Papers by Author: Young Eui Shin

Abstract: In recent years, the replacement repair welding, through which damaged bridges are repaired by replacing of the damaged sections with new steel plates through cutting and welding under in-service conditions, has been highlighted for its brilliant features i.e. it can be achieved without incurring traffic dislocation. However, the validity of the repair method is not fully verified because of its lack of safety and reliability. Moreover, repair method by welding inevitably involves cutting and welding process, and the weldability of the replacement repair welded joints has not yet been clarified. In this paper, the weldability of the replacement-repair-welded joints was investigated in order to improve reliability in the repair welded joints of the aged steel bridge. A Steel plate was extracted from the steel bridge after the period of prolonged service, and was welded with a new plate, using replacement repair welding technique, in order to construct a specimen. Flux Cored Arc Welding (FCAW) process was used to fabricate the double "V" butt joints. Welded joints were characterized on the basis of chemical composition analysis, hardness test and micro-structural analysis.

Abstract: High integration, density and complexity are essential in electronic products such as compact and light mobile phone recently. A new type of development for component and material is also needed. In the PCB design aspect, a technique such as pad on via, trying to keep up with the demand for compact and slim products, is commonly used. In a response to increased pressure from these situations, an enhanced ability on reliability of electronic products is gradually needed. The high cost and risk, however, are higher and higher for the assurance in the reliability. We made a procedure which can evaluate a solder joint reliability on the product development phase. Furthermore, we've pursued a development process for preventing failure as we analyzed the failure mode from predicting the causes of the failure more precisely. In this study, focusing on the joint reliability of the solder interconnection between CSP and pad on via, we have passed through a test procedure as follows. First, making samples applied with high integrated circuit (CSP: 0.65 pitch, via hole size: 165um, solder: Sn-3.0Ag-0.5Cu). Second, operating the thermal cycling experiment (- 45°C~+125°C, 30min/cycle) to find a failure of the test specimen with an electrical method. Third, observation the non-destructive X-ray microscopy and the metallographic cross-sectioning. Fourth, simulation of the finite element model and deformation analysis. Fifth, completion the modeling of the failure mode. Lastly, prediction a lifetime of the solder joint.

Abstract: The reliability of QFP (Quad flat package) solder joint using Sn-8Zn-3Bi solder paste under the thermal shock test was investigated. Considering the environmental restriction such as ROHS, the QFP Cu LF (Lead-frame) was coated with lead-free materials (Sn, Sn-3Bi). To analyze the reliability under thermal shock treatment, the samples were placed in the thermal shock chamber (248K/423K, Dwell time: 30min). During the thermal shock test, the solder joint cross-sections were observed every 500 cycles. No crack initiation and propagation was observed through all type of plated Cu LF. The measured pull strength slightly decreased, as the number of cycles increased. After 1000 cycles, the pull strength of Sn and SnBi plated Cu LF reduced by 30% and 20%, respectively, compared with that of initial condition. Observing the fracture surface morphology by FE-SEM, the fracture mode changed and the IMC fractured area on the both fracture surface was increased. The IMC was identified as γ-Cu5Zn8 by chemical composition analysis using EDS. The reduced pull strength was affected by IMC fracture and fracture mode change. However, the pull strength does not decrease steeply but gradually. Consequently, the Sn-8Zn-3Bi solder joint shows the reliable solder joint strength, fracture mechanism, and compatibility with lead-free plated Cu LF during the thermal shock temperature of 248K to 423K.

Abstract: A novel electrically conductive adhesives (ECAs) with solder particle have been developed. The kinetics of the curing reaction were investigated using isothermal differential scanning calorimeter (DSC). The dynamic DSC scans were also conducted to investigate the temperature dependant curing degree. The temperature dependant viscosity characteristic of polymer was investigated using rheometer. The wetting characteristic of two solders on two kinds of metals were investigated using an optical microscope. It was found that developed resin material showed good fluxing capacity against Sn-Bi solder on Ni/Au and the wetting angles was about 24º.

Abstract: The ultimate goals of this study are to investigate numerically nonequilibrium energy transfer between electrons and phonons, and to predict the crater formation shapes of gold thin film structures irradiated by femtosecond pulse lasers. In particular, the present article expands the onedimensional two-temperature model (1DTTM) to the two-dimensional model (2DTTM) considering wave interference, and it involves the quantum effect to predict thermal and optical properties. The predictions by using 2DTTM are extensively compared with those of 1DTTM, and the influence of film thickness on radiation heat transfer and optical characteristics are also examined. From the results, it is found that the predictions of 2DTTM are in good agreement with those of 1DTTM. As the gold film thickness decreases, the reflectivity decreases dramatically and the absorbed laser intensities at the top surface increases substantially because of wave interference in thin films.

Abstract: Board-level reliability of conventional Sn-37Pb and Pb-free Sn-3.0Ag-0.5Cu solder joints was evaluated using thermal shock testing. In the microstructural investigation of the solder joints, the formation of Cu6Sn5 intermetallic compound (IMC) layer was observed between both solders and Cu lead frame, but any crack or newly introduced defect cannot be found even after 2000 cycles of thermal shocks. Shear test of the multi layer ceramic capacitor (MLCC) joints were also conducted to investigate the effect of microstructural variations on the bonding strength of the solder joints. Shear forces of the both solder joints decreased with increasing thermal shock cycles. The reason to the decrease in shear force was discussed with fracture surfaces of the shear tested solder joints.

Abstract: There have been diverse fatigue models and approaches to properly estimate solder joint reliability. However, it is one of the most difficult problems to determine which solder constitutive models and fatigue models can be applied best. In this paper, both viscoplastic and elastic-plastic-creep solder constitutive models could be utilized to calculate accumulated inelastic response under 208 K to 423 K(-65 °C to 150 °C) thermal cycling condition. And two different fatigue models, Darveaux and creep-fatigue model were applied to find solder joints fatigue life for flip chip assembly. Moreover, each fatigue life was compared to experimental result for the validation of finite element analysis. The actual number of cycles to failure was obtained from cross sectional view of the package with SEM.