Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed

Convective Transport in Nanofluids

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

Empirical correlating equations for predicting the effective thermal conductivity and dynamic viscosity of nanofluids

A review of nanofluid stability properties and characterization in stationary conditions

With the continual increase in cooling demand for microprocessors, there has been an increased focus within the microelectronics industry on developing thermal solutions. Thermal interface materials (TIMs) play a key role in thermally connecting various components of the thermal solution. Review of the progress made in the area of TIMs in the past five years is presented. The focus is on the rheology-based modeling and design of polymeric TIMs due to their widespread use. Review of limited literature on the thermal performance of solders is also provided. Merits and demerits of using nanoparticles and nanotubes for TIM applications are also discussed. I conclude the paper with some directions for the future that I feel are relatively untouched and potentially very beneficial

https://weble.upc.edu/ifsin/Block5/paper_proc7.pdf

Thermal Interface Materials: Historical Perspective, Status, and Future Directions

A testing method for assessing solder joint reliability of FCBGA packages

During the thermal interface material (TIM) development for flip chip packages, it was found that the shear storage modulus, G', was a key parameter impacting the material processing and the package thermal performance. When the G' was too low, the TIM was pumped out easily after curing. When the G' was too high, delamination occurred at the interfaces between the TIM and die or heat spreader, especially at the die corners. Due to the significant warpage caused by the large die size, the TIM needs to be a soft gel. This will allow the material to maintain its interfacial adhesion at large deformation. In this paper, the shear modulus testing method for TIM's development will be discussed

Shear Modulus Measurement for Thermal Interface Materials in Flip Chip Packages

As the bump diameter and bump pitch of flip chip packages get smaller, the underfill becomes more resistant to flow. It is also necessary for the underfill to have low viscosity. High throughput also requires the underfill to have low viscosity. Problems associated with low viscosity underfills include filler settling and flow induced voids due to fast edge flow. In this paper, we discuss how the rheological properties can affect underfill filler settling and flow voids. The viscosities of several underfill materials were measured at both room and dispensing temperatures. The effects of yield stress of underfill on filler settling, and shear thickening of underfill at large shear rate on underfill flow voids, were investigated. SEM and CSAM were used to study the filler settling and flow voids. It was shown that the underfills with small fillers have shear-thickening viscosity and yield stress. The filler settling of underfill with a yield stress was greatly reduced. The flow voids were also reduced for underfill with shear-thickening viscosity.

The effect of flow properties on filler settling of underfill in the flip chip package

Particle laden polymers are one of the most prominent thermal interface materials (TIM) used in electronics cooling. Most of the research groups have primarily dealt with the understanding of the thermal conductivity of these types of TIMs. Thermal resistance is not only dependent on the thermal conductivity but also on the bond line thickness (BLT) of these TIMs. It is not clear that which material property(s) of these particle laden TIMs affects the BLT. This paper discusses the experimental measurement of rheological parameters such as non-Newtonian strain rate dependent viscosity and yield stress for 3 different particle volume fraction and 3 different base polymer viscosity materials. These rheological and BLT measurements vs. pressure will be used to model the BLT of particle-laden systems for factors such as volume fraction.Copyright © 2002 by ASME

Rheological Study of Micro Particle Laden Polymeric Thermal Interface Materials: Part 1 — Experimental

A 3-D flow model, combined with the volume of fluid method, was developed to track capillary driven epoxy flow front, during the underfill process. Die edge effects and fillet spread based on the effect of process parameters were included in this model. Numerical results were validated against flow visualization data. Full field imaging experiments with quartz die were performed to observe flow front as a function of flow time. Development of this new modeling methodology focuses on predicting the capillary underfill flow behavior both in the first level interconnect area between a die and a substrate, and the area surrounding the die. This paper revealed that the edge flow effect contributes to pulling the overall flow front in the bump field area and formation of the fillet spread, which affects the risk associated with underfill delamination in flip chips. A comparative study, which included modeling results and experimental data, clearly indicated that the die edge effect cannot be neglected in order to capture the right trends of flow front shape evolution and an accurate fill time prediction. These comparisons also showed that the model developed in this paper is adequate to approximately simulate the fillet configuration all around the die after encapsulation of flip chips. The modeling methodology developed in this paper provides a fundamental understanding of underfill flow as a function of dispense process parameters, material properties, and package design parameters.

Jinlin Wang

Papers

A testing method for assessing solder joint reliability of FCBGA packages

Shear Modulus Measurement for Thermal Interface Materials in Flip Chip Packages

The effect of flow properties on filler settling of underfill in the flip chip package

Rheological Study of Micro Particle Laden Polymeric Thermal Interface Materials: Part 1 — Experimental

3-D Numerical Simulation and Validation of Underfill Flow of Flip-Chips