Papers by Author: Dae Hoon Lee

Abstract: A preplated frame (PPF) consisting of Au/Pd/Cu-Sn/Cu substrate, i.e., preplated with a Cu-Sn alloy instead of Ni commercially used, was fabricated by electroplating and then the feasibility of the frame as an alternative PPF was investigated. The wettability of the Cu-Sn alloy was better than that of Ni, resulting in excellent contact with the substrate and smoother surface on the upper Au/Pd protective layer. By XPS analyses, it was confirmed that Cu atoms in the Cu-Sn alloy layer did not diffuse through the thin protective layer to the surface at temperatures used in IC assembly. The wire pull-strength and solderability of the Cu-Sn alloy PPF were almost equivalent to those of the Ni PPF. However, the former showed much better corrosion resistance than the latter.

Abstract: Iron and its nitride (e-Fe3N) nanoparticles were fabricated by the CVC using Fe(CO)5 precursor without the aid of LN2 chiller. The iron particles synthesized at 400 oC were a mixture of amorphous and crystalline a-Fe. Fully crystallized iron particles were then obtained above 600 oC. Iron-nitride particles that were easily formed at 500 oC at 1 atm., however, were not fully developed in vacuum unless the reaction temperature reached 850 oC. Nevertheless, the work chamber needed to be maintained in vacuum to obtain finer iron-nitride particles. The synthesized particles possessing the core-shell type structure were all nearly spherical and enclosed with Fe3O4 or Fe3O4-related amorphous layer. The iron nanoparticles (~20 nm) synthesized at 600 oC at 760 torr exhibited iHc ~ 1.0 kOe and Ms ~ 170 emu/g, whereas the iron-nitride particles (~20 nm) synthesized at 850 oC at 0.01 torr exhibited iHc ~ 0.45 kOe and Ms ~ 115 emu/g.

Abstract: Effect of washing step in R-D process on the structure and magnetic properties of Nd-Fe-B powder fabricated by the process using a spray-dried precursor was investigated. Although the powder washed in water contained much more residual CaO than that washed in dilute acetic acid, magnetic properties of the former were much better than those of the latter due to less removal of Nd-rich phase. In the powder washed in water Nd2Fe14B particles were enclosed with thin layer of Nd-rich phase, and each particle consisted of one or more subgrains of the size 50 ~ 100 nm or more. Milling the powder before water washing was effective to remove more CaO, thereby increased the remanence of the powder.