Wong, C. P.

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    Study and Characterization on the Nanocomposite Underfill for Flip Chip Applications
    (Georgia Institute of Technology, 2006-03) Wong, C. P. ; Sun, Yangyang ; Zhang, Zhuqing
    The nanosilica filled composite is a promising material for the no-flow underfill in flip-chip application. However, as the filler size decreases into the nano length scale, the rheological, mechanical, and thermal mechanical properties of the composite change significantly. The filler–filler and filler–polymer interactions have a profound impact on the material properties. The purpose of this paper is to achieve an in-depth understanding of the effect of the filler size and surface treatment on material properties and therefore to design a nanocomposite formulation with desirable material properties for no-flow underfill applications. Mono-dispersed nanosilica filler of 100 nm in size were used in this study. An epoxy/anhydride mixture was used as the base resin formulation. The nanosilica fillers were incorporated into the resin mixture to different filler loadings from 5 wt% to 40 wt% with or without silane coupling agents as the surface treatment. UV-Visible spectroscopy showed that the underfills with nano-size filler were transparent in the visible region even at high filler loading. The curing behavior and the Tg of the nanocomposite were studied using a modulated differential scanning calorimerter. It was found that the presence of the nanosilica could hinder the curing reaction, especially at the late stage of cure. The Tgs of the nanocomposites with untreated silica were found to decrease with the increasing filler loading. The measurement of the dynamic moduli from dynamic mechanical analyzer indicated that there was a secondary relaxation related to the filler–polymer interface. The coefficient of thermal expansion of the nanocomposite was measured using a thermal mechanical analyzer. The rheology of the nanocomposite was studied using a stress rheometer. It was found that the filler treatment could significantly reduce the viscosity of the nanocomposite and improve the processing capability of the underfill. Density measurements and moisture absorption experiments both indicated that the addition of nanosilica could increase the free volume of materials. The dispersion of the nanosilica in the cured composite materials was observed using scanning electron microscopy. Control samples with micron-size silica fillers were formulated and characterized for comparison.
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    Novel Reworkable Fluxing Underfill for Board-Level Assembly
    (Georgia Institute of Technology, 2004-09) Wong, C. P. ; Zhang, Zhuqing ; Li, Haiying
    Underfills are traditionally applied for flip-chip applications. Recently, there has been increasing use of underfill for board-level assembly including ball grid arrays (BGAs) and chip scale packages (CSPs) to enhance reliability in harsh environments and impact resistance to mechanical shocks. The no-flow underfill process eliminates the need for capillary flow and combines fluxing and underfilling into one process step, which simplifies the assembly of underfilled BGAs and CSPs for SMT applications. However, the lack of reworkability decreases the final yield of assembled systems. In this paper, no-flow underfill formulations are developed to provide fluxing capability, reworkability, high impact resistance, and good reliability for the board-level components. The designed underfill materials are characterized with the differential scanning calorimeter (DSC), the thermal mechanical analyzer (TMA), and the dynamic mechanical analyzer (DMA). The potential reworkability of the underfills is evaluated using the die shear test at elevated temperatures. The 3-point bending test and the DMA frequency sweep indicate that the developed materials have high fracture toughness and good damping properties. CSP components are assembled on the board using developed underfill. High interconnect yield is achieved. Reworkability of the underfills is demonstrated. The reliability of the components is evaluated in air-to-air thermal shock (AATS). The developed formulations have potentially high reliability for board-level components.
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    Recent Advances in Flip-Chip Underfill: Materials, Process, and Reliability
    (Georgia Institute of Technology, 2004-08) Wong, C. P. ; Zhang, Zhuqing
    In order to enhance the reliability of a flip-chip on organic board package, underfill is usually used to redistribute the thermomechanical stress created by the coefficient of thermal expansion (CTE) mismatch between the silicon chip and organic substrate. However, the conventional underfill relies on the capillary flow of the underfill resin and has many disadvantages. In order to overcome these disadvantages, many variations have been invented to improve the flip-chip underfill process. This paper reviews the recent advances in the material design, process development, and reliability issues of flip-chip underfill, especially in no-flow underfill, molded underfill, and wafer-level underfill. The relationship between the materials, process, and reliability in these packages is discussed.
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    Modeling of the Curing Kinetics of No-Flow Underfill in Flip-Chip Applications
    (Georgia Institute of Technology, 2004-06) Wong, C. P. ; Zhang, Zhuqing
    No-flow underfill has greatly improved the production efficiency of flip-chip process. Due to its unique characteristics, including reaction latency, curing under solder reflow conditions and the desire for no post-cure, there is a need for a fundamental understanding of the curing process of no-flow underfill. Starting with a promising no-flow underfill formulation, this paper seeks to develop a systematic methodology to study and model the curing behavior of this underfill. A differential scanning calorimeter (DSC) is used to characterize the heat flow during curing under isothermal and temperature ramp conditions. A modified autocatalytic model is developed with temperature-dependent parameters. The degree of cure (DOC) is calculated; compared with DSC experiments, the model gives a good prediction of DOC under different curing conditions. The temperature of the printed wiring board (PWB) during solder reflow is measured using thermocouples and the evolution of DOC of the no-flow underfill during the reflow process is calculated. A stress rheometer is used to study the gelation of the underfill at different heating rates. Results show that at high curing temperature, the underfill gels at a lower DOC. Based on the kinetic model and the gelation study, the solder wetting behavior during the eutectic SnPb and lead-free SnAgCu reflow processes is predicted and confirmed by the solder wetting tests.
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    FEM Modeling of Temperature Distribution of a Flip-Chip No-Flow Underfill Package During Solder Reflow Process
    (Georgia Institute of Technology, 2004-01) Wong, C. P. ; Zhang, Zhuqing ; Sitaraman, Suresh K.
    Flip chip on organic substrate has relied on underfill to redistribute the thermomechanical stress and to enhance the solder joint reliability. However, the conventional flip-chip underfill process involves multiple process steps and has become the bottleneck of the flip-chip process. The no-flow underfill is invented to simplify the flip-chip underfill process and to reduce the packaging cost. The no-flow underfill process requires the underfill to possess high curing latency to avoid gelation before solder reflow so to ensure the solder interconnect. Therefore, the temperature distribution of a no-flow flip-chip package during the solder reflow process is important for high assembly yield. This paper uses the finite-element method (FEM) to model the temperature distribution of a flip-chip no-flow underfill package during the solder reflow process. The kinetics of underfill curing is established using an autocatalytic reaction model obtained by DSC studies. Two approaches are developed in order to incorporate the curing kinetics of the underfill into the FEM model using iteration and a loop program. The temperature distribution across the package and across the underfill layer is studied. The effect of the presence of the underfill fillet and the influence of the chip dimension on the temperature difference in the underfill layer is discussed. The influence of the underfill curing kinetics on the modeling results is also evaluated.
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    Double-Layer No-Flow Underfill Materials and Process
    (Georgia Institute of Technology, 2003-05) Wong, C. P. ; Zhang, Zhuqing
    The no-flow underfill has been invented and practiced in the industry for a few years. However, due to the interfering of silica fillers with solder joint formation, most no-flow underfills are not filled with silica fillers and hence have a high coefficient of thermal expansion (CTE), which is undesirable for high reliability. In a novel invention, a double-layer no-flow underfill is implemented to the flip-chip process and allows fillers to be incorporated into the no-flow underfill. The effects of bottom layer underfill thickness, bottom layer underfill viscosity, and reflowprofile on the solder wetting properties are investigated in a design of experiment (DOE) using quartz chips. It is found that the thickness and viscosity of the bottom layer underfill are essential to the wetting of the solder bumps. Chip scale package (CSP) components are assembled using the double-layer no-flow underfill process. Silica fillers of different sizes and weight percentages are incorporated into the upper layer underfill. With a high viscosity bottom layer underfill, up to 40 wt% fillers can be added into the upper layer underfill and do not interfere with solder joint formation.
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    Double-Layer No-Flow Underfill Process for Flip-Chip Applications
    (Georgia Institute of Technology, 2003-03) Zhang, Zhuqing ; Lu, Jicun ; Wong, C. P.
    No-flow underfill technology shows potential advances over the conventional underfill technology toward a low-cost flop-chip underfill process. However, due to the filler entrapment in between solder bumps and contact pads on board, no-flow underfills are mostly unfilled or filled with very low filler loading. The high coefficient of thermal expansion (CTE) of the polymer material has significantly lowered the reliability of flip chip assembly and has limited its application to large chip assemblies. This paper presents a double-layer no-flow underfill process approach to incorporate silica filler into a no-flow underfill. Two layers of underfills are applied on to the substrate before chip placement. The bottom underfill layer facing the substrate is fluxed and unfilled; the upper layer facing the chip is filled with silica fillers. The total filler loading of the mixture is estimated to be around 55 wt%. The material properties of each layer of underfills, the underfill mixture, and a control unfilled underfill are characterized using differential scanning calorimeter (DCS), thermo-mechanical analyzer (TMA), dynamic mechanical analyzer (DMA), and a stress rheometer. FB250 daisy-chained test chips are assembled on FR-4 boards using the novel approach. A 100% assembly yield of solder Interconnect is achieved with the double-layer no-flow underfill while in the single-layer no-flow underfill process, no solder joint yield is observed. Scanning electronic microscope (SEM) and optical microscope are used to investigate the cross-section of both assemblies. A US provisional patent has been filed for this invention.
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    Assembly of Lead-free Bumped Flip-Chip with No-Flow Underfills
    (Georgia Institute of Technology, 2002-04) Wong, C. P. ; Zhang, Zhuqing
    Lead-free solder reflow process has presented challenges to no-flow underfill material and assembly. The currently available no-flow underfill materials are mainly designed for eutectic Sn–Pb solders. This paper presents the assembly of lead-free bumped flip-chip with developed no-flow underfill materials. Epoxy resin/HMPA/metal AcAc/Flux G system is developed as no-flow underfills for Sn/Ag/Cu alloy bumped flip-chips. The solder wetting test is conducted to demonstrate the fluxing capability of the underfills for lead-free solders. A 100% solder joint yield has been achieved using Sn/Ag/Cu bumped flip-chips in a no-flow process. A scanning acoustic microscope is used to observe the underfill voiding. The out-gassing of HMPA at high curing temperatures causes severe voiding inside the package. A differential scanning calorimeter (DSC) is used to study the curing degree of the underfill after reflow with or without post-cure. The post-curing profiles indicate that the out-gassing of HMPA would destroy the stoichiometric balance between the epoxy and hardener, and result in a need for high temperature post-cure. The material properties of the underfills are characterized and the influence of underfill out-gassing on the assembly and material properties is investigated. The impact of lead-free reflow on the material design and process conditions of no-flow underfill is discussed.
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    Development of Environmental Friendly Non-Anhydride No-Flow Underfills
    (Georgia Institute of Technology, 2002-03) Wong, C. P. ; Zhang, Zhuqing ; Fan, Lianhua
    Most no-flow underfill materials are based on poxy/anhydride chemistry. Due to the sensitizing nature, the use of anhydride is limited and there is a need for a no-flow underfill using nonanhydride curing system. This paper presents the development of novel no-flow underfill materials based on epoxy/phenolic resin system. Epoxy and phenolic resins of different structures are evaluated in terms of their curing behavior, thermo-mechanical properties, viscosity, adhesion toward passivation, moisture absorption and the reliability in flip-chip underfill package. The influence of chemical structure and the crosslinking density of the resin on the material properties is investigated. The assembly with nonanhydride underfill shows high reliability from the thermal shock test. Solder wetting test has confirmed the sufficient fluxing capability of phenolic resins. Results show that epoxy/phenolic system has great potential for an environmental friendly and highly reliable no-flow underfill.
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    Development of No-Flow Underfill Materials for Lead-Free Solder Bumped Flip-Chip Applications
    (Georgia Institute of Technology, 2001-03) Wong, C. P. ; Zhang, Zhuqing ; Shi, Songhua
    No-flow underfill process in flip-chip assembly has become a promising technology toward a smaller, faster and more cost-efficient packaging technology. The current available no-flow underfill materials are mainly designed for eutectic tin-lead (Sn/Pb) solders. With the advance of lead-free interconnection due to the environmental concerns, a new no-flow underfill chemistry needs to be developed for lead-free solder bumped flip-chip applications. Many epoxy resin/hexahydro-4-methyl phthalic anhydride (HMPA)/metal acetylacetonate material systems have been screened in terms of their curing behavior. Some potential base formulations with curing peak temperatures higher than 200 ℃ (based on differential scanning calorimetry at a heating rate of 5 ℃/min) are selected for further study. The proper fluxing agents are developed and the effects of fluxing agents on the curing behavior and cured material properties of the potential base formulations are studied using differential scanning calorimetry (DSC), thermomechanical analyzer (TMA), dynamic-mechanical analyzer (DMA), thermogravimetric analyzer (TGA), and rheometer. Fluxing capability of the developed no-flow formulations is evaluated using the wetting test of lead-free solder balls on a copper board. The developed no-flow underfill formulations show sufficient fluxing capability and good potential for lead-free solder bumped flip-chip applications.