The effect of laser power pattern on the temperature and thermal stress distributions in LED die bonding process is investigated in this work. The wavelength of 940nm diode laser source is used in this study. The laser light is focus on the back of an AlN substrate. The eutectic Au80Sn20 solder metallized between die and substrate is soldered by the heat conducted from the controlled laser power. The finite element package software-MSC. Marc is employed to simulate the laser soldering process. The thermal-elastic-plastic models of the solid elements are used. The temperature dependent material properties are applied to characterize the temperature variation effect during the die bonding. The measured temperature data have also been used to derive the absorption coefficient, conductivity, specific heat of AlN substrate and the convection coefficient in free convection via the inverse engineering process. A difference between the simulated and measured temperature can be kept in 10%. The temperature and thermal stress distributions during the die bonding process have been simulated and studied. The distributions of residual stress induced in this die bonding process have also been studied. The effects of different laser soldering parameters, e.g. focus shift, defocus, inclined angle, on the die bonding are also studied.

When an aluminum alloy plate after rolling, non-uniform residual stress distributions existed inside the plate and defects, such as edge wave, middle wave, of the plate will be induced. Usually, a levelling process will be adopted to modify the plate flatness. By numerically simulating the tension levelling process, the purpose of this thesis is to understand the final dimensions and the residual stress distribution of the aluminum plate subjected to the tension levelling process. This study used the finite element method as the basic theory of the numerical simulation. A 3-D model of a cold-rolled plate with a side wave, subjected to tension levelling process was constructed. Then, the effects of the variations of the tensile ratio and residual stress distribution after rolled on the residual stress distribution after levelling and the improvement of flatness were studied. The simulation results showed that in the wave region, the tension levelling process could eliminate more than 90% of the residual stress, in the flat region was up to 80%.Also, after leveling, the residual stress distribution in the flat region was more uniform than the wave region. After-rolled residual stresses at the wave region affected the final peak position of the wave and the stress eliminated ratio of the wave region, but showed no significant effect on the final plate width and the residual strains. After-rolled residual stresses at the flat region affected the stress elimination ratio of the flat region only. The tensile ratio would affect the plate flatness, the plate width, stress elimination ratio, and the maximum residual stress. The higher of the tensile ratio, the more flatness of the plate would be obtained, but the higher residual strain would be induced and caused the lesser range of available plate.

The effect of residual stress on the post-weld-shift (PWS) of a fiber-solder-ferrule (FSF) under a cyclic thermal load is investigated in this thesis. By using the finite element software MENTAT and MARC, the stress distribution in this model of coupled thermal-elastoplasticity is adopted to solve. The temperature dependent material properties are employed to calculate the residual stresses and the thermal stresses of the solder in the solidification process. The PWS of a fiber and the stress distribution of a solder under different temperature cycles are also investigated in this study. The PWS calculated with and without considering residual stresses are compared with the measured data in this study. Results indicated that the effects of residual stresses introduced in the solder solidification can not be ignored. The temperature dependent material properties, i.e., the melting temperature , Young¡¦s Modulus, coefficient of thermal expansion (CTE) and yield strength at high temperature may affect the residual stress distribution and the PWS of the FSF significantly.

ABSTRACT Wireless communication products require thinner and small packaging to allow for reductions in Cell Phone and PDA product sizes. Currently, the Film-BGA (Ball Grid Array) package is in production for thinner and small case. The Film-BGA package is a thin package that uses polyimide tape as a substrate to reduce the overall package profile to 1 mm and ball pitch is 0.5 mm. The Film substrate can reach 0.11 mm thick or less. Analysis the Film-BGA model consists of a sequentially coupled thermal- mechanical analysis considering epoxy curing, post molding compound and IR-Reflow by MARC. After these processes Film-BGA will occur warpage and residual stress, we will study the sizes of materials on the warpage and residual stress by Taguchi Method. From the MARC analysis, it is found that due to the coefficient of thermal expansion (CTE) mismatch between solder ball, die, molding compound, epoxy and substrate, there exists very high stress near the interfaces of epoxy and results the maximum warpage (y-displacement) that occurs at the edge of the substrate after IR-Reflow. By means of the Taguchi Method, it is found that die size greatly affect the warpage of Film-BGA more than epoxy size and molding compound size. It is also found that epoxy size has more effect than die size and molding compound size on the residual stress of Film-BGA.

The variations of residual stress distributions on solder balls under the cyclic thermal load and aging processes are investigated in this thesis. The solidification phenomena for different shapes and materials of the solder balls during the reflow process are predicted by using the Surface Evolver program. The distribution of residual stress in the solder ball is calculated by employing the MARC finite element package. The temperature dependent material properties of the solders, i.e. 63Sn/37Pb and 96.5Sn/3.5Ag, are used in the residual stresses calculations. The variation of the residual stresses distributions of different solder balls under the temperature cycling test ( between ¡V40¢J and 85¢J ) and the aging test ( at 85¢J ) are simulated and studied. The effects of the solder parameters, i.e. the solder height and the pad geometry shapes on the residual stresses distribution are also studied. Besides, the same simulation and analysis has also applied on the solder ball with an unleaded solder 96.5Sn/3.5Ag. A better understanding about the variation of the residual stress in a solder ball is expected from this analysis.

During the bending of flange plate used for dapped girders some highway bridge fabricators are experiencing cracking of the flange plate particularly when heat is used in assisting the bending process. Due to the extreme strains experienced during the fabrication process, investigating this problem requires the use of a finite element analysis. The fabrication process was broken down into two parts, first the heating of the plate through the use of either a furnace or an acetylene torch (thermal), and the second was the bending process (structural). The five different temperatures collected during the thermal analysis were a uniform temperature of 75oF, a 1100oF uniform temperature as a result of furnace heating, both five and ten minutes of air-cooling after the plate had reached a uniform temperature of 1100oF, and the temperature gradient after heating the flange plate to a surface temperature of 1200oF though the use of an acetylene torch. After the thermal analysis was completed, the resulting temperatures were imported into the structural model. The plate thicknesses analyzed were one, one and a half, and two inches, assuming both 50 and 70 ksi yield strengths. To achieve a 90 degree six-inch radius bend the plate was bent in five separate locations. The result of this analysis showed that with the introduction of temperature gradients into thefabrication process, the strains along the plate??s extreme fibers increased. The model further showed that for both a one and a half and two-inch thick plate the extreme fiber strains exceeded ten percent, which further adds to the increased risk of the flange plate cracking during fabrication. The highest residual stresses through the plate??s thickness occurred during cold bending. The residual stresses through the plate??s thickness decreased when the fabrication process was carried out at elevated temperatures. When steel exceeds a strain of 10 to 16 percent during the fabrication process, the plate becomes susceptible to cracking. This strain limit was exceeded for plate thicknesses of one and a half and two inches.

Recently, wet sand blasting on coated cutting tool inserts has drawn interest to the tooling industry due to its positive effects on cutting performance and tool life. This performance boost has partly been attributed to the buildup of compressive residual stresses in the coating during the blasting process. However, the mechanism of forming residual stresses in ceramic coatings during sand blasting is not fully understood. This work utilize x-ray diffraction as the main tool to study the formation and relaxation of residual stresses after wet sand blasting and annealing on 001, 012 and 110 textured α-Al2O3 coatings. To minimize the influence of stress gradients in the samples, all stress measurements were set up with a fixed analysis depth of 2 µm. Sand blasting was made with an alumina based slurry at 2, 3.2 and 4 bar pressure and the anneal was done at temperatures from 400 to 1000 °C for 2 hours or more. The coating hardness was evaluated by nanoindentation. Finally, the activation energy for the relaxation of residual stresses was estimated using the Zener-Wert-Avrami function. The results reveal the highest compressive residual stress with up to -5.3 GPa for the 012 texture while the stresses for the 001 and 110 textures peaked at -3.1 and -2.0 GPa, respectively. Further, a hardness gradient was present after blasting of the 001 and 012 textured samples indicating a higher stress at the surface of the coating. The 110 textured sample is the most brittle resulting in flaking of the coating during sand blasting. The different deformation mechanisms are related to difference in active slip planes between coatings with different textures. Both the stress and hardness decreased after heat treatment and the activation energy for stress relaxation was found to be as 1.1 ± 0.3 eV, 1.9 ± 0.2 eV and 1.2 ± 0.1 eV for the 001, 012 and 110 textures, respectively.

応用力学論文集v.15

The Thirteenth East Asia-Pacific Conference on Structural Engineering and Construction, September 11-13, 2013, Sapporo, Japan (EASEC-13)

The finite element method is used to investigate the effects of welding-induced residual stress and distortion on the strength and behaviour of ship hull structures. A finite element welding simulation consisting of sequentially coupled transient thermal and nonlinear structural analyses is used to predict the three-dimensional residual stress and distortion fields in welded stiffened plates. Three types of stiffener commonly used in commercial and naval applications are considered. The welding simulation is followed by a 'shakedown' analysis to study the possibility of residual stress relief caused by cyclic loads. The strength and behaviour of stiffened plates under axial load is characterized by normalized plots of average axial stress versus axial strain, commonly referred to as load-shortening curves. These curves are used to evaluate the effects of welding-induced residual stress and distortion on stiffened plate behaviour with and without considering stress relief by shakedown. Load-shortening curves generated by finite element analysis are also compared with load-shortening curves produced using analytical methods including those prescribed in ship structural design standards published by the International Association of Classification Societies (IACS). To conclude, a hull girder ultimate strength analysis is carried out using Smith's method with load-shortening curves generated by several different methods. Results indicate that welding-induced residual stress and distortion decrease the ultimate strength of flat-bar, angle, and tee-stiffened plates investigated in this study by as much as 17%, 15% and 13%, respectively. Stiffened plate ultimate strength values calculated using IACS common structural rules agreed reasonably well with results from numerical models in most cases. There was however, a significant discrepancy between the numerical load-shortening curves and the IACS curves in the post-ultimate regime, where the IACS curves overestimated the post-ultimate strength of stiffened plates by as much as 30%. To investigate stress relief by shakedown, axial stresses of 25% and 50% of the yield stress were applied and residual stresses were reduced by approximately 20% and 40%, respectively. In some cases, these reductions in residual stress led to increases in stiffened plate ultimate strength as high as 7%. Analysis of a box girder using load-shortening curves from a finite element model including residual stresses and distortions predicted by welding simulation predicted a bending moment capacity within 2.7% of the experimentally measured value. Using load-shortening curves from the IACS common structural rules, the ultimate strength was overestimated by 17%. / Thesis .pdf/A