Fundamentals of Thermal Management

1. Fundamentals of Thermal Management What is Thermal Management. Why Thermal Management. Cooling Requirement for Microsystem. Thermal Management Fundamentals. Thermal Management of IC and PWB Packages

2. Introduction • All microelectronic devices require power • A typical microprocessor uses about 100 W by 2005 • This is a high power density that can lead to failure if not handled properly

3. Source : Intel presentation slide Need for Thermal Management • Ensure Long Term Reliability • Guarantee Performance and Functionality

4. What is Thermal Management • Heat is generated by the electrical resistance of leads, poly-silicon and transistors • The absence of cooling will result in constant rate increase in temperature until the component ceases (terhenti) to operate or until it fails • Placing the device in contact with components at lower temperatures (solid or fluid),facilitates the removal of heat flow away from the components. • Presence of cooling, the temperature rise is moderated as it approaches an acceptable steady state value. • At steady state, all heat generated by the component is transferred to the surrounding structure and/or fluid. • Thermal conduction, convection, radiation and “phase change” processes play a role in electronics cooling. • Successful thermal packaging relies on combination of material and heat transfermechanism to stabilize the component temperature at an acceptable level.

5. Why Thermal Management • The primary function of thermal management is to prevent catastrophic failure of a component – an immediate and total loss of electronic function and package integrity. • Catastrophic failures may include: • Associated with very large temperature, which lead to drastic deterioration in semiconductor behavior. • Fracture, delamination, melting, vaporization or combustion (pembakaran) of packaging materials • Individual solid state electronic devices are inherently reliable (very individual low failure rates) • Systems comprise perhaps 15 million transistors, 600 leads and tens of such devices • The system failure rate is the product of the individual failure rates times the number of devices • Achieving failure-free operation over the lifetime is a major challenge • Minimization or elimination of thermally-induced failures often requires the reduction of the temperature rise above the ambient and minimization of temperature variations within the package structure

6. Failure Rate Increase with Temperature • The reliability of a system is defined by the probability that the system will meet the required specifications for a given period of time. • Electronic components do not contain moving parts and can often perform reliably • for many years especially operating at room temperature • In practice, components operate at higher temperatures where most devices are • prone to failure from prolonged exposure at elevated temperature. This accelerated • results from • Mechanical creep in bonded materials • Parasitic chemical reactions • Dopant diffusion • Etc…… • The temperature dependence is often exponential, thus rise in temperature can be • expected to result in increase failure rate  refer next slide • Many package categories, temperature is the strongest contributor to the loss of • reliability  as such thermal management is critical to the success of the electronic system. • Thermal has to be controlled to enhanced reliability of the components  low operating temperature by using fans, pumps and special interface materials

7. Effect of Temperature on Failure Rate

8. Packaging Levels and Heat Removal • The thermal design depends on the relevant packaging level • The chip package is typically put at the bottom of the hierarchy (level 1), the PWB at level 2, the motherboard at level 3, and the box or cabinet at level 4. • The primary thermal transport mechanisms vary substantially from level to level • Level 1 packaging is primary concerned with conducting heat away from the chip to the package and then to the PWB • The most effective method is to reduce the thermal resistance between the die and the outer surface of the package • This can be accomplished by several methods • Passive methods – using conductive die attach adhesive material (level 1)  use of diamond, silver or another conductivity material so called “phase change” material. • Convection methods – attach heat sinks to the surface of the package (level 2), natural air circulation or by air that is blown over the surface. • Conduction methods – PWB with thick, high conductivity power and ground planes/ or embedded heat pipes (level 2) • Active methods – air handling systems, refrigeration system, heat pipes, heat exchangers and pumps (level 3 and 4)

9. Packaging Levels and Heat Removal

10. Packing Levels

11. Cooling Requirements for Microsystems • The automotive category claims the harsh environment title; devices generating 10 -15 W operate at 175oC • Through the 1990’s, the “heat-sink assisted” technology was the primary packaging approach for cost/Performance systems (both notebook and desktop). It relied on clip-attached or adhesive-bonded extruded Al heat sinks, cooled by an external fan • Towards end of 1990s, an attempts to minimize gap between notebook and desktop computers, fan-cooled heat-sink began to appear. • Laptop battery life is not consistent with fan cooling; there are significant cooling problems as we demand more performance in smaller, lighter, machines with longer battery life • Visit http://public.itrs.net/ for power dissipation prediction for details

12. In a typical system, heat removal from the active regions of the chip(s) may require the use of several mechanisms  operating either in series or parallel to transport the heat generated by the chip to the coolant or heat sink. • Heat transfer is energy in transit due to a temperature difference • Temperature gradient has to exist – heat flow will be a direction so as to equalize the temperature at various points • There are three basic thermal transport modes: Conduction (including contact resistance), Convection, Radiation • We may need to use multiple application in any given application Thermal Management Fundamentals

13. One-Dimensional Conduction • Conduction heat transfer – The flow of heat from a region of higher temperature to a region of lower temperature within a solid, stationary liquid, or static gaseous medium. • It occurs as a result of direct energy exchange among molecule. • Conduction is governed by the Fourier equation, which is one-dimensional form. • q = - kA dT • dx q = the heat flow (W) k = thermal conductivity (W/mK) A = cross-sectional area for heat flow (m2) dT/dx = temperature gradient in the direction of heat flow (K/m) k = vary widely among material Integration of the Fourier equation yields, at steady conduction of heat across a distance L T1 – T2 = qL kA Clearly, the larger k is, the smaller the temperature differential

14. 1 9.6 14.6 13.8 12.5 24.6 46 916 6250 5000 16250 12500 83330 One Dimension Conduction The thermal conductivity, k, is a materials property; the values vary widely among Materials

15. Thermal Convection • Convection described the flow of heat from a solid to a fluid in motion • There are two types of convection • Natural – the fluid motion is generated by naturally occurring thermal gradients, as occurs in heat conduction (in solid interface) • Forced -- the fluid motion is forced by stirring or blowing (transport of heat away from the solid surface) • Convection is described by • q = hA (Ts – Tf) • q = heat flow (W) • h = the heat transfer coefficient (W/m2-K) • A = the wetted surface area • Ts and Tf are the surface temperature and the fluid temperature, respectively

16. Thermal Convection • The heat transfer coefficients h, reflect the rapidly, slowly circulation fluid and variations in the convective transport properties. • The range of about 104 as we move from natural convection of gases to boiling liquids

17. Thermal Radiation • Radiation occurs as a results of the emission and absorption of energy contained in electromagnetic radiation waves or photons. • Thermal radiation can occur across a vacuum, or any medium that is transparent to infrared wavelengths (typically larger that 1 µm) • Unlike conduction and convection, the heat transfer is not dependant on the temperature differences. • The heat transfer is • Q = εσA(T41 –T42)F12 • ε = emissitivity σ = Stefan-Boltzmann constant (5.67 x 10-8 W/m2K4) • F12 = radiation view factor between surface 1 and 2. • T1 and T2 are the temperatures of therespective emitting and receiving surfaces • If the surfaces are highly emitting/absorbingand close together, F12 is nearly unity • F12 also approaches unity for heat flow from small, highly emitting surface to a large, highly absorbing surface which surrounds it on all sides.

18. Thermal Resistance • Fourier’s Law suggests analogy between heat transfer and the flow of an electric current through conductor as expressed in Ohm’s Law (∆V = RI) • So the analogy between electrical and thermal conductivity where heat flow (q) is analogous to current (I) and temperature drop (∆T) is analogous to voltage drop (∆V) Then thermal resistance RTH = ∆T • q • Analogy strictly applies only to thermal conduction, but possible to generalize this definition to cover all form of thermal transport. • RTH can be determined: • Experimentally – based on measure value of heat flow and temperature differences. • Analytically – based on theoritical expressions or on correlation.

19. Thermal Resistance (Conduction)

20. k = thermal conductivity (W/mK) A = cross-sectional area for heat flow (m2) h = the heat transfer coefficient (W/m2-K) L = distance ε = emissitivity σ = Stefan-Boltzmann constant (5.67 x 10-8 W/m2K4)

21. End of Chapter 6…..