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CHAPTER 6 Fundamentals of Thermal Management Vincent Wu Jason Mucilli

CHAPTER 6 Fundamentals of Thermal Management Vincent Wu Jason Mucilli. 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 Vincent Wu Jason Mucilli

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  1. CHAPTER 6 Fundamentals of Thermal Management Vincent Wu Jason Mucilli

  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. • Fourier Equation k = thermal conductivity (W/mk) dT/dx = temperature gradient in the direction of heat flow (K/m) q = heat flow (w) A = crossectional area for the heat flow (m) X

  13. 6.4 Thermal Management Fundamental cont. • Thermal Conductivity, k X

  14. 6.4 Thermal Management Fundamental cont. • Low thermal conductivity • Integration of Equation X

  15. 6.4 Thermal Management Fundamental cont. • Example of using Integration Equation • Calculate the temperature difference across a 1 mm thick adhesive of thermal conductivity 1 W/mK. Assume a 1 W heat source spread uniformly over a 1 cm2 area. • Solution • By using this Integration Equation the temperature difference can be calculated by:

  16. 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

  17. 6.4 Thermal Management Fundamental cont. X

  18. 6.4 Thermal Management Fundamental cont. • Heat flow across the interface can be written as: k1 and k2 = thermal conductivities of solid blocks 1 and 2, respectively Kƒ = conductivity of the fluid occupying the gap between the two solids. X

  19. 6.4 Thermal Management Fundamental cont. • Convection • Two mechanism X

  20. 6.4 Thermal Management Fundamental cont. • Newton’s Law of Cooling h (W/m^2K) = heat transfer coefficient Tƒ = bulk temperature of the nearby fluid. A = wetted surface area Ts = surface temperature X

  21. 6.4 Thermal Management Fundamental cont. X

  22. 6.4 Thermal Management Fundamental cont. • Heat Transfer coefficient, h • not a fundamental fluid property like Thermal conductivity. • Many heat transfer literature has many theoretical equations and empirical correlations • Used to determine the prevailing heat transfer coefficient for specified fluids that are flowing within channels or along surfaces of different kinds of geometries. • These relations can be expressed by an non-dimensional form: C = geometric constant Re (Reynolds number ) = non-dimensional velocity Pr (Prandtl number) = non-dimensional fluid characteristic, describing the momentum vs. thermal diffusion characteristics of the fluid. Nu (Nusselt number) = non-dimensional heat transfer coefficient

  23. 6.4 Thermal Management Fundamental cont. X

  24. 6.4 Thermal Management Fundamental cont. X

  25. 6.4 Thermal Management Fundamental cont. • Thermal Radiation • Radiative heat transfer ‘‘view factor’’ between surfaces 1 and 2. Stefan-Boltzmann constant, equal to 5.67 x 108^-8 W/m^2K^4 emissivity X

  26. 6.4 Thermal Management Fundamental cont. • For highly-absorbing surface which surrounds in on all sides, this equation can be used. • where hr is:

  27. 6.4 Thermal Management Fundamental cont. • Lumped Capacity Heating and Cooling • Internal heat generation is absorbed by the solid which the temperature also rises • Internally-heated solid of relatively high thermal conductivity which does not experience external cooling. • Temperature will become a constant rise rate according to: q (heat flow) = rate of internal heating (W) Cp = specific heat of the solid (J /kg K). m = mass of the solid (kg), X

  28. 6.4 Thermal Management Fundamental cont. • Thermal Resistances (a.k.a. Ohm’s Law) • Temperature-difference form of Fourier’s Law • Define Thermal Resistance (Rth) as:

  29. 6.4 Thermal Management Fundamental cont. • Thermal Resistant in Parallel X

  30. 6.4 Thermal Management Fundamental cont. • It can also be written as:

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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.

  37. 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

  38. 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

  39. 6.6 Electronic Cooling Methods cont.

  40. 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

  41. 6.6 Electronic Cooling Methods cont. • Flotherm-3D computational fluid dynamics software • Predicts airflow and heat transfer in electronic models • Conduction, convection and radiation

  42. 6.6 Electronic Cooling Methods • Flowtherm • Model used for Covidien’s ERT project • Sensor module • Completely EM shielded

  43. 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

  44. 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

  45. 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

  46. 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

  47. 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

  48. 6.6 Electronic Cooling Methods cont. • Immersion Cooling

  49. 6.6 Electronic Cooling Methods cont. • Immersion Cooling

  50. 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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