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School of Materials Science and Engineering

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Author: Wong, C. P. × End date: 2009 × Start date: 2009 × search.filters.applied.f.isOrgUnitOfPublicationTitle: School of Materials Science and Engineering ×

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Effect of Permittivity and Permeability of a Flexible Magnetic Composite Material on the Performance and Miniaturization Capability of Planar Antennas for RFID and Wearable Wireless Applications

2009-12 , Martin, Lara J. , Ooi, Sooliam , Staiculescu, Daniela , Hill, Michael D. , Wong, C. P. , Tentzeris, Emmanouil M.

This paper is an investigation of the feasibility of applying a mechanically flexible magnetic composite material to radio frequency identification (RFID) planar antennas operating in the lower ultrahigh-frequency (UHF) spectrum (∼300– 500 MHz). A key challenge is that the magnetic loss introduced by the magnetic composite must be sufficiently low for successful application at the targeted operating frequency. A flexible magnetic composite comprised of particles of Z-phase Co hexaferrite, also known as Co₂Z, in a silicone matrix was developed. To the authors’ knowledge, this is the first flexible magnetic composite demonstrated to work at these frequencies. The benchmarking structure was a quarter-wavelength microstrip patch antenna. Antennas on the developed magnetic composite and pure silicone substrates were electromagnetically modeled in Ansoft High- Frequency Sounder System full wave electromagnetic software. A prototype of the antenna on the magnetic composite was fabricated, and good agreement between the simulated and measured results was found. Comparison of the antennas on the magnetic composite versus the pure silicone substrate showed miniaturization capability of 2.4× and performance differences of increased bandwidth and reduced gain, both of which were attributed in part to the increase in the dielectric and magnetic losses. A key finding of this paper is that a small amount of permeability (μr∼2.5) can provide a substantial capability for miniaturization, while sufficiently low-magnetic loss can be introduced for successful application at the targeted operating frequency. This magnetic composite shows the capability to fulfill this balance and to be a feasible option for RFID, flexible wearable, and conformal applications in the lower UHF spectrum.

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Novel Nano-Scale Conductive Films With Enhanced Electrical Performance and Reliability for High Performance Fine Pitch Interconnect

2009-02 , Li, Yi , Yim, Myung Jin , Moon, Kyoung-Sik , Wong, C. P.

In this paper, a novel nano-scale conductive film which combines the advantages of both traditional anisotropic conductive adhesives/films (ACAs/ACFs) and nonconductive adhesives/films (NCAs/NCFs) is introduced for next generation high-performance ultra-fine pitch packaging applications. This novel interconnect film possesses the properties of electrical conduction along the z direction with relatively low bonding pressure (ACF-like) and the ultra-fine pitch (< 30 μm) capability (NCF-like). The nano-scale conductive film also allows a lower bonding pressure than NCF to achieve a much lower joint resistance (over two orders of magnitude lower than typical ACF joints) and higher current carrying capability. With low temperature sintering of nano-silver fillers, the joint resistance of the nano-scale conductive film was as low as 10―5 Ohm. The reliability of the nano-scale conductive film after high temperature and humidity test (85°C/85% RH) was also improved compared to the NCF joints. The insertion loss of nano-scale conductive film joints up to 10 GHz was almost the same as that of the standard ACF or NCF joints, suggesting that the nano-scale conductive film is suitable for reliable high-frequency adhesive joints in microelectronics packaging.

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Enhanced Electrical Properties of Anisotropic Conductive Adhesive With $pi$ -Conjugated Self-Assembled Molecular Wire Junctions

2009-09 , Zhang, Rongwei , Li, Yi , Yim, Myung Jin , Moon, Kyoung-Sik , Lu, Daoqiang , Wong, C. P.

We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer- sized ( 500 nm in diameter) silver (Ag) particle as conductive filler with the effect of -conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability ( 3400 mA) than those with micro-sized Au-coated polymer particles ( 2000 mA) and Ag nanoparticles ( 2500 mA). More importantly, by construction of -conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.

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Near Void-Free Assembly Development of Flip Chip Using No-Flow Underfill

2009-04 , Lee, Sangil , Yim, Myung Jin , Master, Raj N. , Wong, C. P. , Baldwin, Daniel F.

The advanced flip-chip-in-package (FCIP) process technology, using no-flow underfill material for high I/O density (over 3000 I/O) and fine-pitch (down to 150 μm) interconnect applications, presents challenges for flip chip processing because underfill void formation during reflow drives interconnect yield down and degrades reliability. In spite of such challenges, a high yield, reliable assembly process (> 99.99%) has been achieved using commercial no-flow underfill material with a high I/O, fine-pitch FCIP. This has been obtained using design of experiments with physical interpretation techniques. Statistical analysis determined what assembly conditions should be used in order to achieve robust interconnects without disrupting the FCIP interconnect structure. However, the resulting high yield process had the side effect of causing a large number of voids in the FCIP assemblies. Parametric studies were conducted to develop assembly process conditions that would minimize the number of voids in the FCIP induced by thermal effects. This work has resulted in a significant reduction in the number of underfill voids. This paper presents systematic studies into yield characterization, void formation characterization, and void reduction through the use of structured experimentation which was designed to improve assembly yield and to minimize the number of voids, respectively, in FCIP assemblies.

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