CHAPTER 6 Fundamentals of Thermal Management

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