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CHAPTER 6 Fundamentals of Thermal Management

CHAPTER 6 Fundamentals of Thermal Management. 6.1 WHAT IS THERMAL MANAGEMENT?. Resistance of electrical flow Absence of cooling Contact of Device Cooling roles Steady State Intense Heat Transfer Successful Thermal Packaging. X. 6.2 WHY THERMAL MANAGEMENT?.

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CHAPTER 6 Fundamentals of Thermal Management

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  1. CHAPTER 6 Fundamentals of Thermal Management

  2. 6.1 WHAT IS THERMAL MANAGEMENT? • Resistance of electrical flow • Absence of cooling • Contact of Device • Cooling • roles • Steady State • Intense Heat Transfer • Successful Thermal Packaging X

  3. 6.2 WHY THERMAL MANAGEMENT? • Thermal Management of all microelectronic components is similar • Prevention of Catastrophic failure • Temperature rise • Catastrophic vulnerability X

  4. 6.2 Why Thermal Management cont. • Failure Rate Increases with Temperature • Reliability X

  5. 6.2 Why Thermal Management cont. X

  6. 6.2 Why Thermal Management cont. • The main thermal transport mechanisms and the commonly used heat removal is different in each packaging level. • Level 1 • Level 2 • Level 3 and 4 X

  7. 6.2 Why Thermal Management cont. X

  8. 6.3 Cooling Requirements for Microsystems • Cooling techniques • Buoyancy- induced natural circulation of air • Natural convection cooling • Forced convection • Heat-sink-assisted air cooling

  9. 6.3 Cooling Requirements for Microsystems cont.

  10. 6.4 Thermal Management Fundamental • Electronic cooling, there are three basic thermal transport mode • Conduction (including contact resistance) • Convection • Radiation

  11. 6.4 Thermal Management Fundamental cont. • One-dimensional Conduction X

  12. 6.4 Thermal Management Fundamental cont. • Heat flow across solid interface • Perfect adhering solids • Real Surface Ac = area of actual contact Av = fluid conduction across the open spaces. X

  13. 6.4 Thermal Management Fundamental cont. • Convection • Two mechanism X

  14. 6.4 Thermal Management Fundamental cont. X

  15. 6.4 Thermal Management Fundamental cont. X

  16. 6.4 Thermal Management Fundamental cont. X

  17. 6.4 Thermal Management Fundamental cont. • Thermal Resistant in Parallel X

  18. 6.5 Thermal Management of IC and PWB Packages cont. • Natural Convection air cooling of Electronic equipment still very popular • Simplicity, reliability and low cost • IC packages, PCB’s, heat sinks • Single PWB • Array of PWB’s-array of vertical channels • Nusselt Number: Nu=El/C2A, El=Elenbaas number • Measures the enhancement of heat transfer from a surface that occurs in a real situation, compared to heat transferred if just conduction occurred. Dimensionless quantity

  19. 6.5 Thermal Management of IC and PWB Packages cont. • Optimum Spacing • Isothermal arrays the optimum spacing maximizes the total heat transfer • Optimum PWB spacing where max power can be dissipated in the PWB’s • Limitations-closely spaced PWB’s tend to under predict heat transfer • Due to between package “wall flow” and the non smooth nature of channel surfaces

  20. 6.5 Thermal Management of IC and PWB Packages cont. • PWB’s in Forced Convection • Most applications • Laminar Flow- the flow of cooling air proceeds downstream between the PWB’s in “sheet-like” fashion. • Forced laminar flow in long, or narrow parallel plate channels the heat transfer coefficient has an asymptotic value of: h=4kf/de. Where de=Hydraulic diameter

  21. 6.6 Electronic Cooling Methods • Heat Sinks • Convective thermal resistance can be reduced by • Increasing heat transfer coefficient or • Increasing heat transfer area • Coefficient is function of flow conditions which are fixed • Most applications-increase heat transfer area provides only means to reduce convective thermal resistance- by use of extended surfaces or fins

  22. 6.6 Electronic Cooling Methods cont. • Heat Sinks continued: • The temperature of the fin is expected to decrease from the base temperature as move toward the fin tip • Amount of convective heat transfer depends on the temperature difference between the fin and ambient • Heat transfer from fin area: • q=ηhAf(Tb-Ta) • Af Base area • Η fin efficiency • Tb base temperature • Single plate fin, most thermally effective use of fin material achieved when efficiency is 0.63

  23. 6.6 Electronic Cooling Methods Cont. • Heat Sinks continued: • “extended” surfaces • Manufacturer provides heat sink thermal resistance for range of flow rates • Most common are extruded heat sinks • Limitation on fin height to fin gap due to structural strength.

  24. 6.6 Electronic Cooling Methods cont. • Thermal Vias cont. • Large number of Vias-Qzz model to determine thermal conductivity: kzz=kMaM + k1(1 – aM) • kM & k1 are the thermal conductivity of the metal and insulator and aM is the fraction of cross-sectional conductivity in Z-direction • Sparse amt. of vias-Qxyz model: • “In-plane” thermal conductivity to first approximation-combination of vias may be neglected

  25. 6.6 Electronic Cooling Methods cont. • Thermal Vias • VIA • PCB design-pad with plated hole that connects copper tracks from one layer of the board to other layers • Help to reduce resistance in heat flow • Examine thermal conductivity both analytically and experimentally

  26. 6.6 Electronic Cooling Methods cont.

  27. 6.6 Electronic Cooling Methods cont. • Thermal Vias cont. • Trace layers • Can help to transport heat to the edges of the board • Finite Element model simulation

  28. 6.6 Electronic Cooling Methods cont. • Flotherm-3D computational fluid dynamics software • Predicts airflow and heat transfer in electronic models • Conduction, convection and radiation

  29. 6.6 Electronic Cooling Methods • Flowtherm • Model used for Covidien’s ERT project • Sensor module • Completely EM shielded

  30. 6.6 Electronic Cooling Methods cont. • Heat Pipe Cooling • Thermal transport device uses phase change processes and vapor diffusion to transfer large quantities of heat over substantial distances with no moving parts and constant temp • Use is increasing especially in laptops • High effective thermal conductivity of heat pipe at low weight

  31. 6.6 Electronic Cooling Methods cont. • Heat Pipe Cooling cont • 3 sections • Evaporator-heat absorbed and fluid vaporized • Condenser-vapor condensed and heat rejected • Adiabatic-vapor and the liquid phases of the fluid flow in opposite directions through the cork and wick

  32. 6.6 Electronic Cooling Methods cont. • Heat Pipe Cooling • Most cylindrical in shape • Variety of shapes possible • Right angle bends, S-turns, spirals… • .3cm minimum thickness • Concerns • Degradation over time • Some fail just after a few months operation • Contamination and trapping of air that occur during fabrication process

  33. 6.6 Electronic Cooling Methods cont. • Jet Impingement Cooling • Used when high convective heat transfer rates required • For unpinned heat sink, the multiple jets yield higher convective coefficients that single jet by a factor of 1.2 • In presence of pins, almost no difference is seen

  34. 6.6 Electronic Cooling Methods cont. • Immersion Cooling • Dates back to 1940’s • Mid 80’s- used in Cray 2 and ETA010 supercomputers • Well suited to cooling of advanced electronics under development • Operate in closed loop

  35. 6.6 Electronic Cooling Methods cont. • Immersion Cooling

  36. 6.6 Electronic Cooling Methods cont. • Immersion Cooling

  37. 6.6 Electronic Cooling Methods cont. • Thermoelectric Cooling • TEC-Thermal electric cooler-solid state heat pump • Potential placed across 2 junctions-heat absorbed into one junction and expelled from another • Most obvious in P-N junctions • e- transported from p-side to n-side, transported to higher energy state and absorb heat thus cooling surrounding area • From n-side to p-side they release heat • Common materials- bismuth telluride, lead telluride, and silicon germanium • Selected from performance and COP (coefficient of performance) curves

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