Sang-Soo Chee

Preparation and oxidation behavior of Ag-coated Cu nanoparticles less than 20 nm in size

This study examines the oxidation behavior of Ag-coated Cu (Cu@Ag) nanoparticles (NPs) less than 20 nm in size synthesized using a solvothermal method and an immersion process with varying Ag shell quality. First, the mechanism for the formation of Cu NPs is discussed with respect to increasing reaction temperature and time. It was found that La Mers model and the digestive-ripening mechanism affected the size and the morphology of the Cu NPs. Spherical Cu NPs were first observed after 15 min at 240 °C. Moreover, Cu@Ag NPs synthesized with varying immersion temperatures were observed by elemental mapping and line profile imaging using scanning transmission electron microscopy. The average thickness and density of the Ag shell increased with increasing immersion temperature. Based on the results, we evaluated the anti-oxidation property of samples immersed at 80 and 150 °C using high-temperature X-ray diffraction. The sample immersed at 150 °C exhibited an enhanced anti-oxidation property mainly as a result of the thicker and denser Ag shell. In particular, we observed a difference in the amount of free Ag NPs as a result of dewetting as the Ag shell structure changed. The anti-oxidation property of Cu@Ag NPs was strongly dependent on the Ag shell quality.

Reduction synthesis of tin nanoparticles using various precursors and melting behavior

To achieve a more significant melting point drop through finer particles, chemical reduction synthesis of tin nanoparticles were conducted using four tin precursor agents: tin(II) acetate, tin(II) chloride, tin(II) sulfate, and tin(II) 2-ethylhexanoate. Depending on the precursor type, the sizes and size distributions of the synthesized Sn nanoparticles were highly diverse. Tin nanoparticles synthesized with tin(II) sulfate or tin(II) 2-ethylhexanoate displayed characteristics of monodispersity at reduced size. The nanoparticles had average diameters of just ∼3 nm and ∼6 nm, respectively, and exhibited melting points of 102.2°C and 131.1°C, which represented extreme drops by 130.4°C and 101.5°C in comparison with the melting point of bulk tin.

Effects of process parameters in synthesizing Sn nanoparticles via chemical reduction

In order to prepare solder particles for fine pitch interconnections, Sn nanoparticles were synthesized via chemical reduction methods. A number of the process parameters, i.e., injection rate of a precursor solution, application of sonication, reaction temperature, types of reaction medium and capping agent, and drying temperature, are varied in order to study their effect on this process. Using a methanol solution containing 1,10-phenathroline monohydrate, the size of Sn nanoparticles collected after the synthesis decreases as the injection rate increases. An increase in the drying temperature strengthens the degree of agglomeration between Sn nanoparticles, and, in addition, the application of sonication accelerates the process of agglomeration and aggregation between nanoparticles during synthesis. Much smaller Sn nanoparticles are synthesized in diethylene-glycol solutions containing PVP, compared to the methanol solutions with 1,10-phenathroline monohydrate. In the synthesis using diethylene-glycol solutions, the Sn nanoparticle size increases quickly with the reaction temperature.

Synthesis of tin nanoparticles through modified polyol process and effects of centrifuging and drying on nanoparticles

Abstract Sn nanoparticles with average diameter of 13.1 nm were synthesized by modified polyol process, and their melting point was observed to be about 40 °C less than that of bulk Sn. Even though Sn nanoparticles are solid and covered with thin PVP capping and oxide layer, Sn nanoparticles in solution agglomerate upon centrifugation, resulting in a melting point increase. Owing to stickiness of the PVP capping layer, extensive aggregation between Sn nanoparticles was observed after drying. However, the aggregation behavior did not further influence the melting point, because actual agglomeration did not occur in the aggregates.

Reduction synthesis of silver nanoparticles anchored on silver micro-flakes and electrical resistivity of isotropic conductive adhesives at percolation threshold

Reduction synthesis of silver in an ethanol solution without polyvinyl pyrrolidone was performed to anchor synthesized nanoparticles on micro-sized silver flake surfaces in order to enhance the electrical conductivity of isotropic conductive adhesives (ICAs). Although some shape-separated silver particles were formed, the synthesis of particles anchored on the flake surfaces was successful. The cured ICAs containing the silver flakes anchored with 1 wt. % nanoparticles indicated enhanced electrical conductivity due to their filling the gaps between noncontacting silver flakes at the beginning region of the percolation threshold. However, the anchored nanoparticles had a detrimental effect on the conductance when the content of anchored nanoparticles or flakes was higher.

Fabrication of Cu-Ni mixed phase layer using DC electroplating and suppression of Kirkendall voids in Sn-Ag-Cu solder joints

A solderable layer concurrently containing Cu-rich and Ni-rich phases (mixed-phase layer, MPL) was fabricated by direct current electroplating under varying process conditions. Current density was considered as the main parameter to adjust the microstructure and composition of MPL during the electroplating process, and deposit thickness were evaluated as functions of plating time. As a result, it was observed that the coral-like structure that consisted of Cu-rich and Ni-rich phases grew in the thickness direction. The most desirable microstructure was obtained at a relatively low current density of 0.4 mA/cm2. In other words, the surface was the smoothest and defect-free at this current density. The electroplating rate was slightly enhanced with an increase in current density. Investigations of its solid-state reaction properties, including the formation of Kirkendall voids, were also carried out after reflow soldering with Sn-3.0 Ag-0.5 Cu solder balls. In the solid-state aging experiment at 125°C, Kirkendall voids at the normal Sn-3.0 Ag-0.5 Cu solder/Cu interface were easily formed after just 240 h. Meanwhile, the presence of an intermetallic compound (IMC) layer created in the solder/MPL interface indicated a slightly lower growth rate, and no Kirkendall voids were observed in the IMC layer even after 720 h.

Synthesis of Sub-Micron 2SnO·(H 2 O) Powders Using Chemical Reduction Process and Thermal Calcination

Synthesis of sub-micron powders by chemical reduction process was performed at room temperature as function of viscosity of methanol solution and molecular weight of PVP (polyvinylpyrrolidone). Tin(II) 2-ethylhexanoate and sodium borohydride were used as the tin precursor and the reducing agent, respectively. Simultaneous calcination and sintering processes were additionally performed by heating the powders. In the synthesis of the powders, it was possible to control the powder size using different combinations of the methanol solution viscosity and the PVP molecular weight. The molecular weight of PVP particularly influenced the size of the synthesized powders. A holding time of 1 hr in air at sufficiently transformed the into phase; however, most of the PVP (molecular weight: 1,300,000) surface-capped powders decomposed and was removed after heating for 1 h at . Hence, heating for 1 h at made a porous film containing residual PVP, whereas dense films with no significant amount of PVP formed after heating for 1 h at .