Person:

Wong, C. P.

Permanent Link

https://hdl.handle.net/1853/71846

Associated Organization(s)

Organizational Unit

School of Materials Science and Engineering

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Publication Search Results

Now showing 1 - 10 of 34

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.
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.
Moisture Absorption in Uncured Underfill Materials

(Georgia Institute of Technology, 2004-06) Wong, C. P. ; Luo, Shijian

This paper presents a systematic study on moisture absorption in uncured underfill based on epoxy cured with acid anhydride [methylhexahydrophthalic anhydride (MHHPA)] and epoxy cured with non-acid anhydride curing agent. The influence of absorbed moisture on curing properties, thermomechanical property, and adhesion property of underfill after curing has been investigated. For epoxy cured with non-acid anhydride, the moisture absorption is low, and the absorbed moisture has no significant effect on the properties of cured underfill materials. For epoxy cured with acid anhydride, the moisture absorption before curing can be more than 2.0%, and the absorbed moisture can affect the properties significantly. The absorbed moisture can catalyze the curing reaction between acid anhydride and epoxy. The glass transition temperature of the cured samples is reduced after the underfill absorbs the moisture before curing. The adhesion strength decreases dramatically after the underfill absorbs the moisture before curing.
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.
Next Generation of 100-μm-Pitch Wafer-Level Packaging and Assembly for Systems-on-Package

(Georgia Institute of Technology, 2004-05) Wong, C. P. ; Kang, E. T. ; Tay, Andrew A. O. ; Wong, E. H. ; Swaminathan, Madhavan ; Iyer, Mahadevan K. ; Rotaru, Mihai D. ; Tummala, Rao R. ; Doraiswami, Ravi ; Ang, Simon S. ; Kripesh, V.

According to the latest ITRS roadmap, the pitch of area array packages is expected to decrease to 100 μm by 2009. Simultaneously, the electrical performance of these interconnections needs to be improved to support data rates in excess of 10 Gbps, while guaranteeing thermomechanical reliability and lowering the cost. These requirements are challenging, thus, needing innovative interconnection designs and technologies. This paper describes the development of three interconnection schemes for wafer-level packages (WLPs) at 100-μm pitch, involving rigid, compliant, and semicompliant interconnection technologies, extending the state of the art in each. Extensive electrical and mechanical modeling was carried out to optimize the geometry of the interconnections with respect to electrical performance and thermomechanical reliability. It was found that the requirements of electrical performance often conflict with those of thermomechanical reliability and the final “optimum” design is a tradeoff between the two. For the three interconnection schemes proposed, it was found that the electrical requirements can be met fairly well but acceptable mechanical reliability may require organic boards with coefficient of thermal expansion of 10 ppm/K or lower.
High dielectric insulation coating for time domain reflectometry soil moisture sensor

(Georgia Institute of Technology, 2004-04) Fujiyasu, Y. ; Pierce, C. E. ; Fan, L. ; Wong, C. P.

Insulation coating is often applied to time domain reflectometry (TDR) soil moisture sensors to reduce the conduction loss of energy. However, because of low dielectric constants of common plastic insulation materials, sensitivity is reduced, and the measurements are strongly dependent on coating thickness. Analytical studies clearly showed that these unwanted features could be mitigated if a high dielectric material is used for coating. An epoxy-ceramic nanocomposite was selected as a high dielectric insulation material because of its high dielectric constant and high adhesion. Experimental work using this composite indicated that the material has the potential to be used as a high dielectric coating for TDR soil moisture probes. On the basis of the results of these analytical and experimental studies a framework for designing an improved epoxy-ceramic composite for coating TDR soil moisture sensors is suggested.
A Reworkable Epoxy Resin for Isotropically Conductive Adhesive

(Georgia Institute of Technology, 2004-02) Wong, C. P. ; Li, Haiying

Electrically conductive adhesive (ECA) is a promising alternative to the toxic eutectic tin-lead solder as an interconnect material. Typical ECAs use epoxy resin as their matrix, which has superior properties over other polymers, such as high adhesion, and low dielectric constant. However, once cured, it is not reworkable. In this study, a liquid diepoxide was designed and synthesized, and used in isotropically conductive adhesive (ICA) formulations. This diepoxide has a molecular structure able to thermally decompose at mild temperature that allows selective individual removal of the bad component without damaging the board and its surroundings. The characterizations including proton and carbon 13 nuclear magnetic resonance, infrared spectroscopy indicated the success of the synthesis. A dual-epoxy system containing this secondary diepoxide and an equivalent bisphenol-A diepoxide were formulated and cured with an anhydride hardener and an imidazole catalyst. Thermal analyses, such as differential scanning calorimetry, thermo-gravimetry analysis (TGA), thermo-mechanical analysis (TMA) and dynamic mechanical analysis (DMA) were employed for the curing kinetics, thermal degradation behavior, glass transition temperature, coefficient of thermal expansion (CTE), and mechanical modulus, respectively. The dual-epoxy system showed two exothermal curing peaks at 140°C and 180°C, respectively. The thermoset of this dual-epoxy system has a decomposition temperature around 234°C, a glass transition temperature around 80 to 90°C, and CTEs of 74 ppm/°C and 225 ppm/°C below and above its Tg, respectively. The rework test on a surface mount component bonded to copper surface showed this ECA can be easily and quickly removed from the copper surface. The bulk resistance and contact resistance of ICAs were measured before and during an accelerated aging process in a temperature/humidity chamber (85°C/85% RH). The ECA showed good bulk resistivity and contact resistance comparable to its control and a commercial ECA on gold and copper surface finishes.
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.
Surface Property of Passivation Layer on Integrated Circuit Chip and Solder Mask Layer on Printed Circuit Board

(Georgia Institute of Technology, 2003-10) Wong, C. P. ; Luo, Shijian

Adhesion of underfill to passivation layer on integrated circuit chip and solder mask layer on printed circuit board is critical to the reliability of an underfilled flip chip package. In this study, the surface properties of solder mask and four passivation materials: benzocyclobutene (BCB), polyimide (PI), silicon dioxide (SiO₂)and silicon nitride (SiN) were investigated. A combination of both wet and dry cleaning processes was very effective to remove contaminants from the surface. The element oxygen, introduced during O₂plasma treatment or UV/O₃treatment, led to the increase of the base component of surface tension. X-ray photoelectron spectroscopy (XPS) experiments confirmed the increase of oxygen concentration at the surface after UV/O₃treatment. Wetting of underfill on passivation and solder mask was slightly improved at higher temperatures. Although UV/O₃ cleaning and O₂plasma treatment significantly improved the wetting of underfill on passivation materials, they did not improve adhesion strength of epoxy underfill to passivation. Therefore, the wetting was not the controlling factor in adhesion of the system studied.
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.