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Wong, C. P.

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Author: Tison, Christopher K. × Author: Wong, C. P. × search.filters.applied.f.isOrgUnitOfPublicationTitle: College of Engineering ×

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    A Novel Aluminum-Filled Composite Dielectric for Embedded Passive Applications
    (Georgia Institute of Technology, 2006-05) Wong, C. P. ; Xu, Jianwen ; Moon, Kyoung-Sik ; Tison, Christopher K.
    This paper presents the development of a novel aluminum- filled high dielectric constant composite for embedded passive applications. Aluminum is well known as a low-cost and fast self-passivation metal. The self-passivation forms a nanoscale insulating boundary outside of the metallic spheres, which has dramatic effects on the electrical, mechanical, and chemical behaviors of the resulting composites. Influences of aluminum particle size and filler loading on the dielectric properties of composites were studied. Because of the self-passivated insulating oxide layer of fine aluminum spheres, a high loading level of aluminum can be used while the composite materials continues to be insulating. Dielectric property measurement demonstrated that, for composites containing 80wt%3.0 maluminum, a dielectric constant of 109 and a low dissipation factor of about 0.02 can be achieved. The dielectric constant of epoxy-aluminum composites increased almost 30 times as compared with that of the pure epoxy matrix, which is about 3.5. Die shear tests showed that at such loading level, materials still had good processability and good adhesion toward the substrate. Bulk resistivity measurement, high-resolution transmission electron microscope (HRTEM) observation, and thermogravimetric analysis (TGA) were conducted to characterize the aluminum powders in order to understand the dielectric behavior of aluminumfilled composites. Bimodal aluminum-filled composites were also systematically studied in order to further increase the dielectric constant. Ouchiyama–Tanaka’s model was used to calculate the theoretical maximum packing fraction (MPF) of bimodal systems. Based on the calculation, rheology studies were performed to find the optimum bimodal filler volume fraction ratio that led to the best packing efficiency of bimodal fillers. It was found that the viscosity of polymer composites showed a minimum at optimum bimodal filler volume fraction ratio. A high dielectric constant of 160 (@10 kHz) with a low dissipation factor of less than 0.025 was achieved with the optimized bimodal aluminum composites. The developed aluminum composite is a promising candidate material for embedded capacitor applications.
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    Study on Underfill/Solder Adhesion in Flip-Chip Encapsulation
    (Georgia Institute of Technology, 2002-11) Wong, C. P. ; Fan, Lianhua ; Tison, Christopher K.
    Underfill materials are employed in flip-chip assemblies to enhance solder joint reliability performance. The adhesion of underfills with solders is important to the integrity of the flip-chip structure. We have studied the adhesion strength of two underfill samples with tin/lead (Sn/Pb) eutectic solder and tin/copper (Sn/Cu) lead-free solder, benchmarked with a copper surface. It was found that the adhesion of underfills and both solder materials was about 1/3 of the adhesion between underfills and copper. The effect of temperature and humidity aging as well as flux residue on adhesion strength was also investigated. A loss of adhesion was observed after the pressure cooker test, but 85 ℃/85% RH aging and flux residue revealed only a slight influence on adhesion strength. Surface analysis was performed on solid surfaces including copper, Sn/Pb eutectic solder, Sn/Cu lead-free solder and cured underfills by using the three-liquid-probe three-component surface tension method with a goniometer. The surface tension of liquid underfills was measured by the pendent drop method, and their contact angles on copper, Sn/Pb eutectic solder and Sn/Cu lead-free solder were also measured with a goniometer. The thermodynamic work of adhesion for underfills with copper and solder surfaces of different conditions was then calculated following these two surface analysis approaches. It was found that the thermodynamic work of adhesion was not correlated with the lap shear strength of underfills with copper and solder materials. Thus, the wetting property of an underfill on a substrate is not the determining factor for its practical adhesion strength. Various possible techniques for improving the adhesion of underfills and solder materials were then considered, and the use of additives in underfill formulations was experimented. However, we have not observed any significant effect of adhesion strength enhancement from any of these additives. Further tests of these additives with the base underfill formulation seemed to reveal a slight possibility to enhance adhesion of underfills and solders by proper manipulation of the underfill and/or flux formulation.