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School of Materials Science and Engineering

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College of Engineering

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Separation of Low Molecular Siloxanes for Electronic Application by Liquid-Liquid Extraction

(Georgia Institute of Technology, 1999-10) Wong, C. P. ; Urasaki, Naoyuki

Silicone resins are widely used for electronic packaging as potting and encapsulating materials. Silicone resins have many advantages for electronic packaging applications such as superior electrical properties, thermal stability, low water absorption, etc. Furthermore, silicone resins are not only used as protective materials for integrated circuit (IC) devices but also as conducting materials for interconnection. However, silicone resins have two big drawbacks: low adhesion strength and low molecular weight creep. A simple liquid-liquid extraction method has been developed to purify silicone resins, which will improve adhesion strength and eliminate low molecular weight creep. This paper describes the results of the liquid-liquid extraction method to remove low molecular weight cyclic siloxanes. Fourier transform-infrared (FT-IR) spectroscopy was used to monitor the removal rate of low molecular weight cyclic siloxanes. Thermogravimetric analysis (TGA) was used to evaluate the purity of silicone resin. Gas chromatography-mass spectrometry (GC/MS) was used to identify the low molecular weight cyclic siloxanes. Thermomechanical analyzer (TMA), dynamic mechanical analyzer (DMA), and die shear test were used for evaluate the properties of silicone resin.
A Study of Lubricants on Silver Flakes for Microelectronics Conductive Adhesives

(Georgia Institute of Technology, 1999-09) Wong, C. P. ; Lu, Daoqiang

Conductive adhesives are composites of polymer matrixes and metal fillers (conductive elements). Silver (Ag) flakes are widely used as fillers for electrically conductive adhesives (ECA’s). Generally, there is a thin layer of organic lubricant coated on the commercial Ag flake surface. This lubricant layer is needed for eliminating the Ag particle agglomeration while dispersing the Ag filler into the polymeric resin. Therefore the lubricant influences rheology, conductivity, and other properties of ECA’s. The nature of the lubricant on a Ag flake and the interaction between the lubricant and the Ag flake surface were studied by diffuse reflectance infrared spectroscopy (DRIR). Thermal decomposition of the lubricant was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, the effects of some chemical compounds on lubricant removal and the enhancement of conductivity of the ECA were also investigated. It was found that 1) a chemical bonding was formed on the Ag flake surface between the lubricant and Ag; 2) the short chain acids replaced the long chain lubricants; 3) an ether and a poly(ethylene glycol) enhanced electrical conductivity by partially removing the Ag flake lubricants.
Electron dynamics in gold and gold–silver alloy nanoparticles: The influence of a nonequilibrium electron distribution and the size dependence of the electron–phonon relaxation

(Georgia Institute of Technology, 1999-07-15) Link, S. ; Burda, C. ; Wang, Z. L. (Zhong Lin) ; El-Sayed, Mostafa A.

Electron dynamics in gold nanoparticles with an average diameter between 9 and 48 nm have been studied by femtosecond transient absorption spectroscopy. Following the plasmon bleach recovery after low power excitation indicates that a non-Fermi electron distribution thermalizes by electron–electron relaxation on a time scale of 500 fs to a Fermi distribution. This effect is only observed at low excitation power and when the electron distribution is perturbed by mixing with the intraband transitions within the conduction band (i.e., when the excitation wavelength is 630 or 800 nm). However, exciting the interband transitions at 400 nm does not allow following the early electron thermalization process. Electron thermalization with the lattice of the nanoparticle by electron–phonon interactions occurs within 1.7 ps under these conditions, independent of the excitation wavelength. In agreement with the experiments, simulations of the optical response arising from thermalized and nonthermalized electron distributions show that a non-Fermi electron distribution leads to a less intense bleach of the plasmon absorption. Furthermore, the difference between the response from the two electron distributions is greater for small temperature changes of the electron gas (low excitation powers). No size dependence of the electron thermalization dynamics is observed for gold nanoparticles with diameters between 9 and 48 nm. High-resolution transmission electron microscopy (HRTEM) reveals that these gold nanoparticles possess defect structures. The effect of this on the electron–phonon relaxation processes is discussed. 18 nm gold–silver alloy nanoparticles with a gold mole fraction of 0.8 are compared to 15 nm gold nanoparticles. While mixing silver leads to a blue-shift of the plasmon absorption in the ground-state absorption spectrum, no difference is observed in the femtosecond dynamics of the system.
Mechanisms Underlying the Unstable Contact Resistance of Conductive Adhesives

(Georgia Institute of Technology, 1999-07) Wong, C. P. ; Lu, Daoqiang ; Tong, Quinn K.

One critical obstacle of current conductive adhesives is their unstable contact resistance with nonnoble metal finished components during high temperature and humidity aging. It is commonly accepted that metal oxide formation at the interface between the conductive adhesive and the nonnoble metal surface is responsible for the contact resistance shift. Two different mechanisms, simple oxidation and galvanic corrosion, both can cause metal oxide formation, but no prior work has been conducted to confirm which mechanism is the dominant one. Therefore, this study is aimed at identifying the main mechanism for the metal oxide formation and the unstable contact resistance phenomenon of current conductive adhesives. A contact resistance test device, which consists of metal wire segments and conductive adhesive dots, is specially designed for this study. Adhesives and metal wires are carefully selected and experiments are systematically designed. Based on the results of this systematic study, galvanic corrosion has been identified as the underlying mechanism for the metal oxide formation and for the observed unstable contact resistance phenomenon of conductive adhesives.
Conductivity Mechanisms of Isotropic Conductive Adhesives (ICA’s)

(Georgia Institute of Technology, 1999-07) Wong, C. P. ; Lu, Daoqiang ; Tong, Quinn K.

Isotropic conductive adhesives (ICA’s) are usually composites of adhesive resins with conductive fillers (mainly silver flakes). The adhesive pastes before cure usually have low electrical conductivity. The conductive adhesives become highly conductive only after the adhesives are cured and solidified. The mechanisms of conductivity achievement in conductive adhesives were discussed. Experiments were carefully designed in order to determine the roles of adhesive shrinkage and silver (Ag) flake lubricant removal on adhesive conductivity achievement during cure. The conductivity establishment of the selected adhesive pastes and the cure shrinkage of the corresponding adhesive resins during cure were studied. Then conductivity developments of some metallic fillers and ICA pastes with external pressures were studied by using a specially designed test device. In addition, conductivity, resin cure shrinkage, and Ag flake lubricant behavior of an ICA which was cured at room temperature (25 ℃) were investigated. Based on the results, it was found that cure shrinkage of the resin, rather than lubricant removal, was the prerequisite for conductivity development of conductive adhesives. In addition, an explanation of how cure shrinkage could cause conductivity achievement of conductive adhesives during cure was proposed in this paper.
Novel Bi-Layer Conformal Coating for Reliability Without Hermeticity MEMS Encapsulation

(Georgia Institute of Technology, 1999-07) Wong, C. P. ; Wu, Jiali ; Pike, Randy T.

A flexible, smooth, and low profile conformal coating was developed to accomplish the encapsulation of a microelectromechanical system (MEMS) device that will be applied to sense the static pressure on aircraft during real flight testing. The encapsulant should be able to protect the MEMS device and the multichip module (MCM) from adverse environmental conditions, i.e., mechanical shock, temperature fluctuation, engine fuel and oil contamination, and moisture/mobile ion permeation. Presently, conventional packaging schemes for electronics cannot satisfy this specific outdoor application, and a new encapsulation combination has been designed in accord with the requirement of reliability without hermeticity (RWOH). A bi-layer structure was selected because of property limitations of a single material. Pliable elastomeric silicones, are typically flexible, water repellant, and abrasion resistant. The silicone encapsulant will be first applied to planarize the MEMS surface and function as durable dielectric insulation, stress-relief, and shock/vibration absorbers over a wide humidity/temperature range. To compensate for the deficiency of silicone on engine fuel/oil contamination, Parylene C is to be deposited afterward. This bi-layer coating can achieve excellent bulk properties, such as moisture and mobile ion barrier resistance, chemical compatibility, and electrical insulation characteristics. However, the poor adhesion of Parylene C to silicone greatly restricts its application. To address this problem, silane coupling agents were used as an adhesion promoter. Significant adhesion im provement was achieved by placing an interlayer silane coupling agent to provide interfacial bonding to the silicone elastomeric surface and the Parylene C film. Furthermore, a possible mechanism of adhesion enhancement will also be presented in this study. Index Terms— Bi-layer conformal coating, micro-electromechanical system (MEMS), multichip module, Parylene C, reliability without hermeticity (RWOH), silane coupling agent, silicone elastomer.
Plasmon energy shift in mesoporous and double length-scale ordered nanoporous silica

(Georgia Institute of Technology, 1999-05-03) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)

Electron energy-loss spectroscopy studies are reported on three different types of structures: solid silica spheres, mesoporous silica, and the double length-scale ordered (DLSO) porous silica. The mesoporous silica has porosity at the length scale of nanometers. The DLSO porous silica has an additional ordering on submicron hollows created by the template polystyrene spheres. The plasmon energy of the porous silica shows a significant shift in comparison to that of the bulk, suggesting that the local density of the bound electrons in the porous structures is lower than that in the bulk. This gives the possibility of tuning the electronic structure of silica by varying its porosity, leading to even lower dielectric loss.
Synthesis and properties of Sr₂CeO₄ blue emission powder phosphor for field emission displays

(Georgia Institute of Technology, 1999-03-22) Jiang, Yongdong ; Zhang, Fuli ; Summers, Christopher J. ; Wang, Z. L. (Zhong Lin)

A blue emission powder phosphor Sr₂CeO₄ for field emission displays was prepared using a chemical coprecipitation technique, which is most suitable for large-scale production. The powders were fired at different temperatures to optimize the properties. Firing the powder at 1200 °C for 2 h gave the highest luminescence efficiency of 5.4 lm/W at 4 kV and 29.0 lm/W at 10 kV. The emission peak of this phosphor is at ~ 470 nm and Commission International de l'Eclairage coordinates are x = 0.19, y = 0.26.
Recent Advances in Plastic Packaging of Flip-Chip and Multichip Modules (MCM) of Microelectronics

(Georgia Institute of Technology, 1999-03) Wong, C. P. ; Wong, Michelle M.

The success in consumer electronics in the 1990’s will be focused on low-cost and high performance electronics. Recent advances in polymeric materials (plastics) and integrated circuit (IC) encapsulants have made high-reliability very-large-scale integration (VLSI) plastic packaging a reality. High-performance polymeric materials possess excellent electrical and physical properties for IC protection. With their intrinsic low modulus and soft gel-like nature, silicone gels have become very effective encapsulants for larger, high input/output (I/O) (in excess of 10 000), wire-bonded and flip-chip VLSI chips. Furthermore, the recently developed silica-filled epoxies underfills, with the well controlled thermal coefficient of expansion (TCE), have enhanced the flip-chip and chip-on-board, direct chip attach (DCA) encapsulations. Recent studies indicate that adequate IC chip surface protection with high-performance silicone gels and epoxies plastic packages could replace conventional ceramic hermetic packages. This paper will review the IC technological trends, and IC encapsulation materials and processes. Special focus will be placed on the high-performance silicone and epoxy underfills, their chemistries and use as VLSI device encapsulants for single and multichip module applications.
Comparative Study of Thermally Conductive Fillers for Use in Liquid Encapsulants for Electronic Packaging

(Georgia Institute of Technology, 1999-02) Wong, C. P. ; Bollampally, Raja Sheker

Thermal management plays a very vital role in the packaging of high performance electronic devices. Effective heat dissipation is crucial to enhance the performance and reliability of the packaged devices. Liquid encapsulants used for glob top, potting, and underfilling applications can strongly influence the package heat dissipation. Unlike molding compounds, the filler loading in these encapsulants is restrained. This paper deals with the development and characterization of thermally conductive encapsulants with relatively low filler loading. A comparative study on the effect of different ceramic fillers on the thermal conductivity and other critical properties of an epoxy based liquid encapsulant is presented.