Enhancement of hardness of bulk solder by doping Cu nanoparticles at the interface of Sn/Cu solder joint

© 2019 In this study, Cu NPs prepared by chemical reduction method,were doped into flux at weight proportions (0, 0.2, 0.5, 1 and 2 wt.%) and then reflow soldering was performed for the pure Sn solder (initial diameter = 1400 μm) with Cu substrate at 250 °C for 120 s. The presence of Cu nanoparticles (NPs) in soldering flux was observed to enhance the spreading of solder on Cu substrate. Solder with larger base spread area, characterized with faster diffusion of Cu from substrate attained quicker supersaturation during reflow and larger precipitation of primary Cu 6 Sn 5 intermetallics (IMCs) upon air cooling. The solders containing greater area proportion of primary IMCs are characterized with enhanced effective elastic modulus. Experimentally, the solders prepared with Cu nanoparticles composed flux,were characterized with the increase in Vicker's microhardness. Solder processed with flux containing 2 wt.% Cu nanoparticles possessed Vicker's hardness of 18.6 HV, whereas solder prepared with undoped flux only had hardness of 13.8 HV. Numerical computation of the Cu diffusion and the elasticity have been done using finite element method.status: publishe

Geometrical effects on growth kinetics of interfacial intermetallic compounds in Sn/Cu joints reflowed with Cu nanoparticles doped flux

© 2018 Elsevier B.V. In this study, Cu nanoparticles prepared by chemical reduction method, were doped into flux (0–2 wt%) and disseminated to the pure Sn solder ball at 250. The enhanced spreading rate due to use of nanoparticles, increased the base diameter (W) and decreased the height (H) of the solder at constant volume. The finite element analysis for Cu concentration, temperature and velocity; in relation to the magnitudes of W and H show that larger W is responsible for enhancement of supersaturation and radial thermal gradient, whereas smaller H is responsible for reduction in flow velocity. The growth kinetics of interfacial Cu 6 Sn 5 film during isothermal reflow is proportional to W 2/3 during isothermal reflow whereas linearly dependent on H during air cooling. As the ripening at isothermal stage and precipitation at cooling stage contribute to the gross growth behavior of Cu 6 Sn 5 intermetallic compounds layer growth, the combined geometrical effect of base diameter and height of the solder specimen renders solder corresponding to flux with 2.0 wt% nanoparticles to have the overall thickest intermetallics.status: publishe

Synthesis of Cu@Ag core-shell nanoparticles for characterization of thermal stability and electric resistivity

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. A two-step synthetic method has been utilized to prepare copper–silver (Cu@Ag) core–shell particles with thin Ag shell coated over a Cu core of initial diameter of 80 ± 5 nm. The formation of core–shell particles is characterized by transmetallation reaction on the surface of the Cu particles, where copper atoms function as the reducer for silver ions. The morphological characterization of Cu@Ag reveals that excess supply of Ag-based reagent produces nanostructures with enhanced core–shell diameter, increased shell thickness, and agglomeration of Ag in the bulk surface, whereas limited supply of Ag species results in nanoparticles with imperfect enveloping of Cu core—making them susceptible to oxidation. Experiments with TGA and DSC verify that thermal stability of core–shell nanoparticles is achieved for the specimen undisturbed by agglomeration and imperfect enveloping effects. Though the electrical resistivity of Cu@Ag nanoparticles increases in general with larger molar proportion of Cu, its increment rate is small for the limit [Cu]:[Ag]=4:1 and then higher beyond it. The sample with [Cu]:[Ag]=4:1, characterized by higher thermal stability, slowest oxidation speed, lower electric resistivity(64.24 μΩ cm), and negligible agglomeration effect, is recommended for industrial applications.status: publishe

Size effect on interface reaction of Sn-xCu/Cu solder joints during multiple reflows

© 2019, Springer Science+Business Media, LLC, part of Springer Nature. At present, electronic products are developing in the direction of miniaturization and integration, which leads to the downsizing of solder bump in the packaging process. Moreover, micro solder bumps often require undergoing multiple reflow processes due to the improvement of packaging technology, which has a great influence on interface reaction. Hence, it is necessary to study the effects of solder composition, bump size and reflow cycle on interfacial reaction between solder alloys and Cu substrates. In this experiment, Sn–xCu (x = 0, 0.7, 2.0 wt%) alloys with diameter of 200 µm, 500 µm, and 800 µm were soldered to Cu substrates at 250 °C for 1 min, and then reflowed 20 cycles totally. The size effect of micro solder joints on the growth of IMC after multiple reflows was analyzed. At the same time, the impact of Cu concentration inside the bulk solder on the interfacial reaction during multiple reflows was explored. This experiment finds that the diameter of IMC grains increases with the decrease of solder ball diameter after one reflow cycle, and a significant size effect occurs in Sn/Cu solder bump. As the number of reflow cycle increases, the size effect on interface reaction is more pronounced. The most direct kinetic factor of this phenomenon is that the average Cu concentration in the small-sized solder ball rises faster than the others. When the number of reflow cycle reach to nine times, the lateral growth rate of IMC grains begins to surpass the longitudinal growth rate, and the morphology of IMC grains becomes flat. This phenomenon is especially evident in the small-sized solder ball. Moreover, the addition of Cu element in solder promotes ripening reaction resulting in the lateral growth of IMC. Cu 6 Sn 5 micro particles appearing at the Sn/Cu, Sn–0.7Cu/Cu interface hinder the grain boundary motion and inhibit the lateral annexation of IMC grains, thereby suppressing the lateral growth of IMC.status: publishe

All-round suppression of Cu6Sn5 growth in Sn/Cu joints by utilizing TiO2 nanoparticles

© 2018, Springer Science+Business Media, LLC, part of Springer Nature. Both TiO 2 and Cu nanoparticles, when added at the interface of the Sn/Cu joints during reflow soldering at 250 °C for 10, 60, 120 s and subsequent air cooling, are observed to reduce the growth of Cu 6Sn 5 whiskers. Though both reinforcments reduce the vertical growth of intermetallic compound (IMC), only TiO 2 could suppress the lateral growth of the compound. The solder saturation level with Cu at different reflow duration can influence final dimensions as well as morphology of IMC and occurrence of screw dislocation. The addition of TiO 2, by intervening the mechanism of ostwald ripening during reflow can be utilized to enhance the solder-IMC bonding at interface.status: publishe