why thermal management? - Department of Electrical, Computer
Download
Report
Transcript why thermal management? - Department of Electrical, Computer
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