Je-Young Chang

The demands for both thinner bare-die ball grid array (BGA) packages and thinner thermal solutions have added complexity for the thermal enabling design and material options associated with system on chip packages in mobile personal computer (PC) platforms. The thermomechanical interactions between the bare-die package and the thermal solution are very critical, creating the needs for: (1) an in-depth thermomechanical characterization to understand their impacts on product quality and performance and (2) a simple and yet robust modeling methodology to analyze design parameters using a commercially available software. In this paper, experimental metrologies and modeling methodology are developed with the details of contents documented. Validation of the newly developed tools and recommendation/guidance are also discussed for detailed assessments of thermomechanical tradeoffs for optimal design spaces for next-generation mobile platforms.

This paper reports the test results of vapor chambers using copper post heaters and silicon die heaters. Experiments were conducted to understand the effects of nonuniform heating conditions (hot spots) on the evaporative thermal performance of vapor chambers. In contrast to the copper post heater, which provides ideal heating, a silicon chip package was developed to replicate more realistic heat source boundary conditions of microprocessors. The vapor chambers were tested for hot spot heat fluxes as high as 746 W/cm2. The experimental results show that evaporator thermal resistance is not sensitive to nonuniform heat conditions, i.e., it is the same as in the uniform heating case. In addition, a model was developed to predict the effective thickness of a sintered-wick layer saturated with water at the evaporator. The model assumes that the pore sizes in the sintered particle wick layer are distributed nonuniformly. With an increase of heat flux, liquid in the larger size pores are dried out first, followed by drying of smaller size pores. Statistical analysis of the pore size distribution is used to calculate the fraction of the pores that remain saturated with liquid at a given heat flux condition. The model successfully predicts the experimental results of evaporative thermal resistance of vapor chambers for both uniform and nonuniform heat fluxes.

Pumped-loop liquid-cooled system involving phase-change (two-phase) is a promising technology for removing excess heat from high-density electronics. However, no consensus has been reached so far regarding the hydraulic and heat transfer characteristics of such two-phase (TP) micro flow passages with hydraulic diameters (Dh) on the order of several hundred micrometers or smaller. In a previous paper [Tong et al., ASME IMECE2007-42027], we reported flow boiling experiments for several microchannel (MC) cold plate devices with channel widths ranging from 61 to 340 μm (hydraulic diameters from 100 to 337 μm) and a micro-pin-fin array (μPFA) device. Two working fluids, deionized water and HFE-7100, were tested respectively. In this paper, we focus on the highspeed visualization study of the phase-change phenomena inside the MC cold plates. Features of the major TP flow patterns are discussed for each MC unit with varying sizes and with the two different working fluids. From direct visualization, we report the liquid thin film evaporation rate for the pulsating annular flow regions in MCs. The observed liquid film evaporation rate is much higher than previous predictions/assumptions from a steady-state annular flow picture as assumed by most theoretical investigations so far. The local transient heat flux is thus much higher than the average heat flux provided, especially for water and small hydraulic diameter MCs.