EFFECTIVE MEDIUM APPROXIMATION FOR CONDUCTIVITY OF COATED-INCLUSION COMPOSITES WITH ANISOTROPIC COATING

Effective medium approximations are developed to estimate the macroscopic conductivity of coated-inclusion composites with thin anisotropic coating. The two-phase coated-inclusion are substituted by equivalent one-phase inclusion, using the multi-coated spheres assemblage and the differential substitution approaches. Then, the usual effective medium approximation schemes are applied to the equivalent medium to estimate the conductivity of original three-phase composites. The results obtained were compared with the numerical simulation by finite element method in 2D show the effectiveness of the methods.

Aluminum Coated Micro Glass Spheres to Increase the Infrared Reflectance

The reflective properties of micro glass spheres (MGS) such as Solid Micro Glass Spheres (SMGS, “glass beads”) and Micro Hollow Glass Spheres (MHGS, “glass bubbles”) are utilized in various applications, for example, as retro-reflector for traffic road stripe paints or facade paints. The reflection behavior of the spheres can be further adapted by coating the surfaces of the spheres, e.g., by titanium dioxide or a metallic coating. Such coated spheres can be employed as, e.g., mid infrared (MIR)-reflective additives in wall paints to increase the thermal comfort in rooms. As a result, the demand of heating energy can be reduced. In this paper, the increase of the MIR-reflectance by applying an aluminum coating on MGS is discussed. Aluminum coatings are normally produced via the well-known Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD). In our work, the Liquid Phase Deposition (LPD) method, as a new, non-vacuum method for aluminum coating on spherical spheres, is investigated as an alternative, scalable, and simple coating process. The LPD-coating is characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and reflection measurements. The results are compared to a reference PVD-coating. It is shown that both sphere types, SMGS and MHGS, can be homogeneously coated with metallic aluminum using the LPD method but the surface morphology plays an important role concerning the reflection properties. With the SMGS, a smooth surface morphology and a reflectance increase to a value of 30% can be obtained. Due to a structured surface morphology, a reflection of only 5% could be achieved with the MHGS. However, post-treatments showed that a further increase is possible.

Solder paste wicking in socketable BGAs

The trend of the electronics industry to miniaturize package design has caused the need to adopt BGA packages for a variety of applications. OEM microprocessors have conventionally used LGA designs press fitted into sockets for ease of reworkability. However, BGA packages, which have been widely used for surface mount (SMT) applications, face challenges when they are used in sockets. One of the major challenges faced is the formation of intermetallics between the bare solder ball and the gold paddle of the socket. To address these challenges, Georgia Tech is developing a universal solution for socketing and SMT applications by surface modification of solder spheres. In this new approach, the solder spheres have an outermost noble metal layer which prevents any damage to the BGA and doesn't react with the gold paddle, when placed in a socket. The surface coating would also collapse with the solder ball under specific conditions during reflow when used in SMT applications. This paper focuses on the process of attaching these modified solder spheres on the package and subsequent assembly of the package on the board. We evaluated high and low melting solder pastes to attach the coated spheres to the package. A major challenge observed was the wicking of the solder paste to the entire surface of the solder sphere during reflow. This was addressed by studying the wetting characteristics of the solder paste on different metal surfaces and controlling the volume of solder paste required. Optimization of solder paste volume was done to control wicking and at the same time, achieve a strong and stable joint. Microstructure of the solder joint was analyzed to determine its effect on the joint stability. It was found that the amount of wicking is a strong function of the composition of the solder paste, the reflow time, and the material in surface with which the solder paste is in contact with. This work advances the assembly requirements of socketable BGAs.

Simulating the Impact of the Internal Structure of Phytoplankton on the Bulk Particulate Backscattering Ratio using a Two-Layered Spherical Geometry for Cells

The bulk backscattering ratio ($\tilde{b_{bp}}$) is commonly used as a descriptor of the bulk real refractive index of the particulate assemblage in natural waters. Based on numerical simulations, we analyze the impact of heterogeneity of phytoplankton cells on $\tilde{b_{bp}}$. $\tilde{b_{bp}}$ is modeled considering viruses, heterotrophic bacteria, phytoplankton, detritus, and minerals. Three study cases are defined according to the relative abundance of these different components. Two study cases represent typical situations in open ocean, outside (No-B/No-M) and inside bloom (B/No-M). The third study case is typical of coastal waters with the presence of minerals. Phytoplankton cells are modeled by a two-layered spherical geometry representing a chloroplast surrounding the cytoplasm. The $\tilde{b_{bp}}$ values are higher when heterogeneity is considered because the contribution of coated spheres to backscattering is higher than homogeneous spheres. The impact of heterogeneity is however strongly conditioned by the hyperbolic slope $\xi$ of the particle size distribution. Even if the relative concentration of phytoplankton is small (<1%), $\tilde{b_{bp}}$ increases by about 60% (for $\xi=4.3$ and for the No-B/No-M water body), when the heterogeneity is taken into account, in comparison with a particulate population only composed by homogeneous spheres. As expected, heterogeneity has a much smaller impact (about 5$\%$ for $\xi=4.3$) on $\tilde{b_{bp}}$ when minerals are added.