CHAPTER 6
Fundamentals of Thermal
Management
6.1 WHAT IS THERMAL MANAGEMENT?

Resistance of electrical flow

Absence of cooling

Contact of Device
•
•
Steady State

•

Cooling
roles
Intense Heat Transfer
Successful Thermal Packaging
X
6.2 WHY THERMAL
MANAGEMENT?

Thermal Management of all
microelectronic components is similar
Prevention of Catastrophic failure
 Temperature rise
 Catastrophic vulnerability

X
6.2 Why Thermal Management
cont.

Failure Rate Increases with Temperature

Reliability
X
6.2 Why Thermal Management
cont.
X
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
6.2 Why Thermal Management
cont.
X
6.3 Cooling Requirements for
Microsystems

Cooling techniques
Buoyancy- induced natural circulation of air
 Natural convection cooling
 Forced convection
 Heat-sink-assisted air cooling

6.3 Cooling Requirements for
Microsystems cont.
6.4 Thermal Management
Fundamental

Electronic cooling, there are three basic
thermal transport mode
Conduction (including contact resistance)
 Convection
 Radiation

6.4 Thermal Management
Fundamental cont.

One-dimensional Conduction
X
6.4 Thermal Management
Fundamental cont.



Heat flow across solid interface
Perfect adhering solids
Ac = area of actual
Real Surface
contact
Av = fluid conduction
across the open
spaces.
X
6.4 Thermal Management
Fundamental cont.

Convection

Two mechanism
X
6.4 Thermal Management
Fundamental cont.
X
6.4 Thermal Management
Fundamental cont.
X
6.4 Thermal Management
Fundamental cont.
X
6.4 Thermal Management
Fundamental cont.

Thermal Resistant in Parallel
X
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

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
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
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

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
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.
k xyz 
tM
kM

1
(1  t M )

k1
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 Zdirection

Sparse amt. of vias-Qxyz model:

“In-plane” thermal conductivity to first approximationcombination of vias may be neglected
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

6.6 Electronic Cooling Methods
cont.

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
6.6 Electronic Cooling Methods
cont.

Flotherm-3D computational fluid dynamics
software
Predicts airflow and heat transfer in electronic
models
 Conduction, convection and radiation

6.6 Electronic Cooling Methods

Flowtherm

Model used for Covidien’s ERT project
 Sensor

module
Completely EM shielded
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
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
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
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
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
6.6 Electronic Cooling Methods
cont.

Immersion Cooling
6.6 Electronic Cooling Methods
cont.

Immersion Cooling
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