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Mechanical Engineer´s Handbook P73

CHAPTER 54 COOLING ELECTRONIC EQUIPMENT Allan Kraus Allan D. Kraus Associates Aurora, Ohio 54.1 THERMAL MODELING 54. 1 . 1 Introduction 54. 1 .2 Conduction Heat Transfer 54.1.3 Convective Heat Transfer 54.1.4 Radiative Heat Transfer 54.1.5 Chip Module Thermal Resistances HEAT-TRANSFER CORRELATIONS FOR ELECTRONIC EQUIPMENT COOLING 54.2.1 Natural Convection in Confined Spaces 1649 1 649 54.3 1649 1652 1655 1656 54.2.2 Forced Convection 1662 1667 1672 1672 1674 THERMAL CONTROL TECHNIQUES 54.3.1 Extended Surface and Heat Sinks 54.3.2 The Cold Plate 54.3.3 Thermoelectric Coolers 54.2 1661 1661 54.1 THERMAL MODELING 54.1.1 Introduction To determine the temperature differences encountered in the flow of heat within electronic systems, it is necessary to recognize the relevant heat. | CHAPTER 54__ COOLING ELECTRONIC EQUIPMENT Allan Kraus Allan D. Kraus Associates Aurora Ohio 54.1 THERMAL MODELING 1649 54.2.2 Forced Convection 1662 54.1.1 Introduction 1649 54.1.2 Conduction Heat 54.3 THERMAL CONTROL Transfer 1649 TECHNIQUES 1667 54.1.3 Convective Heat 54.3.1 Extended Surface and Transfer 1652 Heat Sinks 1672 54.1.4 Radiative Heat Transfer 1655 54.3.2 The Cold Plate 1672 54.1.5 Chip Module Thermal Resistances 1656 54.3.3 Thermoelectric Coolers 1674 54.2 HEAT-TRANSFER CORRELATIONS FOR ELECTRONIC EQUIPMENT COOLING 1661 54.2.1 Natural Convection in Confined Spaces 1661 54.1 THERMAL MODELING 54.1.1 Introduction To determine the temperature differences encountered in the flow of heat within electronic systems it is necessary to recognize the relevant heat transfer mechanisms and their governing relations. In a typical system heat removal from the active regions of the microcircuit s or chip s may require the use of several mechanisms some operating in series and others in parallel to transport the generated heat to the coolant or ultimate heat sink. Practitioners of the thermal arts and sciences generally deal with four basic thermal transport modes conduction convection phase change and radiation. 54.1.2 Conduction Heat Transfer One-Dimensional Conduction Steady thermal transport through solids is governed by the Fourier equation which in onedimensional form is expressible as q -kA W dx 54.1 where q is the heat flow k is the thermal conductivity of the medium A is the cross-sectional area for the heat flow and dT dx is the temperature gradient. Here heat flow produced by a negative temperature gradient is considered positive. This convention requires the insertion of the minus sign in Eq. 54.1 to assure a positive heat flow q. The temperature difference resulting from the steady state diffusion of heat is thus related to the thermal conductivity of the material the cross-sectional area and the path length L. according to Tt T2 cd q K .