Electronics Cooling MPE 635

Mechanical Power Engineering Dept.

1. To establish fundamental understanding of heat transfer in electronic equipment.

2. To select a suitable cooling processes for electronic components and systems.

3. To increase the capabilities of post-graduate students in design and analysis of cooling of electronic packages.

4. To analysis the thermal failure for electronic components and define the solution.

Course Goals

• Part-A• Main topics • Introduction to electronics cooling and thermal

packaging• Introduction to basic modes of heat transfer • Conduction heat transfer and extended surfaces in

electronic devices• Transient conduction• Natural convection heat transfer (i.e. PCB cooling) • Forced convection heat transfer (Internal and External

flow )• Fan performance• Radiation heat transfer and its applications in electronic

devices• Solving the electronics cooling problems with EES

software• Electronics cooling problems• Solution of selected electronics cooling problems

3. Basics of Heat Transfer

Modes of heat transfer

Conduction

• Conduction heat transfer as diffusion of energy due to molecular activity.

• Conduction in liquids and solids ascribed to molecules vibration (solids), translational and rotational (liquids)

Conduction

• Fourier’s law

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dTkqx

L

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Thermal convection

• The heat transfer by convection is described by the Newton's law of cooling:

)( TThAq WA ir m o v e m e n t d u e t o t e m p e ra t u re d iffe re n c e

(a )F re e c o n v e c t io n o n e le c t ric c o m p o n e n t s c h ip s

A ir

(b )F o rc e d c o n v e c t io n o n e le c t ric c o m p o n e n t s c h ip s

F o rc e d fa n

Thermal convection

• convection heat transfer ranges Process h(w/m2.k)

Free convection - gases 2-25 - liquids 50-1000Forced convection - gases 25-250 - liquids 50-20,000Convection with two phase - boiling or condensation 2500-100,000

Thermal convection

• Example 3.1: An electric current is passed through a wire 1mm diameter and 10 cm long. This wire is submerged in liquid water at atmospheric pressure, and the current is increased until the water boils. For this situation h = 5000 W/m2.oC. And the water will be 100 oC. How much electric power must be supplied to the wire to maintain the wire surface at 114 oC?

Schematic:

Electric wire

Water

Thermal convection

• Solution:• The total convection loss from the wire is

given by.

• For this problem the surface area of the wire is A= π d L = π (1 x 10-3) (10 x 10-2) = 3.142 x10-4 m2

• The heat transfer is therefore

• And this is equal to the electric power which must be applied.

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Thermal radiation• The mechanism of heat transfer by radiation

depends on the transfer of energy between surfaces by electromagnetic waves in the wave length interval between 0.1 to 100 μm.

• Radiation heat transfer can travel in vacuum such as solar energy.

• Radiation heat transfer depends on the surface properties such as colors, surface orientation and fourth power of the absolute temperature (T4) of the surface.

• The basic equation for radiation heat transfer between two gray surfaces is given by

)( 42

41 TTfAq

Thermal radiation

• Example 3.2. A horizontal steel pipe having a diameter of 10 cm is maintained at a temperature of 60 oC in a large room where the air and wall temperature are at 20 oC with average heat transfer coefficient 6.5 W/m2K. The emissivity of the steel is 0.6 calculate the total heat lost from the pipe per unit length.

Thermal radiation

• Solution:• The total heat lost from the pipe due

to convection and radiation

• Because the pipe in a large enclosure then the geometrical factor ƒ = 1

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• There exists an analogy between the diffusion of heat and electrical charge. Just as an electrical resistance is associated with the conduc tion of electricity, a thermal resistance may be associated with the conduction of heat.

Analogy between Heat Transfer and Electric Circuits

Analogy between Heat Transfer and Electric Circuits

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Series Circuits:

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Combined Modes of Heat Transfer

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• Combined Convection and Radiation

Now if we define the arithmetic mean temperature as:

If further Ts-Te<<Ts then

So we may define the radiation heat transfer coefficient as:

And finally;

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Combined Modes of Heat Transfer

Combined Modes of Heat Transfer

• Combined Convection and Conduction

• This combination is likely to occur with the use of extended surfaces where the primary surface exchanges heat by convection to the adjacent fluid flow and by conduction through the extended surfaces. This case may be considered in a similar manner as the above, but here the problem doesn't need extra work as the conduction thermal resistance is predefined.

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Overall Heat Transfer Coefficient

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Fluid combination U, W/m2.ºK.

Water to water 850-1700

Water to oil 110-350

Steam condenser, water in tube 1000-6000

Ammonia condenser, water in tube 800-1400

Finned tube heat exchanger, water in tubes air in cross flow

25-50