Papers by Author: Kyu Hwan Oh

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Authors: S.H. Joo, H.J. Chang, Woong Ho Bang, Heung Nam Han, Kyu Hwan Oh
Abstract: A computational model for roll pressing of powder was developed based on an elastoplastic finite element method, and was applied to predict the alligatoring behavior at roll nip during powder compacting process. The yield criterion for powder has been implanted for the simulation of the roll pressing of Direct Reduced Iron powder with both flat roller and indentationtype roller. Calculated results could well explain the experimental observation that the indentationtype roller is more useful to hinder in alligatoring.
Authors: J.H. Cho, Soo Jin Park, Kyu Hwan Oh
Authors: H.J. Chang, Heung Nam Han, M.W. Moon, Kwang Hee Lee, Kyu Hwan Oh
Abstract: An analysis for non steady state heat transfer of a hot pressing roller was suggested in 1-dimensional model. The surface temperature on hot pressing roller was predicted by using surface contact heat transfer coefficient calculated with induced analytic solution. We calculated the size of iron powder, influencing on surface contact heat transfer coefficient. Since coarse iron powder has reduced heat transfer coefficient during contacting on roll surface with smaller contact area, temperature on roller surface has been expected to decrease. This predicted temperature by the analytic model was fairly reasonable in comparison with experimental data and finite element model.
Authors: Jung Suk Lee, J.P. Jung, Woong Ho Bang, Kyu Hwan Oh
Abstract: TLP method was applied to the lead free soldering process; Sn-3.5Ag core solder (Tm of 221°C) / Sn-Bi coating layer (Tm of 139°C at eutectic) was reflowed on UBM pad. Sn-Bi was formed by electroplating method at current range of 2~6A/dm2, which had varied the coating layer composition from 96wt% to 28wt% Bi. Specimens were reflowed on copper pad at temperature range of 200 and 220°C, and then interface reaction was examined. It was claimed that high Bi adding in the plated layer improved the wetting performance on Cu. Even at the reflow temperature of 200°C, SEM observation revealed well-defined interface junctions between solder and copper pad, i.e. nucleation and growth of CuxSn intermetallic compound. After heating for 5min at 220°C, Bi was resolved into Sn-3.5Ag core solder and completely homogenized, which is favorable from the viewpoint of long term fatigue reliability.
Authors: K.I. Kang, J.P. Jung, Woong Ho Bang, J.H. Park, Kyu Hwan Oh
Abstract: Bending fatigue behavior of eutectic Sn-3.5Ag solder bump bonded on FR4-PCB was characterized by experimental and finite element method (FEM). To investigate an effect of stress state on bump failure, which had not been weighed in conventional Coffin-Manson model of Nf=K ⋅εp -1~-2, ‘fatigue frequency variable’ and ‘bump viscoplasticity’ were included in analysis procedure. As experimental results, with increasing fatigue cycles from 3,000 to 10,000, bond strength decreased from 98.9% to 76.5%, and from 97.5% to 67.1% at the fatigue frequencies of 2.5Hz and 5.0Hz, respectively. Stress state could be critical components to determine fatigue life, which should be combined in Coffin-Manson criteria. FEM calculation showed that higher bending frequency led to higher normal stress development at the solder and IMC interface, but smaller plastic strain in bump. However, bending fatigue experiment revealed discrepant results from that of Coffin-Manson criteria. Higher bending frequency, which was predicted to give rise to smaller εp at solder, showed dramatic bond deterioration of solder bump on UBM (under bump metallurgy). This was confirmed experimentally through SEM (scanning electron microscopy) observation as cracks were found at the solder bump and UBM interfacial IMC, Ni3Sn4, in case of the higher bending frequency.
Authors: Jae Hyung Cho, Jong Soo Cho, Jung Tak Moon, J. Lee, Young Hee Cho, Anthony D. Rollett, Kyu Hwan Oh
Authors: D.J. Seol, S.Y. Hu, Yong Li Li, Li Qing Chen, Kyu Hwan Oh
Authors: Suk Hoon Kang, Jae Hyung Cho, Joon Sub Hwang, Jong Soo Cho, Yong Jin Park, Jung Tak Moon, Kyu Hwan Oh
Abstract: Cold drawn gold wires are widely applied in electronic packaging process to interconnect micro-electronic components. They basically provides a conducting path for electronic signal transfer, and experience thermo-mechanical loads in use. The mechanical stability of drawn gold wires is a matter of practical concern in the reliable functioning of electronic devices. It is known that mechanical properties of materials are deeply related to the microstructure. With appropriate control of deformation and heat processes, the mechanical properties of final products, such as tensile strength and elongation can be improved. Severe plastic deformation by torsion usually contributes to grain refinement and increment of strength. In this study, microstructure variations with torsion strain followed by drawing and heat treatment were investigated. Analyses by focused ion beam (FIB) and electron backscattered diffraction (EBSD) were carried out to characterize the effect of deformation and heat treatment on the drawn gold wires. Pattern quality of EBSD measurements was used as a quantitative measure for plastic deformation.
Authors: Jae Hyung Cho, Seong Jun Park, S.H. Choi, Kyu Hwan Oh
Authors: Hyo Tae Jeong, Dong Nyung Lee, Kyu Hwan Oh
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