Papers by Author: Chang Woo Lee

Abstract: The Electron Beam Melted (EBM) method is one of the attractive attention thing additive manufacturing methods. By using an EBM additive manufacturing method, CP Ti and Ti-6Al-4V specimen were fabricated with a certain processing parameters. The mechanical properties such as fatigue limit, tensile properties including microstructural characteristics of CP Ti and Ti-6Al-4V specimens fabricated by EBM were confirmed and were compared with the conventional Ti alloys. Additive manufacturing was obtained high strength by creating martensite due to rapid cooling. On the other hand, void occurrence cannot be avoided by the method of using powder, accordingly it had a low fatigue strength value. Therefore, this study focused on that the values of fatigue characteristics of the EBM specimens and conventional specimens were compared and analyzed. EBM CP Ti had good mechanical properties such as yield strength, ultimate tensile strength, elongation and fatigue limits, approximately as same as casting CP Ti. EBM Ti-6Al-4V showed good mechanical properties, but fatigue limits were lower than wroughtTi-6Al-4V. That resulted from the formation of several kinds of internal pores which caused to increase the crack initiation and propagation.

Abstract: The development of 3D integration is necessarily required for high speed, high density, small size, and multi-functional electronic devices. Through silicon via (TSV) technology has been rapidly developed to fulfill the demand of the next generation of multifunctional electronic systems as one of the most alternative applications for 3D packaging. In this study, low cost and high speed molten solder-filling of TSV and bonding process by using micro bump was investigated. Micro bumps were formed with two step, Cu pillar bump and Sn-Ag cap bump by using electroplating. The size of micro-bumps was 10 and 20um and the precise content of the bump was developed with good planarity by adding reflow process. Additionally, the SiC nanoparticle composite solder was fabricated for low CTE filling material. From the results, it is possible that the CTE of composite solder added 1.0wt% SiC nanoparticles had decreased until 15.0ppm/°C. Comparing CTE of pure Sn, which was about 24.0ppm/°C, it was very low CTE and lower then CTE of Cu (16.5ppm/°C). Even if the electrical resistance of nanopowder composite solder is increased, the increasing rate is very slow until 1.0 wt%. However, the resistance changing rate is rapidly increased over the 1.0wt%. From the result, it is expected that nanopowder composite solder can contribute high reliability of molten solder filling TSV.

Abstract: The effect of SiC nanoparticle dispersion was investigated for microstructure change and mechanical properties of Sn-Bi electroplated alloys. The diameters of SiC nanoparticle in this study were 45-55 nm. The SiC nanoparticles were mixed with Sn-Bi electroplating and then the nanoparticles were dispersed with ultrasonic vibrator. After the dispersion, the SiC dispersed Sn-Bi alloys were electroplated on Cu deposited Si wafer. The microstructure and mechanical properties of the sample were evaluated by FE-TEM, FE-SEM, EDS, and shear tester. For TEM observation, the specimens were prepared by ultramicrotome and FIB. The SiC nanoparticles were well-dispersed in Sn-Bi alloy. SiC particles were located near grain boundaries or grain inside. The average grain size of the solder alloy was decrease about 30% compared with the grain size of Sn-Bi alloy prepared in the same condition. Due to the grain refinement and dispersion hardening by SiC nanoparticles, the SiC dispersed Sn-Bi alloy is expected to obtain high reliability and joining strength when it applied to interconnection materials.

Abstract: Vibration characteristics of 5μm- thick Ni film were investigated with applying acoustic wave to the Ni diaphragm of 2mm x 2mm unit size. In the modal analysis, the first resonance mode of the diaphragm showed an out-of-plane piston-like movement and the first natural frequency was 1,643 Hz, whereas in this experiment, the first natural frequency appears at about 1,300 Hz under sound pressure of 0.2 Pa. The amplitudes of diaphragm increase with increase of sound pressure level in the applied frequency range from 300 Hz to 1,000 Hz, indicating that area of diaphragm influences directly the amplitude.

Abstract: The “stop-action” technique was employed to study grain structure evolution during friction-stir welding of AZ31 magnesium alloy. The grain structure formation was found to be mainly governed by the combination of the continuous and discontinuous recrystallization but also involved geometric effect of strain and local grain boundary migration. Orientation measurements showed that the deformation mode was very close to the simple shear associated with the rotating pin and material flow arose mainly from basal slip.

Abstract: The growth behaviour of the intermetallic compounds (IMCs) in Pb-free solder bump is investigated. The Pb-free micro-bump, Sn-50%Bi, was fabricated by binary electroplating for flip-chip bond. The diameter of the bump is about 506m and the height is about 60 6m. In order to increase the reliability of the bonding, it is necessary to protect the growth of the IMCs in interface between Cu pad and the solder bump. For control of IMCs growth, SiC particles were distributed in the micro-solder bump during electroplating. The thickness of the IMCs in the interface was estimated by FE-SEM, EDS, XRF and TEM. From the results, The IMCs were found as Cu6Sn5 and Cu3Sn. The thickness of the IMCs decreases with increase the amount of SiC particles until 4 g/cm2. The one candidate of the reasons is that the SiC particles could decrease the area which be reacted between the solder and Cu layer. And another candidate is that the particle can make to difficult inter-diffusion within the interface.