Sangil Lee

Source apportionment of fine particulate matter in the southeastern United States

Abstract Particulate matter (PM) less than 2.5 μm in size (PM2.5)source apportionment by chemical mass balance receptor modeling was performed to enhance regional characterization of source impacts in the southeastern United States. Secondary particles, such as NH4HSO4, (NH4)2SO4,NH4NO3, and secondary organic carbon (OC) (SOC), formed by atmospheric photochemical reactions, contribute the majority (<50%) of ambient PM2.5 with strong seasonality. Source apportionment results indicate that motor vehicle and biomass burning are the two main primary sources in the southeast, showing relatively more motor vehicle source impacts rather than biomass burning source impacts in populated urban areas and vice versa in less urbanized areas. Spatial distributions of primary source impacts show that each primary source has distinctively different spatial source impacts. Results also find impacts from shipping activities along the coast. Spatiotemporal correlations indicate that secondary particles are more regionally distributed, as are biomass burning and dust, whereas impacts of other primary sources are more local.

Void Formation Study of Flip Chip in Package Using No-Flow Underfill

The advanced flip chip in package (FCIP) process using no-flow underfill material for high I/O density and fine-pitch interconnect applications presents challenges for an assembly process that must achieve high electrical interconnect yield and high reliability performance. With respect to high reliability, the voids formed in the underfill between solder bumps or inside the solder bumps during the no-flow underfill assembly process of FCIP devices have been typically considered one of the critical concerns affecting assembly yield and reliability performance. In this paper, the plausible causes of underfill void formation in FCIP using no-flow underfill were investigated through systematic experimentation with different types of test vehicles. For instance, the effects of process conditions, material properties, and chemical reaction between the solder bumps and no-flow underfill materials on the void formation behaviors were investigated in advanced FCIP assemblies. In this investigation, the chemical reaction between solder and underfill during the solder wetting and underfill cure process has been found to be one of the most significant factors for void formation in high I/O and fine-pitch FCIP assembly using no-flow underfill materials.

Assessment of Secondary Organic Carbon in the Southeastern United States: A Review

Abstract Organic carbon (OC) is one of the major components of ambient PM2.5 (particulate matter [PM] ≤ 2.5 µm in aerodynamic diameter) and a significant portion of OC is from secondary organic aerosol (SOA) formation in the southeastern United States. Various approaches (based on measurement and modeling results) are applied to estimate secondary organic carbon (SOC) and its origins in the region. SOC estimates by various methods are consistent as to clear seasonal variation (i.e., relatively higher SOC in summer) and little spatial variability (i.e., a regional characteristic of SOC). However, there are differences as to the origins of SOC. SOA organic tracer and emission-based modeling studies indicate that the biogenic origin of SOC is dominant in the Southeast, showing that biogenic-origin SOC accounts for 90% of SOC in summer and more than 70% even in other seasons. However, results from other studies suggest that the anthropogenic origin of SOC is dominant, significant amounts of anthropogenic-origin SOC, or important roles of anthropogenic pollutants for SOA formation, especially at urban areas, as strong correlations between water-soluble OC (an indicator of SOC) and anthropogenic pollutants, considerable amounts of fossil water-soluble OC, and significant contributions of fossil SOC (37–52% in summer months, 70–73% in winter months) are observed. Therefore, more studies are needed to reconcile the differences in the source attribution of SOC measurements.

Void Formation Mechanism of Flip Chip in Package Using No-Flow Underfill

This paper investigates the void formation mechanism induced by chemical interaction between eutectic solder (Sn63/Pb37) wetting and no-flow underfill material curing during flip chip in package assembly. During the process, low weight molecular components, such as fluxing agents and water molecules, could be induced by the chemical interaction between solder wetting and underfill curing when these components are heated to melt and cure, respectively. The low weight molecular components become volatile with exposure to temperatures above their boiling points; this was found to be the main source of the extensively formed underfill voiding. This mechanism of chemically and thermally induced voids was explained using void formation study, differential scanning calorimetry thermogram comparison, and gas chromatography and mass spectroscopy chemical composition identification on the suggested chemical reaction formula. This finding can enhance understanding of the mechanism that drives no-flow underfill voiding and can develop a void-free flip chip assembly process using no-flow underfill material for cost effective and high performance electronics packaging applications. Furthermore, this study provides the design guideline to develop an advanced no-flow underfill having high performance at high temperature range for the lead-free application.

A numerical study of void nucleation and growth in a flip chip assembly process

In this study, we develop mathematical models and numerical simulations of void nucleation and growth induced by the chemical reaction in the flip chip package assembly process using a no-flow underfill. During the thermal assembly process, the underfill chemically reacts to the oxidation of solders I/O on the chip, achieving interconnection between chip and substrate. The chemical reaction causes a large number of voids in the thermal reflow process. The voids have been considered as a critical defect, reducing the life of the thermal reliability. This study investigates the mechanism of void nucleation and growth based on classical bubble nucleation theory and bubble dynamics, respectively. This knowledge can provide a theoretical foundation to achieve a void-free assembly process and high reliability performance.

Sensitivity analysis of Pb free reflow profile parameters toward flip chip on silicon assembly yield, reliability and intermetallic compound characteristics

Flip chip process excels due to its low cost, fine pitch, small form factor and its ready-adaptation to the conventional Surface Mount Technology (SMT) process, in the fact that the reflow is often used to form the solder joint. As the use of Pb free solder is legislated today, it is vital to understand the impact of reflow process conditions on the formation of the flip chip solder joint, so that the assembly process of the flip chip can be better controlled. This paper introduces a comprehensive experimental study on the impact of Pb free reflow profile parameters towards flip chip on silicon assembly solder joint formation characteristics as well as the reliability performance. The reflow parameters studied include the soak time, peak temperature and time above liquidus. Three levels of each reflow parameter are investigated. The Response Surface Methodology (RSM) is used for Design of Experiment (DOE) to explore the quadratic effect of the investigated parameters. Results studied include the package assembly yield, package shear strength, intermetallic compound thickness as well as the package reliability performance. Study results show that the fine pitch flip chip on silicon package has a wide reflow process window to achieve 100% yield, if reflowed in a Nitrogen environment. Yield loss was found when the packages are reflowed in air. With the fifteen reflow profiles studied, it was found that the reflow parameters are not significant in terms of the package shear strength. For the intermetallic compound thickness, it was found that the time above liquidus is a significant factor, with a 99.9% confidence level. No statistical difference was found among packages assembled under different reflow conditions up to 2500 liquid to liquid thermal shock reliability testing.

Development of traceable precision dynamic dilution method to generate dimethyl sulphide gas mixtures at sub-nanomole per mole levels for ambient measurement

Dimethyl sulphide (DMS) is an important compound in global atmospheric chemistry and climate change. Traceable international standards are essential for measuring accurately the long-term global trend in ambient DMS. However, developing accurate gas standards for sub-nanomole per mole (nmol/mol) mole fractions of DMS in a cylinder is challenging, because DMS is reactive and unstable. In this study, a dynamic dilution method that is traceable and precise was developed to generate sub-nmol/mol DMS gas mixtures with a dynamic dilution system based on sonic nozzles and a long-term (>5 years) stable 10 μmol/mol parent DMS primary standard gas mixtures (PSMs). The dynamic dilution system was calibrated with traceable methane PSMs, and its estimated dilution factors were used to calculate the mole fractions of the dynamically generated DMS gas mixtures. A dynamically generated DMS gas mixture and a 6 nmol/mol DMS PSM were analysed against each other by gas chromatography with flame-ionisation detection (GC/FID) to evaluate the dilution system. The mole fractions of the dynamically generated DMS gas mixture determined against a DMS PSM and calculated with the dilution factor agreed within 1% at 6 nmol/mol. In addition, the dynamically generated DMS gas mixtures at various mole fractions between 0.4 and 11.7 nmol/mol were analysed by GC/FID and evaluated for their linearity. The analytically determined mole fractions showed good linearity with the mole fractions calculated with the dilution factors. Results showed that the dynamic dilution method generates DMS gas mixtures ranging between 0.4 nmol/mol and 12 nmol/mol with relative expanded uncertainties of less than 2%. Therefore, the newly developed dynamic dilution method is a promising reference method for generating sub-nmol/mol DMS gas standards for accurate ambient measurements.

Heterogeneous Void Nucleation Study in Flip Chip Assembly Process Using No-Flow Underfill

No-flow underfill process has exhibited a narrow feasible process window due to electrical assembly yield loss or underfill voiding. In general, the assembly yield can be improved using reflow process designed at high temperature, while the high temperature condition potentially causes serious underfill voiding. Typically, the underfill voiding can result in critical defects, such as solders fatigue cracking or solders bridge, causing early failures in thermal reliability. Therefore, this study reviews a classical bubble nucleation theory to model voids nucleation during reflow process. The established model designed a reflow process possibly preventing underfill voiding. The reflow process was validated using systematic experiments designed on the theoretical study with a commercial high I/O counts (5000>), fine-pitch (<150 μm) flip chip. The theoretical model exhibits good agreement with experimental results. Thus, this paper presents systematic studies through the use of structured experimentation designed to achieve a high, stable yield, and void-free assembly process on the classical bubble nucleation theory.

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

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 mum) 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.

Assembly Yields Characterization of High IO Density, Fine Pitch Flip Chip in Package Using No-Flow Underfill

The application of no-flow underfills for high IO density, fine-pitch, flip chip in package (FCIP) applications is analyzed. A number of commercially developed no flow underfills are evaluated. Process parameters for improved assembly yields depend strongly on the underfill materials characteristics and particularly the reflow profile. The test vehicles used in this study incorporate high I/O density, large chip size, and small interconnect pitch. This paper presents a methodology for evaluating new commercial no flow underfill materials, techniques for establishing baseline reflow profiles for yielding FCIP devices, and initial yield sensitivity analysis for the FCIP assembly process.