Download Electronics Cooling MEP 635

Electronics Cooling
Mechanical Power Engineering
Dept.
1. Introduction to electronics
cooling and thermal packages
• Thermal management importance in the
electronic product development
• Heat sources in electronic products:
-Power dissipated through electric resistances
P=I2R
-Switching power dissipation in transistors
Automotive Electronics
Electronic content in cars and trucks has significantly increased
in the last 30 years. Much of the functional content of these
vehicles is now generated or controlled by electronic systems.
History of typical engine control modules (ECMs)
Examples of thermal requirements
for various products
• Cost/Performance 2004 RF Chip
Thermal Requirements
-
Power Dissipation - 100W
Temperatures: Junct = 150C; Ambient = 45C
Chip Size - 3mm x 1mm 0.3mm
Wireless Module = 10 Chips, 1kW
Thermal “Space Claim” - 150 x 150 x 150mm
Thermal Resistances:



Spreading (Chip Level) = 0.6K/W
Internal Convective (Chip Level) = 0.2K/W
External Convective (Module Level) = 0.25K/W
Thermal Packaging, Future
Forecasting
•
Future Thermal Packaging Needs
-
•
Higher Power Dissipation
Higher Volumetric Heat Density
Market-Driven Thermal Solutions
Air as the Ultimate Heat Sink
Environmentally-Friendly Design
Future Thermal Packaging Solutions
-
Thermo-fluid Modeling Tools
Integrated Packaging CAD
Compact Heat Exchanger Technology
Design for Manufacturability/Sustainability
“Commodity” Refrigeration Technology
Thermal Packaging Options and Trends
Aims of thermal control
• PREVENT CATASTROPHIC FAILURE
- Electronic Function
- Structural Integrity
• PROVIDE ACCEPTABLE MICROCLIMATE
- Device Reliability
- Packaging Reliability
- Prevent Fatigue, Plastic Deformation and Creep
• SYSTEM OPTIMIZATION
- Fail Safe or Graceful Degradation
- Multilevel Design
- Reduction of “Cost of Ownership”
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
dT
q x   k
dx
T2  T1
qx  k
L
Thermal convection
• The heat transfer by convection is described by the
Newton's law of cooling:
q  hA(TW  T )
Air movement due to temperature difference
Forced fan
Air
(b)Forced convection on electric components chips
(a)Free convection on electric components chips
Thermal convection
• convection heat transfer ranges
Process
Free convection
- gases
- liquids
Forced convection
- gases
- liquids
Convection with two phase
- boiling or condensation
h(w/m2.k)
2-25
50-1000
25-250
50-20,000
2500-100,000
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
q   fA(T  T )
4
1
4
2
Analogy between Heat Transfer
and Electric Circuits
• There exists an analogy between the
diffusion of heat and electrical charge.
Just as an electrical resistance is
associated with the conduction of
electricity, a thermal resistance may be
associated with the conduction of heat.
Series Circuits:
E
E1  E2
i 

Re Re,1   Re, 2   Re,3 
• By analogy
q 
Toverall
Rt
T1  T 2
q 
Rt ,conv   Rt ,cond   Rt ,conv 
q 
T1  T 2
 1   L   1 

  

  
 h1 A   kA   h2 A 
Parallel Circuit:
L1, k1, A1
T T
qi  ki Ai

Li Rt ,i
qtot  qi
T1
T2
L3, k3, A3
qtot
L4, k4, A4
qtot
L5, k5, A5
 1

1
1
1
1
1
1

 qtot  T  






R
R
R
R
R
R
R
t
,
1
t
,
2
t
,
3
t
,
4
t
,
5
t
,
6
t
,
7


T
 qtot 
Rt ,tot
1
1

Rt ,tot
Rt ,i
L2, k2, A2
qtot
L6, k6, A6
L7, k7, A7
q1
q2
q2
R2
q3
R3
q3
q4
R4
q4
q5
R5
q5
q6
R6
q6
q7
R7
q7
qtot
Combined Modes of Heat Transfer
•
Combined Convection and Radiation
qnet  qconv  qrad
q rad  hr  A  (Ts  T f )
q rad
(Ts4  Te4 )
hr 
     Fse 
A  (Ts  T f )
(Ts  T f )
(Ts4  T f4 )
hr      Fse 
(Ts  T f )
 hr      Fse 
(Ts2  T f2 )  (Ts  T f )  (Ts  T f )
(Ts  T f )
Combined Modes of Heat Transfer
htot  hconv  hrad
q net  htot  A  (Ts  T f )