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 Steady State Intense Heat Transfer Intense Heat Transfer Successful Thermal Packaging Successful Thermal Packaging X

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

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

5. 6.2 Why Thermal Management cont. X

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

7. 6.2 Why Thermal Management cont. X

8. 6.3 Cooling Requirements for Microsystems Cooling techniques Cooling techniques Buoyancy- induced natural circulation of air Buoyancy- induced natural circulation of air Natural convection cooling Natural convection cooling Forced convection Forced convection Heat-sink-assisted air cooling 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 Electronic cooling, there are three basic thermal transport mode Conduction (including contact resistance) Convection Radiation Radiation

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

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

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

14. 6.4 Thermal Management Fundamental cont. X

15. X

16. X

17. Thermal Resistant in Parallel Thermal Resistant in Parallel X

18. Natural Convection air cooling of Electronic equipment still very popular Natural Convection air cooling of Electronic equipment still very popular Simplicity, reliability and low cost Simplicity, reliability and low cost IC packages, PCB’s, heat sinks IC packages, PCB’s, heat sinks Single PWB Single PWB Array of PWB’s-array of vertical channels Array of PWB’s-array of vertical channels Nusselt Number: Nu=El/C 2 A, El=Elenbaas number Nusselt Number: Nu=El/C 2 A, 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 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 6.5 Thermal Management of IC and PWB Packages cont.

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

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

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

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

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

24. 6.6 Electronic Cooling Methods cont. Thermal Vias cont. Thermal Vias cont. Large number of Vias-Q zz model to determine thermal conductivity: k zz =k M a M + k 1 (1 – a M ) Large number of Vias-Q zz model to determine thermal conductivity: k zz =k M a M + k 1 (1 – a M ) k M & k 1 are the thermal conductivity of the metal and insulator and a M is the fraction of cross-sectional conductivity in Z- direction k M & k 1 are the thermal conductivity of the metal and insulator and a M is the fraction of cross-sectional conductivity in Z- direction Sparse amt. of vias-Q xyz model: Sparse amt. of vias-Q xyz model: “In-plane” thermal conductivity to first approximation- combination of vias may be neglected “In-plane” thermal conductivity to first approximation- combination of vias may be neglected

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

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

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

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

30. 6.6 Electronic Cooling Methods cont. Heat Pipe Cooling 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 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 Use is increasing especially in laptops High effective thermal conductivity of heat pipe at low weight High effective thermal conductivity of heat pipe at low weight

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

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

33. Jet Impingement Cooling Jet Impingement Cooling Used when high convective heat transfer rates required 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 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 In presence of pins, almost no difference is seen

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

35. 6.6 Electronic Cooling Methods cont. Immersion Cooling Immersion Cooling

36. 6.6 Electronic Cooling Methods cont. Immersion Cooling Immersion Cooling

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