Showing papers in "Journal of Electronic Packaging in 2004"

Transport Phenomena in Two-Phase Micro-Channel Heat Sinks

Abstract: The design and reliable operation of a two-phase micro-channel heat sink require a fundamental understanding of the complex transport phenomena associated with convective boiling in small, parallel coolant passages. This understanding is the primary goal of this paper. This goal is realized by exploring the following aspects of boiling in micro-channels: hydrodynamic instability, two-phase flow patterns, pressure drop, and convective boiling heat transfer. High-speed photographic methods were used to determine dominant flow patterns and explore as well as characterize hydrodynamic instabilities. Two types of dynamic instability were identified, a severe pressure drop oscillation and a mild parallel channel instability, and a simple method is recommended to completely suppress the former. Predictions of three popular two-phase pressure drop models and correlations were compared to micro-channel water data, and only a separated flow (Lockhart-Martinelli) correlation based on the assumption of laminar flow in both phases gave acceptable predictions. Several popular heat transfer correlations were also examined and deemed unsuitable for micro-channel heat sinks because all these correlations are based on turbulent flow assumptions, and do not capture the unique features of micro-channel flow such as abrupt transition to slug flow, hydrodynamic instability, and high droplet entrainment in the annular regime. These findings point to the need for further study of boiling behavior and new predictive tools specifically tailored to micro-channel heat sinks.Copyright © 2002 by ASME

Compact Modeling of Fluid Flow and Heat Transfer in Straight Fin Heat Sinks

Abstract: This work was supported by KISTEP ~Korea Institute of Science & Technology Evaluation and Planning! under grant number 2-578 through the National Research Lab Program.

Design of Cooling Systems for Electronic Equipment Using Both Experimental and Numerical Inputs

Abstract: This paper presents a methodology for the design and optimization of the cooling system for electronic equipment. In this approach, inputs from both experimentation and numerical modeling are to be used concurrently to obtain an acceptable or optimal design. The experimental conditions considered are driven by the numerical simulation, and vice versa. Thus, the two approaches are employed in conjunction, rather than separately, as is the case in traditional design methods. Numerical simulation is used to consider different geometries, materials and dimensions, whereas experiments are used for obtaining results for different flow rates and heat inputs, since these can often be varied more easily in experiments than in simulations. Also, transitional and turbulent flows are more accurately and more conveniently investigated experimentally. Thus, by using both the approaches concurrently, the entire design domain is covered, leading to a rapid, convergent, and realistic design process. Two simple configurations of electronic cooling systems are used to demonstrate this approach.Copyright © 2003 by ASME

Cluster of High-Powered Racks Within a Raised-Floor Computer Data Center: Effect of Perforated Tile Flow Distribution on Rack Inlet Air Temperatures

Abstract: This paper focuses on the effect on inlet rack air temperatures as a result of maldistribution of airflows exiting the perforated tiles located adjacent to the fronts of the racks. The flow distribution exiting the perforated tiles was generated from a computational fluid dynamics (CFD) tool called Tileflow (Trademark of Innovative Research, Inc.). Both raised floor heights and perforated tile free area were varied in order to explore the effect on rack inlet temperatures. The flow distribution exiting the perforated tiles was used as boundary conditions to the above floor CFD model. A CFD model was generated for the room with electronic equipment installed on a raised floor. Fourty racks of data processing (DP) equipment were arranged in rows in a data center cooled by chilled air exhausting from perforated floor tiles. The chilled air was provided by four A/C units placed inside a room 12.1 m wide × 13.4 m long. Since the arrangement of the racks in the data center was symmetric only one-half of the data center was modeled. The numerical modeling for above the raised floor was performed using a commercially available finite control volume computer code called Flotherm (Trademark of Flomerics, Inc.). The flow was modeled using the k-e turbulence model. Results are displayed to provide some guidance on the design and layout of a data center.© 2003 ASME

The Challenges of Electronic Cooling: Past, Current and Future

Abstract: This paper represents my personal recapitualation of my 4 decades of continuous involvement in all phases of electronic cooling, from conceptual design, through engineering development to product implementation. The cooling designs that we applied successfully in the past are reviewed chronologically. The challenges we are currently facing are also discussed and an attempt is made to forecast the challenges that will confront the electronics cooling community in the near and distant future. The paper includes a summary of IBM sponsored research spanning a period of 25 years at 12 universities on a wide range of topics related to electronic cooling technology.Copyright © 2003 by ASME

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

Abstract: Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406 × 2.032 mm cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal that the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Annual flow is identified as the dominant flow pattern for conditions relevant to two-phase micro-channel heat sinks, and forms the basis for development of a theoretical model for both pressure drop and heat transfer in micro-channels. Features unique to two-phase micro-channel flow, such as laminar liquid and gas flows, smooth liquid-gas interface, and strong entrainment and deposition effects are incorporated into the model. The model shows good agreement with experimental data for water-cooled heat sinks.Copyright © 2003 by ASME

Temperature Dependent Deformation Analysis of Ceramic Ball Grid Array Package Assembly Under Accelerated Thermal Cycling Condition

Abstract: A robust scheme of moireinterferometry for real-time observation is employed to study the temperature dependent thermo-mechanical behavior of a ceramic ball grid array package assembly. The scheme is implemented with a convection-type environmental chamber that provides the rapid temperature control required in accelerated thermal cycling. Thermal deformations are documented at various temperatures. Thermal-history dependent analyses of global and local deformations are presented. A significant nonlin- ear global behavior is documented due to complete stress relaxation at the maximum temperature. An analysis of solder interconnections reveals that inelastic deformation accumulates at the bottom eutectic solder fillet only at high temperatures. @DOI: 10.1115/1.1646426#

Environmental Aging and Deadhesion of Polyimide Dielectric Films

Abstract: One possible failure mechanism of products containing polyimide dielectric is deadhesion of polyimide from neighboring metallization. Deadhesion usually occurs due to the combined damage mechanisms of environmental aging and fatigue. In this paper, the rate of aging of Kapton-E polyimide is quantified as a function of temperature and humidity exposure using peel and tensile tests. An accelerated test methodology that accounts for both aging and fatigue and that can be used to evaluate the resistance of electronic products to polyimide deadhesion is then proposed.

Mechanical Characterization of the Heat Affected Zone of Gold Wirebonds Using Nanoindentation

Abstract: Abstrac t -With increasing miniaturization in microelectronics the wirebonds used in IC packages are witnessing a thrust towards fine pitch wirebonding. To have a precise control over loop height of the wirebond for fine pitch wirebonding, it is imperative to do mechanical characterization of the wirebond. The present work studies the mechanical properties of gold wire and wirebond using nanoindentation. The wirebond specimen surface was planarized using mechanical polishing. The loop height of the gold wirebond is directly proportional to the length of the heat affected zone (HAZ) above the ball of gold wirebond. Metallographic preparation of gold wirebond cross section reveals the presence of undesirable coarse grain structure in HAZ due to recrystallization and grain growth in the gold wire adjacent to the ball. The recrystallization temperature of our gold wire was found using D.S.C. to be 340.66C. The doping elements present in the gold wire used, were identified using TOF-SIMS. Nanoindentation of the gold wire was done at different maximum loads to observe the hardness variation with load. The nanoindentation of gold wirebond has confirmed a v-shaped hardness profile in the HAZ. The hardness minima for the particular gold wire used with a ball size ratio of 2.4 was observed at distance of 160-170 μm from the neck of the ball. The elastic modulus was found to vary randomly and to be independent of the microstructure in the wirebond. A yield stress profile based on empirical hardness-yield strength correlation has been predicted for the gold wirebond.

A Hybrid Thermal Energy Storage Device, Part 2: Thermal Performance Figures of Merit

Abstract: Two figures of merit for hybrid Thermal Energy Storage (TES) units are developed volumetric figure of merit, V ̃ , and the temperature control figure of merit, DT̃. A dimensional analysis shows that these quantities are related to the performance specificat the storage unit and its physical design. A previously benchmarked semi-empirical volume model is used to study the characteristics of various plate-type TES-unit de A parametric study is used to create a database of optimal designs, which is then u form simple correlations of V ̃ and DT̃ in terms of design requirements and attributes. preliminary design procedure utilizing these figures of merit is suggested. Sample c lations show that these correlations can be used to quickly determine the design attr of a plate-type TES-unit, given design requirements. @DOI: 10.1115/1.1646420 #

An Analytical Study of the Optimized Performance of an Impingement Heat Sink

Abstract: The results of a study of a new and unique high performance air-cooled impingement heat sink are presented An extensive numerical investigation of the heat sink performance is conducted and is verified by experimental data The study is relevant to cooling of high power chips and modules in air-cooled environments and applies to workstations or mainframes In the study, a rectangular jet impinges on a set of parallel fins and then turns into cross-flow The effects of the fin thickness and gap nozzle width and fin shape on the heat transfer and pressure drop are investigated It is found that pressure drop is reduced by cutting the fins in the central impingement zone without sacrificing the heat transfer due to a reduction in the extent of the stagnant zone A combination of fin thicknesses of the order of 05 mm and channel-gaps of 08 mm with appropriate central cut-out yielded heat transfer coefficients over 1500 W/m2 K at a pressure drop of less than 100 N/m2 , as is typically available in high-end workstations A detailed study of flow-through heat sinks, subject to the same constraints as the impingement heat sink showed that the flow-through heat sink could not achieve the high heat transfer coefficients at a low pressure dropCopyright © 2003 by ASME

Characterization of Substrate Materials for System-in-a-Package Applications

Abstract: System-in-a-Package (SiP) aims to integrate the entire system functions within a system-level package containing multiple ICs and other components interconnected in a high-density substrate. A structure based on the SiP concept is proposed in this paper. Based onthis SiP structure, four substrate candidates, namely FR4, liquid crystal polymer (LCP),teflon (PTFE) and low temperature co-fired ceramic (LTCC) were compared by assessingand measuring their mechanical reliability, electrical performance and environmentalinfluence. First, to evaluate long-term reliability, the 3D finite element method (FEM) wasused to calculate the stress distribution and warpage of the whole package. Both three-point bending and cooling in the manufacturing process were taken into consideration.The LCP has a coefficient of thermal expansion (CTE) close to that of the silicon chip,and a Young’s module close to that of FR4, which gave the best reliability in both bendingand cooling situations. Next, the dielectric constants and the loss tangent for the foursubstrates were evaluated in the electrical performance investigation. The LCP has a lowrelative dielectric constant and a low dissipation factor for the frequency range 1 GHz to35 GHz, making it a good substrate for high frequency applications. An environmentalassessment included several environmental impact categories; this assessment indicatedthat LCP is the most environmentally acceptable substrate. @DOI: 10.1115/1.1648057#

Effect of Underfill Entrapment on the Reliability of Flip-Chip Solder Joint

Abstract: The application of underfill materials to fill up the room between the chip and substrate is known to substantially improve the thermal fatigue life of flip chip solder joints. Nowadays, no-flow underfill materials are gaining much interest over traditional underfill as the application and curing of this type of underfill can be undertaken before and during the reflow process and thus aiding high volume throughput. However, there is always a potential chance of entrapping no-flow underfill in the solder joints. This work, attempts to find out the extent of underfill entrapment in the solder joints and its reliability effect on the flip chip packages. Some unavoidable underfill entrapments at the edges of the joint between solder bumps and substrate pads are found for certain solder joints whatever bonding conditions are applied. It is interesting to report for the first time that partial underfill entrapment at the edges of the solder joint seems to have no adverse effect on the fatigue lifetime of the samples since most of the first solder joint failure in the no-flow flip chip samples during thermal cycling are not at the site of solder interconnection with underfill entrapment. Our modeling results show good agreement with the experiment that shows underfill entrapment can actually increase the fatigue lifetime of the no-flow flip chip package. @DOI: 10.1115/1.1756590#