena that are detrimental for the performance and adversely affect the efficiency of systems on which the bump configuration is employed. Only damped global eigenmodes have been identified at the parameters examined, pointing to the possibility of the above mentioned unsteadiness being the result of algebraic instability.

Hiding a Bump on a PEC Plane by Using an Isotropic Lossless Dielectric Layer

An infinite perfect electric conducting (PEC) plane on top of which lies a 2-D PEC object (bump) is excited by a plane wave. The main purpose of this work concerns the determination of the appropriate permittivity er and thickness H of a lossless dielectric superstrate slab layer, placed on top of the overall structure of the PEC bump and the PEC plane, such that two objectives are fulfilled: 1) the plane wave reflection by the grounded slab configuration in the absence of the bump resembles closely that by the infinite PEC plane, and simultaneously 2) the contribution due to the presence of the bump itself on the scattered far-field radiated over all directions becomes as small as possible. A semianalytic integral equation methodology is developed to determine the field scattered by the bump configuration. Numerical results are presented demonstrating that indeed for certain regions of values of the superstrate's parameters both objectives 1) and 2) are met within significant numerical accuracy. Particularly, it is shown that may offer optimal cloaking characteristics. Finally, the results of the analysis as well as of the developed two objectives strategy are verified by appropriate computational simulations.

Contact Resistance of Flip-Chip Joints in Wearable Electronic Textiles

Flip-chip bonding to a Cu lead frame transferred to a fabric was achieved by use of a non-conducting adhesive. Average contact resistance of the flip-chip joints was evaluated on variation of the Cu and Sn thickness of Cu/Sn bumps of size 150 × 220 μm2. The total thickness of the Cu/Sn bumps was fixed at 15 μm. The average contact resistance of the flip-chip joints on the fabric was 5.4–10.8 mΩ, depending on the Sn thickness of the Cu/Sn bumps; this was lower than for flip-chip joints on a rigid Si substrate (15.6–26.5 mΩ). The average contact resistance of flip-chip joints on the fabric decreased from 10.8 mΩ to 5.5 mΩ when the chip–bump configuration was changed from 15-μm-thick Sn to 7-μm-thick Cu/8-μm-thick Sn. The contact resistance of flip-chip joints bonded with the 7-μm-thick Cu/8-μm-thick Sn bumps remained below 10 mΩ for up to 750 h in the 85°C/85% relative humidity test and even decreased to below 4 mΩ in the storage test at 125°C for up to 1000 h.

Analysis of thermal characteristics and mechanism of degradation of flip-chip high power LEDs

The purpose of this study is to investigate the thermal behavior at the die-attached interfaces of flip-chip GaN high-power light emitting diodes (LEDs) using a combination of theoretical and experimental analyses. The results indicate that contact thermal resistance increased dramatically at the die-attached interfaces with aging time and stress, degrading the luminous flux. The junction temperature and thermal uniformity of the flip-chip structure both strongly depend on the arrangement of gold bumps. Local hot spots effectively reduce light output under high electric and thermal stress, influencing the long-term performance of the LED device. The results were validated using finite element analysis and in experiments using an infrared and an emission microscope. A two-step thermal transient degradation mode was identified under various aging stresses. A simulation further optimized the bump configuration that was associated to yield a low junction temperature and high temperature uniformity of the LED chip. Accordingly, the results are helpful in enhancing the performance and reliability of high-power LEDs.

A chip-level electrothermal-coupled design model for high-power light-emitting diodes

An advanced three-dimensional electrothermal-coupled simulation model basing on finite-element method numerical simulation is developed to study the electrical and thermal properties of chip-level high-power GaN-based light-emitting diodes (LEDs). The current spreading, heat generation, and transfer in the device are comprehensively considered in this model. The current-spreading effect of the transparent current-spreading layer and the thermal performance of LEDs with interdigitated-electrodes are investigated. The simulation results prove that the temperature distribution in the active layer is strongly affected by the electrode pattern. The obvious heat accumulation in LEDs with conventional interdigitated-electrode patterns can be seen both in the simulated results and the infrared measured results. The heat transfer efficiency can be improved by using a symmetry electrode pattern design. The thermal management of the bump configurations in flip-chip LEDs is also studied. A more reasonable and thermal effective bump configuration is presented, and the simulated results show that a lower average temperature and more uniform heat distribution in the chips can be obtained.

Thermal Study of High-Power Nitride-Based Flip-Chip Light-Emitting Diodes

This paper presents a chip-level thermal study of high-power nitride-based flip-chip (FC) light-emitting diodes (LEDs). In order to understand the thermal performance of the high-power FC LEDs thoroughly, a quantitative parametric analysis of the thermal dependence on the chip contact area, bump configuration, and bump defects was performed by finite-element model (FEM) numerical simulation and thermal infrared (IR) microscopy testing, respectively. FEM numerical simulation results proved that the optimized bump configuration design was essential to get a uniform temperature distribution in the active layer and improve the thermal performance of the FC LED. IR microscopy testing results recognized that bump defects formed in the LED chip solder processing would lead to surface hot spots around the vicinity of these bump defects, particularly under high-current working conditions. In addition, a light-emitting dark zone was also observed in the optical field for FC LEDs with bump defects. In summary, optimized LED FC bump configuration design and good bonding quality in the chip bonding process are proved to be critical for improving the thermal performance and extending the operating longevity of high-power FC LEDs.

Application of Al/PI composite bumps to COG bonding process

This work demonstrates the probing, testability and applicability of Al/PI (aluminum/polyimide) composite bumps to the chip on-glass (COG) bonding process for liquid crystal display (LCD) driver chip packaging. The experimental results showed that the thickness of Al overlayer on PI core of the bump, the location of pin contact, and the bump configuration affect bump probing testability. The bump with type IV configuration prepared in this work exhibited excellent probing testability when its Al overlayer thickness exceeded 0.8 /spl mu/m. We further employed Taguchi method to identify the optimum COG bonding parameters for the Al/PI composite bump. The four bonding parameters, bonding temperature, bonding time, bonding pressure and thickness of Al overlayer are identified as 180/spl deg/ C, 10 s, 800 kgf/cm/sup 2/ and 1.4 /spl mu/m, respectively. The optimum bonding condition was applied to subsequent COG bonding experiments on glass substrates containing Al pads or indium tin oxide (ITO) pads. From the results of resistance measurement along with a series of reliability tests, Al pad is found to be a good substrate bonding pad for Al/PI bump to COG process. Excellent contact quality was observed when the bumps had Al overlayer thickness over 1.1 /spl mu/m. As to the COG specimens with substrate containing ITO pads, high joint resistance suggested that further contact quality refinement is necessary to realize their application to COG process.

Optimization of the HBT and bump configuration for bump heat sink structure and application to HBT power MMICs

Suitable HBT and bump configuration for the BHS (Bump Heat Sink) structure are experimentally investigated. The best results in terms of thermal and electrical characteristics is obtained from the combination of a single emitter finger and an “H” shaped bump. Power HBTs are demonstrated with typical power added efficiencies η add of 70% at 8.0 W and a linear power vs transistor size relation up to 10 W at 0.9 GHz CW with V cc = 6 V, using the optimized structure. A three-stage HBT MMIC power amplifier and a power amplifier module for GSM class 4 are demonstrated with η add of 53% at 5.0 W, for V cc = 6 V CW operation, also using the optimized structure.

Compliant Foil Bearing Structural Stiffness Analysis—Part II: Experimental Investigation

This paper describes the second part of an investigation into the mechanism of deformation of the corrugated foil (bump foil) strips used in compliant surface foil bearings. In the earlier work, a theoretical model was developed to predict the structural characteristics of bump foil strips under various loads, including the effects of the friction forces between the compliant elements, local interaction forces, load distribution profiles, and bump configurations. In the experiments described here in, two-dimensional deflections of bump foils were recorded via an optical tracking system for a wide range of operating conditions to verify the feasibility of the theoretical model. Test results corroborate the theoretical model for the linear regions of load and the deflection parameters. The effects of the bearing design parameters, such as bump configuration, load profile, and surface coating and lubricant, on the structural characteristics of the bump foil strip were investigated. In addition, the source and mechanism of nonlinear behavior of the bump foil strips under light load conditions were examined, and more effective methods of achieving both Coulomb damping and optimum structural compliance were investigated. An understanding of the analytical and semi-empirical relations resulting from this work offers designers the potential for enhancing the design of high-performance compliant foil bearings.

AN IMPLICIT FLUX‐DIFFERENCE SPLITTING METHOD FOR SOLVING THE EULER EQUATIONS ON ADAPTIVE TRIANGULAR GRIDS

An implicit method for the solution of transonic flows modelled by the time‐dependent Euler equations is presented. The method is characterized by a robust linearization for first‐ and second‐order versions of Roe's flux‐difference splitting scheme, an implicit treatment of the boundary conditions and the implementation of an adaptive grid strategy for global efficiency. The performance of the method is investigated for the GAMM test circular‐arc bump configuration and for the RAE 2822 aerofoil.