Electronics Cooling Using a Self-Contained, Sub-Cooled Pumped Liquid System

Evaluation of analytical models for thermal analysis and design of electronic packages

Recently energy saving is most important concept for all electric products and production. Especially, in Data-Center cooling system, power consumption of current air cooling system is increasing. For not only improving thermal performance but also reducing electric power consumption of this system, liquid cooling system has been developed. This paper reports the development of cold plate technology and vapor chamber application by using micro-channel fin. In case of cold plate application, micro-channel fin technology is good for compact space design, high thermal performance, and easy for design and simulation. Another application is the evaporating surface for vapor chamber. The well-known devices for effective heat transfer or heat spreading with the lowest thermal resistance are heat pipes and vapor chamber, which are two-phase heat transfer devices with excellent heat spreading and heat transfer characteristics. Normally, vapor chamber is composed of sintered power wick. Vapor chamber container is mechanically supported by stamped pedestal or wick column or solid column, but the mechanical strength is not enough strong. So far, the application is limited in the area of low strength assembly. Sometime the mechanical supporting frame is design for preventing deformation. In this paper, the testing result of sample is described that thermal resistance between the heat source and the ambient can be improved approximately 0.1°C/W by using the micro-channel vapor chamber. Additionally, authors presented case designs using vapor chamber for cooling computer processors, and proposed ideas of using micro-channel vapor chamber for heat spreading to replace the traditional metal plate heat spreader.

Originally published in IEEE Transactions on Components and Packaging Technology Vol. 20 No. 2. IEEE holds all copyright of this article. IEEE allows the final published version of author's own work to be deposited in institutional repositories.

With increased performance requirements for smaller, more capable, and more efficient electronic systems, thermal challenges have become critical issues in electronic packaging design. Breakthroughs are needed in advanced cooling solutions and pragmatic design at chip, board, and system package levels. In response to these critical needs, advanced materials and process improvements in packaging and cooling technology are required to provide high thermal transfer efficiency, well-controlled thermal transient behavior, environmental compatibility, low weight, and acceptable material and fabrication cost. The lowest device temperature and expected reliability will be achieved when the lowest package thermal resistance is achieved. This chapter provides an outline of thermal challenges in advanced electronic packaging, heat transfer theory, and its application in thermal design, development of thermal management solutions, advanced thermal management materials selection and component design guideline, as well as environmental compatibility requirements in thermal management of electronic packaging.

The development of high power and compact size semiconductor packages lead to the challenge in the need of high thermal dissipation design. For package level thermal solution, metal (especially copper) heat spreader and metal/metal-oxide filler thermal interface material are widely adopted. However, the thermal performance of packages is limited by isotropic conductive behavior of most of these materials. The thermal conducting performance could be further improved by directing the heat transportation in the direction desired, for instances the in-plane dissipation for heat spreader and the out-of-plane dissipation for thermal interface material. This present study reported the successful adoption of anisotropic thermal conductive pyrolytic graphite sheet as heat spreader and vertically aligned graphite sheet as thermal interface material in semiconductor package, individually. The Joint Electron Device Engineering Council standard still air junction to ambient measurement experiment shows that a flip chip ball grid array package incorporated with a pyrolytic graphite sheet as heat spreader improved thermal resistance of the module by 20%. The thermal dissipation behavior of the module was simulated with a well validated computational thermal dynamics model based on the experimental results. The anisotropic thermal conductive vertically aligned graphite sheet was also adopted as thermal interface material to replace the nowadays grease type thermal interface material. A thermal test chip assembled large size high performance flip chip ball grid array package mounted on a Joint Electron Device Engineering Council defined 4 layers thermal test printed circuit board was chosen as the test vehicle for thermal interface material performance test. The junction to case measurement experiment shows that the package with the anisotropic conductive thermal interface material improved the thermal resistance by 40%. Some novel designs were proposed basing on the application of this anisotropic thermal conductive material as validated by thermal simulation study. Package design guideline basing on the implementation of the anisotropic thermal conductive material will be discussed in this paper.

Aiming at the temperature control of the liquid cooling system of aircraft electronic equipment, the thermal characteristics of the system under different temperature conditions are analyzed through model simulation, and a calculation method is given for the thermal design of the liquid cooling system. This paper simplifies the system and establishes a numerical calculation model for the heat transfer of the main components (such as liquid storage tanks, gear pumps, radiators, etc.). The temperature changes in the main components of the system within the operating temperature range of the system (-40 °C-50 °C) were calculated, and the thermal characteristics of the cold plate were obtained by using the AMESim software. Further, temperature control schemes of several liquid cooling systems are compared for their working efficiency under low temperature and over-temperature conditions based on the original model. The results show that the temperature index of the airborne liquid cooling system basically meets the technical requirements of the cold plate inlet temperature (5 °C-30 °C) under typical operating conditions. Under low-temperature conditions, the rapid heating of the cold plate can be achieved through the electric heater control scheme. Under high-temperature circumstances, opening the ram air port scheme has a better cooling effect than increasing the fan speed scheme, but the former needs to consider the flight conditions to avoid the cold plate temperature exceeding the target range.

This paper presents the results of thermal performance and mechanical reliability co-design of flip chip devices by a comprehensive evaluation of package structure and material effects. The study provided valuable insights that can assist to identify potential package solutions and enhancement options by considering process, materials, thermal performance and mechanical reliability. With concurrent thermal and mechanical analysis, the thermal trends obtained from this study can be used to enhance the selection of thermal options and the reliability insights gained allowed further mechanical optimizations to identified solutions. The study evaluates both the larger sized FC-BGA and smaller sized FC-CSP using selected production, development and potential devices. A total of 6 FC-BGA and 5 FC-CSP test vehicles were selected for the study. The UTAC-patented XP (eXtra Performance) embedded heat spreader was also assessed and benchmarked against the other test vehicles. Thermal performance analysis, detailed package level stress and warpage analysis were conducted for all the identified test vehicles. For both FC-BGA and FC-CSP devices, analysis results showed that the bare die structure has the highest warpage and highest junction-to-ambient thermal resistance. The addition of mold compound is able to improve warpage for small die FC-BGA packages. However, mold compound results in insignificant warpage improvements when die-topackage ratio is large, such as in the FC-CSP. Concurrent thermal analysis shows that mold compound slightly enhances the thermal performance of both FC-BGA and FC-CSP devices under no/low wind velocities. Heat spreaders or heat sinks are comparatively more effective thermal enhancement solutions. The effects of heat spreaders were analyzed using several different package structures. Other than the thermal benefits reaped, heat spreaders brought about significant warpage control. Especially in the case of XP, the combined effect of the mold compound and the embedded heat spreader remarkably reduced warpage even in package with large dieto-package ratio. However, stress analysis showed a 10-25% increase in die stress for both small and large die packages when the heat spreader is used. With the recent interest and migration to the use of moldable underfill material (MUF), its performance was also benchmarked against the conventional capillary underfill (CUF) and mold compound. Thermal analysis results shows that MUF results in minor thermal enhancements compared to the capillary underfill (CUF). Mechanical analysis results showed that the MUF improves package coplanarity when the package exhibited smiling warpage behaviour at room temperature. The shift in critical die and underfill stresses was also observed with the change in material set. The thermal performance and mechanical reliability costudy adopted a holistic approach that obtained the trends and insights learnt that enabled informed choices to be made in materials selection and identification of potential packages. The approach and the insights obtained are thus presented in this paper.

Thermal simulation had been conducted on flip chip packages with copper heat spreader and better thermal performance was achieved. The copper heat spreader was able to enhance the heat spreading from the die but thermal performance was affected due to the thin layer of thermal interface materials. It was observed that thermal performance gain can be as high as 30% from heat spreader. Ansys thermo-mechanical simulation showed that heat spreader was reduced the overall package warpage but stresses in the solder bump increased due to the stiffer heat spreader. The solder bump joint fatigue life of the heat spreader package was subsequently reduced. A submodeling technique was developed in a three dimensional true symmetry global model at the critical joint location to find out the fatigue life of the package.

The 45。heat spreading angle is familiar among thermal designers. This angle has been used for thermal design of electronic devices, and provides a heat spreading area inside a board, e.g. printed circuit board, which is placed between a heat dissipating element and a relatively large heat sink. By using this angle, the heat transfer behavior can be estimated quickly without using high-performance computers. In addition, the rough design can be made easily by changing design parameters. This angle is effective in a practical situation; however, the discussion has not been made sufficiently on the applicability of the 45。heat spreading angle. In the present study, therefore, the extensive numerical investigation is conducted for the rational thermal design using the 45。heat spreading angle. The two-dimensional mathematical model of the board is considered; the center of the top is heated by a heat source while the bottom is entirely cooled by a heat sink. The temperature distribution is obtained by solving the heat conduction equation numerically with the boundary conditions. From the numerical results, the heat transfer behavior inside the board is shown and its relation with the design parameters is clarified. The heat transfer behavior inside the 45。heat spreading area is also evaluated. The applicability is moreover discussed on the thermal resistance of the board obtained by the 45。heat spreading angle. It is confirmed that the 45。heat spreading angle is applicable when the Biot number is large, and then the equations are proposed to calculate the Biot number index to use the 45。angle. Furthermore, the validity of the 45。heat spreading angle is also confirmed when the isothermal boundary condition is used at the cooled section of the board.

<div class="htmlview paragraph">Being able to plan each powertrain performance parameter with improved accuracy is an important factor for shortening the development period. In this regard, advance temperature predictions are particularly important because of the strong correlation between the thermal design and the vehicle layout, engine proper and other related parts.</div> <div class="htmlview paragraph">At present, it is still difficult to make thermal performance predictions for the engine oil temperature. There are many examples of studies in which the oil temperature was predicted in a one-dimensional circuit. In those studies, either experimental data were used for the heat release from engine components to the oil or calculations were made with a model that was converted empirically from three-dimensions to one-dimension. Because the mechanisms of oil temperature changes inside the engine are not well understood in detail, it is not possible to examine how modifications of the engine structure might affect the oil temperature.</div> <div class="htmlview paragraph">In this work, an effort was made to resolve such problems by making three-dimensional oil temperature predictions. While three-dimensional predictions present problems with respect to the calculation time and model size, those problems were overcome by performing a coupled thermal fluid-structural simulation using a general-purpose software code. Separate calculations were run to find the boundary conditions, including the flow rates of the circulating coolant and engine oil and the combustion gas condition. The calculated boundary conditions were then corrected using experimental data and theoretical considerations before inputting them into three-dimensional models of the cylinder head, block, water jacket and oil passages. This method made it possible to shorten the calculation time and reduce the model size. The results obtained with this method showed good agreement with the experimental data, making it possible to improve the accuracy of the thermal performance design.</div>

Early-stage thermal performance design of thermo-active walls implemented in underground infrastructures

In this paper the focus is on thermal and electrical design aspects of a NbTi-based demonstrator magnet for magnetic density separation (MDS) that is being constructed at the University of Twente. MDS is a recycling technology that allows the separation of non-magnetic particles based on their mass density, using a vertical magnetic field gradient and a ferrofluid. To minimize the distance between the planar array of racetrack coils and the ferrofluid bath, the system is conduction-cooled. First the thermal design is presented, which shows that the coils can operate below 4.5 K with sufficient margin using a single cryocooler. High-purity aluminium heat drains enable a low thermal gradient across the cold mass. The current path is introduced, as well as the adopted protection scheme. The magnet’s stored energy can safely be dumped in the coils. Diodes are placed (anti-)parallel to the coils in the cold to prevent high terminal voltages. In the case of a quench in the superconducting part of the current leads or an external anomaly, a switch is opened and the current is forced through a resistor in series with the diodes, causing a deliberate transition of the coils to the normal state and thus a fast ramp-down.

For the same blocking voltage rating and on-state resistance, Silicon Carbide (SiC) MOSFETs die is thinner and smaller compared to Si counterpart due to SiC material's superior electric-field withstanding capability. This smaller die size improves the switching performance significantly but deteriorates the thermal performance of the device packaging. This makes thermal design more challenging when using SiC MOSFETs. When using surface-mount (SM) packaging, printed circuit board (PCB) design plays a key role in the overall thermal design. Understanding the PCB design choices is necessary to maximize the performance of SM SiC MOSFET. This paper investigates several key factors in the thermal design of FR4 PCB to reduce junction temperature (Tj) of surface-mount (SM) SiC MOSFETs in steady-state and overload transient conditions. The factors of interest include heat sinks, copper area per device, and copper thickness. In experiments, doubling PCB copper area can reduce Δ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{j}$</tex> by > 40% in steady-state operation, making it the dominant factor. Experiments are performed on a two-layer PCB with a 900 V, 20 mΩ SiC device in D2Pak-7L package. For steady-state performance, doubling PCB copper area reduced the junction temperature rise by > 40%, making it the dominant design factor for steady-state thermal performance. For transient tests, copper thickness is the most critical factor. Junction temperature rise reduced by > 30% when PCB copper thickness is increased from 2 ounces (oz) to 6 oz. Experimental data is provided at various current stress levels for nominal steady-state and overload transient conditions. The findings provide thermal design guidelines of FR4 PCB for SM SiC MOSFETs in various applications.

&lt;div class="htmlview paragraph"&gt;NASA launched two rovers in June and July of 2003 as a part of the Mars Exploration Rover (MER) project. MER-A (Spirit) landed on Mars in Gusev Crater at 15 degrees South latitude and 175 degrees East longitude on January 4, 2004 (&lt;span class="xref"&gt;Squyres, et al., Dec. 2004&lt;/span&gt;). MER-B (Opportunity) landed on Mars in Terra Meridiani at 2 degrees South latitude and 354 degrees East longitude on January 25, 2004 (&lt;span class="xref"&gt;Squyres, et al., Aug. 2004&lt;/span&gt;). Both rovers have well exceeded their design lifetime (90 Sols) by more than a factor of 5. Spirit and Opportunity are still healthy and continue to execute their roving science missions at the time of this writing. This paper discusses rover flight thermal performance during the surface missions of both vehicles, covering roughly the time from the MER-A landing in late Southern Summer (aereocentric longitude, Ls = 328, Sol 1A) through the Southern Winter solstice (Ls = 90, Sol 255A) to nearly Southern Vernal equinox (Ls = 160, Sol 398A).&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;This paper describes the MER rover thermal design, its implementation and performance on Mars. The rover surface thermal design performance was better than pre-landing predictions. The very successful thermal design allowed a high level of communications immediately after landing without overheating and required a minimal amount of survival heating in the dead of winter.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;An analytical thermal model developed for the rover was used to predict surface operations performance. A reduced-node version of this model was integrated into the mission planning tool to achieve the proper balance between: 1) a desired science and communications operating profile, 2) the energy available from the power system and 3) requirements to maintain rover hardware within prescribed temperature limits. One of the more challenging thermal problems during surface operations, predicting the performance of actuator and camera electronics warmup heaters, was automated by using heater lookup tables that were periodically updated based on flight telemetry.&lt;/div&gt; &lt;div class="htmlview paragraph"&gt;Specific MER rover thermal flight experiences are discussed in this paper. Lessons learned and suggestions for improving future Mars surface vehicle designs are presented.&lt;/div&gt;

Thermal hydraulic performance of printed circuit heat exchanger with various channel configurations and arc ribs for SCO2 Brayton cycle