Wong, C. P.

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Now showing 1 - 7 of 7
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    Improved Stability of Contact Resistance of Low Melting Point Alloy Incorporated Isotropically Conductive Adhesives
    (Georgia Institute of Technology, 2003-06) Wong, C. P. ; Moon, Kyoung-Sik ; Wu, Jiali
    With the driving force of “green” revolution in the electronics industry, tremendous efforts have been made in pursuing lead-free alternatives. Although lately lead-free alloys have drawn a lot of attention, their technical weaknesses, such as high processing temperature, poor wetting and high surface tension, limit their applications on the thermally sensitive, flexible, nonsolderable substrates and the ultra-fine pitch size flip chip interconnection. Conventional isotropically conductive adhesives (ICAs) have been used widely in surface mount and die-attach technologies for electrical interconnection and heat dissipation. The low temperature processing of ICAs is one of the major advantages over lead-free solders, which brings a low system stress, simple manufacture process and the like. In order to enhance the contact resistance of ICAs, the low melting point alloy (LMA) incorporating technology has been developed by our group. In this paper, LMA fusing methods were studied, since nonfused LMA in ICAs after a curing process can adversely affect the physical property and contact resistance stability. A differential scanning calorimeter (DSC) was used for the basic examination of depleting rate of LMAs in the typical ICAs. The cross-sectional morphology, LMA distribution and intermetallic compound were investigated by a scanning electron microscope (SEM). In addition, contact resistance for the ICA formulation incorporated with LMAs under elevated temperature and humidity was evaluated.
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    Development of High Performance Interfill Materials for System Chips Technology
    (Georgia Institute of Technology, 2002-06) Wong, C. P. ; Wu, Jiali ; Bhattacharya, Swapan ; Lloyd, Courtney ; Pogge, H. Bernhard ; Tummala, Rao R.
    An innovative precisely interconnected chip (PIC) technology is currently under development at IBM to seek more effective means of creating system chips. The objective of this research is developing fabrication methods to permit the realization of high yielding large area chips, as well as chips that may contain very diverse technologies. This paper reports the use of a high-performance interfill material based on epoxy resin, which is used to connect the different chip sector macros that make up the system chip. This novel interfill material remains thermally stable through the subsequent processing temperature hierarchies during the interchip interconnection fabrication. Spherical SiO2 powders are incorporated into the epoxy resin to improve its mechanical properties, reduce coefficient of thermal expansion, and increase thermal conductivity. Adhesion and rheology of the formulated interfill materials are evaluated. Microstructure of SiO2 filled epoxy system is also investigated to confirm the reliability of the composite before and after thermal aging. Initial results indicate that the formulated EPOXY A resin composite is qualified for the system chip manufacturing process in terms of the dispensing processability, structural and mechanical integrity, and reliability.
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    Development of New Low Stress Epoxies for MEMS Device Encapsulation
    (Georgia Institute of Technology, 2002-06) Wong, C. P. ; Wu, Jiali
    In this study, a series of new low stress epoxies was introduced as conformal encapsulants, which show a high promise to meet all the requirements for the protection of the pressure sensor system. Mechanical properties such as initial Young’s modulus, toughness and ultimate tensile stress were evaluated. The more critical issue of material’s contamination resistance to the jet fuel was improved. And the mechanism behind materials lowstress and toughness behaviors was investigated from the viewpoint of microstructure.
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    Evaluation and Characterization of Reliable Non-Hermetic Conformal Coatings for Microelectromechanical System (MEMS) Device Encapsulation
    (Georgia Institute of Technology, 2000-11) Wong, C. P. ; Wu, Jiali ; Pike, Randy T. ; Kim, Namsoo P. ; Tanielian, Minas H.
    The thrust of this project was to evaluate commercial conformal encapsulation candidates for low cost aerospace applications. The candidate conformal coatings evaluated in this study included silicone elastomers, epoxies, and Parylenes with bi-layer or tri-layer designs. Properties characterized in this study included mobile ion permeation and moisture ingress resistance, interfacial adhesion variation through thermal shock cycling and 85 C/85% RH aging. Surface Insulation Resistance (SIR), Triple Track Resistance (TTR) and die shear strength were used for the corresponding electrical and physical property characterizations. Parylene F displayed excellent properties for environmental protection. Silicone elastomers displayed less resistance to the harsh environment as compared to the Parylene family (N, C, D types), but it could provide advantages for low residual stress applications. The change in adhesion strength between Parylene C and silicone elastomers after exposure to thermal shock cycling or 85 C/85%RH aging for different time periods were conducted from die shear test in terms of the interfacial failure. SIR values of all the candidate materials after 1000 h exposure to 85 C/85%RH, with 100V dc for resistance measurement, range from 1 108–1 109. Leakage current values after 1000 h exposure to 85 C/85%RH, 175 V bias, are in the range of 10 9 to 10 11 Amp. The bi- or tri-layer conformal coating combination investigated in this study showed significant promise for encapsulation of the microelectromechanical system (MEMS) devices.
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    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.
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    Correlation of Flip Chip Underfill Process Parameters and Material Properties with In-Process Stress Generation
    (Georgia Institute of Technology, 1999-01) Wong, C. P. ; Palaniappan, Prema ; Baldwin, Daniel F. ; Selman, Paul J. ; Wu, Jiali
    Electronic packaging designs are moving toward fewer levels of packaging to enable miniaturization and to increase performance of electronic products. One such package design is flip chip on board (FCOB). In this method, the chip is attached face down directly to a printed wiring board (PWB). Since the package is comprised of dissimilar materials, the mechanical integrity of the flip chip during assembly and operation becomes an issue due to the coefficient of thermal expansion (CTE) mismatch between the chip, PWB, and interconnect materials. To overcome this problem, a rigid encapsulant (underfill) is introduced between the chip and the substrate. This reduces the effective CTE mismatch and reduces the effective stresses experienced by the solder interconnects. The presence of the underfill significantly improves long term reliability. The underfill material, however, does introduce a high level of mechanical stress in the silicon die. The stress in the assembly is a function of the assembly process, the underfill material, and the underfill cure process. Therefore, selection and processing of underfill material is critical to achieving the desired performance and reliability. The effect of underfill material on the mechanical stress induced in a flip chip assembly during cure was presented in previous publications. This paper studies the effect of the cure parameters on a selected commercial underfill and correlates these properties with the stress induced in flip chip assemblies during processing.
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    Use of Compliant Adhesives in the Large Area Processing of MCM-D Substrates
    (Georgia Institute of Technology, 1998-10) Wong, C. P. ; Wu, Jiali ; Pike, Randy T.
    Large area substrate processing is a key solution for improving the productivity of multichip module deposition (MCM-D) technology. This project is focused on high temperature polymeric adhesives for attachment of silicon tiles to suitable pallets to facilitate large area film processing of MCM structures. Current polymeric high temperature adhesives are predominately polyimide-based that are not reworkable, which places an obstacle to remove the coated substrates and to reuse the high cost pallets. However, an approach will be presented in this paper to address this demand by introducing thermally cleavable links in the thermoset polyimide-amide resin. A series of novel reworkable high temperature (in excess of 350–400 ℃) adhesives have been developed, that can meet the requirements of adhesion, viscosity, thermal stability, and reworkability of the MCM-D production. Furthermore, scanning electron microscopy (SEM) microstructure images are presented for intuitive reworkability analysis.