Encyclopedia of Thermal Packaging

The following sections are included:

• Foreword to the Encyclopedia of Thermal Packaging

• Preface

• List of Symbols

• Contents

The inexorable rise in chip power dissipation and emergence of on-chip "hot spots" continues to fuel attention toward direct cooling with dielectric liquids. Use of dielectric liquids in intimate contact with the heat dissipating surfaces can eliminate the deleterious effects of solid-solid interface resistances and harnesses highly-efficient phase-change processes to the critical thermal management of advanced IC chips. As an introduction to direct liquid cooling, this chapter begins with a discussion of its advantages, followed by a summary of available cooling modes and techniques. This chapter ends with a description of the history of direct liquid cooling of electronics and a brief overview of the remainder of the volume.

The direct cooling of microelectronic components imposes stringent chemical, electrical, and thermal requirements on the fluids to be used in this thermal control mode. Direct liquid cooling of microelectronic components requires compatibility between the liquid coolant and a system-specific combination of the chip, chip package, substrate, and printed circuit board materials, e.g., silicon, silicon dioxide, silicon nitride, alumina, o-rings, plastic encapsulants, solder, gold, and epoxy-glass. A liquid coolant must possess the dielectric strength needed to provide electrical isolation between adjacent power/ground conductors and signal lines operating at a potential of approximately 1 V to 5 V and spaced as close as 0.05 mm. It is also desirable that the liquid's dielectric constant be close to unity to avoid introducing significant propagation delays. Furthermore, such coolants must be non-toxic and chemically inert.

Despite increasing performance demands and advances in thermal management technology, single phase convective cooling of electronic equipment (most often with air) dominates the product landscape. Single phase natural convection is the quietest, least expensive, easiest to implement, and most reliable implementation of direct fluid cooling. In addition, single phase natural convection liquid cooling provides a convenient lower bound for boiling and a benchmark solution for other advanced cooling approaches.

As discussed in Chapter 2, the electrical characteristics of candidate fluids that make them suitable for direct contact with electronic devices also tend to be accompanied by relatively poor thermal properties, at least compared to water. As a consequence, the natural convection heat transfer coefficients explored in Chapter 3 result in cooling rates of less than one watt per square centimeter from the immersed surfaces. These heat fluxes may be adequate for memory chips, but are insufficient to meet the demanding requirements of high performance microprocessor chips. Fortunately, the highly efficient nucleate boiling process may be employed to dramatically increase heat transfer rates from microelectronics immersed in dielectric liquids. Further, the naturally generated agitation of the boiling liquid, or "bubble pumping," can assist in mixing and spreading of heat within an immersion module.

Unlike the heat transfer phenomena of the previous two chapters, where fluid motion is driven by density differences associated with temperature and/or phase variations, attention now shifts to active cooling methods based on forced circulation of the working fluid. Forced convection treatments are generally divided between external flow along surfaces and across bodies, and internal flow within ducts and channels. Whereas for relatively low velocities and short flow paths, fluids generally flow in smooth layers or laminae along surfaces, at higher velocities and/or longer flow paths, a churning motion may develop. Thus, a second distinction is made between laminar and turbulent convection. This chapter is focused on single phase forced convection inside channels and in the target area of jets. The following chapter, Chapter 6, considers forced flow with liquid-to-vapor phase change.

The relatively high heat transfer rates attainable with nucleate pool boiling and forced convection of dielectric liquids flowing through miniature channels makes the forced evaporative flow of such liquids in heated channels a most promising candidate for the thermal management of advanced semiconductor devices. Regrettably, few validated correlations exist for flow boiling in miniature channels, but understanding and identification of the prevailing flow regimes (i.e. stratified, intermittent, bubble, annular) can facilitate a regime-informed extrapolation of available data and traditional flow boiling correlations to the analysis and evaluation of the two-phase heat transfer coefficients in miniature channels.

The challenges posed by high chip heat fluxes and ever-more stringent performance and reliability constraints make thermal management a key enabling technology in the development of electronic systems. The cooling of microelectronic components by immersion in dielectric liquids provides a most attractive alternative to more conventional electronic thermal management approaches, but regrettably the industry trend away from large mainframe computers and supercomputers has blunted the expansion of direct liquid cooling. Workstation, personal computer, and telecommunication equipment developers, who have historically relied on air cooling, have been reluctant to embrace liquid immersion due to concerns about the complexity and reliability of pumped liquid systems. Passive immersion cooling modules (PIMs), internally-filled with a dielectric fluid while externally cooled by air, provide a simple, highly reliable, and most effective thermal management alternative. Inclusion of fins on a PIM's "submerged condenser" can substantially expand the thermal performance envelope of the module. This chapter explores PIM system performance.

Chip stacking is a crucial building block in advanced 3D microsystem architectures and can accommodate shorter interconnect distances between devices, leading to reduced power dissipation and improved electrical performance. Although enhanced conduction can serve to transfer the dissipated heat to the top and sides of the package and/or down to the underlying PCB, effective thermal management of stacked chips remains a most difficult challenge. Immersion cooling techniques, which provide convective and/or ebullient heat transfer, along with buoyant fluid flow, in the narrow gaps separating adjacent chips, are a most promising alternative to conduction cooling of three-dimensional chip stacks. Application of the available theories, correlations, and experimental data are shown to reveal that passive immersion cooling—relying on natural convection and/or pool boiling—could provide the requisite thermal management capability for 3D chip stacks anticipated for use in much of the portable equipment category. Alternatively, pumped flow of dielectric liquids through the microgaps in 3D stacks, providing single phase and/or flow boiling heat absorption, could meet many of the most extreme thermal management requirements for high-performance 3D microsystems.

The following sections are included:

• Appendix: Fluid Property Data

• Author Index

• Subject Index

• About the Authors

• About the Editor-in-Chief