Thermal Management for
Electronic Packaging
03/02/2006
Guoping Xu
Sun Microsystems
CSE291: Interconnect and Packaging, UCSD, Winter 2006
Outline
Introduction
Heat transfer theory
Thermal resistance in electronic packaging
Thermal design
Thermal modeling
Thermal measurement
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Functions of Electronic Packaging
Package protection
Signal distribution
Power distribution
Heat dissipation
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Packaging Hierarchy
Chip
Package
Board
System
Rack
Room
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
High end chip power trend
600
CPU Power, (W)
500
400
UltraSparc [1]
Power 4 [2]
Itanium 2 [3]
ITRS 2002 [4]
ITRS 2005
300
200
100
0
1990
1995
2000
2005
2010
2015
2020
Year
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Cost performance chip power trend
160
140
ITRS 2005
CPU Power, (W)
120
100
80
60
40
20
0
2004
2006
2008
2010
2012
Year
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2014
CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Power density in datacom equipment
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Power density in datacom equipment
Total power: 24KW
Footprint: 15 sq. ft
Power density: 1600W/sq. ft
Sun Fire E25K
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Introduction
Impact of Device junction temperature
Computing performance
Reliability
Fire hazard and/or Safety issues
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Conduction
Definition: Conduction is a mode of heat transfer in which heat flows from a
region of higher temperaure to one of lower temperature within a medium (solid,
liquid, or gases) or media in direct physical contact
Fourier's law: Q = -KA(dT/dX)
1-D conduction: Q = -KA (T1-T2)/L
Thermal resistance: R = (T1-T2)/Q = L/(KA)
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Conduction
Contact thermal resistance
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Thermal conductivity of various packaging materials
Material
Aluminum (pure)
Aluminum Nitride
Alumina
Copper
Diamond
Epoxy (No fill)
Epoxy (High fill)
Epoxy glass
Gold
Lead
Silicon
Silicon Carbide
Silicon Grease
Solder
W/mK
216
230
25
398
2300
0.2
2.1
0.3
296
32.5
144
270
0.2
49.3
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Convection
Convection: is a mode of heat transport from a solid surface to a
fluid and occurs due to the bulk motion of the fluid.
Newton's law: Q= hA (Tw- Tf)
Convective thermal resistance: R= 1/(hA)
Effects of heat transfer coefficient
Convetion mode: Natural convection, Foreced convection, phase change
Flow regime: Laminar, Turbulent flow
Flow velocity
y Tf
Fluid
y
Surface condition
Fluid
Velocity
distribution
u(y)
Heated
surface
Temperature
distribution
Tw
T(y)
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Typical values of the heat transfer coefficient
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Heat Transfer Theory
Radiation
Definition: Radiation heat transfer occurs as a result of radiant energy emitted from a
body by virtue of its temperature.
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Resistance
-Package without heat sink
Ta
Tb
Chip
Tt
Tj
Package
PCB
Rja: Junction to air thermal resistance
Rja= (Tj-Ta)/P
Low value is good thermal perfromance
Rjc: Junction to case thermal resistance
Rjc = (Tj-Tc)/P
Ψjt: Thermal characterization parameter: Junction to package top,
NOT thermal resistance.
Ψjb: Thermal characterization parameter: Junction to board
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Resistance
-Package with heat sink
Ta
Heat sink
Package
Die
Ts
Tc
Tj
Rja: Junction to air thermal resistance
Rja= (Tj-Ta)/P=Rjc +Rcs+Rsa
Rjc: Junction to case thermal resistance
Rjc = (Tj-Tc)/P
Rsa: External heat sink thermal resistance
Rsa= (Ts-Ta)/P
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Resistance
-PBGA package example
Tair
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Resistance
-Impact factors for package without heat sink
Die size
Package size, lead count
Packaging material thermal condunctivity
Material thickness in major heat flow path
Number of vias
Heat spreader or heat slug
Air velocity and temperature
PC Board size
Board configuration and material
Board layout
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Conduction application
Single material
Q
Die
Substrate
Tc
Tj
Tc = Tj - QL/(KA)
ti
Composite material
Kin-plane
Layer 1
Layer 2
Layer i
Layer N
KTthrough
Uniform heating on the die
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Conduction application
Heat spreader
Die
Substrate
Ab  As kb Ab R ba  tanh H b 
Rsb 
kb Ab As 1  kb Ab R ba tanh H b 
23
1


Ab
As
Ab: Heat spreader base area
As: Heat source area
Hb: Heat spreader thickness
Kb: Heat spreader thermal conductivity
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Convection application-Heat sink design
Hf
b
H
tf
Fins
Hb
L
Heat sink base
W
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Convection application-Heat sink design
Thermal resistance:
Heat transfer:
Rba
Tb  Tinlet


Q
hDh
Nu 
 7.54
k air
Fin efficiency:
o  1 
At
1   f 
.
.
m c p (1  e
 hAto m c p
Laminar flow
hDh
Nu 
 0.024 Re 0.786 Pr 0.45
k air
Af
1
Turbulent flow
f 
tanh( mH f )
mH f
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Convection application-Heat sink design
Total static pressure loss:

 U ch 2
L
P   K c  4 f app
 K e  
Dh
2


Culham and Muzychka (2001)
Apparent friction
factor
fapp
calculation
Fully
developed flow
friction factor f
 

f app Re  3.44 L*


L
Dh Re


2
  f Re  

 2
 40.829b H f 3  22.954b H f 4
5
 6.089b H f 
f Re  24  32.527 b H f  46.721 b H f
Contraction loss
coefficient Kc
Expansion
loss
coefficient Ke
L* 
0.5  2
David Copeland (2000)
12
  1
f app
 

Re  3.2 L*

 0.57  2

2
  f Re  

b H f 2  1
f Re  4.7  19.64
b H f  12
K c  0.421   
K c  0.8  0.4 2
K e  1   2
K e  1   2  0.4
Nt f
W

b
tf b
12
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Convection application- Heat sink design Impact
factors
Air flow rate
Available space
Heat sink base and fin material
Fin pitch and fin thickness
Heat flux
Heat sink technologies
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Thermal Design
Design methodology
Define requirements
Analyze given package design
Identify major heat paths and paths for improvements
Consider and assess potential improvements
Detail analysis/modeling
Build prototypes
Thermal testing
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Modeling
Finite Element Method (FEM)
Software: ANSYS
Solve conduction problem within package or board
Require input data: material propoerties, package/board
construction/geometry
Boundary conditions:
Heat source distribution on the die or board
Effective convective heat transfer coefficient on the surface of the package or
board
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Modeling
Finite Element Method
Procedure:
Create package/board geometry or import from CAD file
Mesh
Input material properties and assign boundary conditions
Solve
Post-process
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Modeling
Finite Difference Method (FDM )
Computational Fluid Dynamics (CFD)
Commercial software: Flotherm, Fluent
Solve the temperature field and flow field
Not only solve the conduction, also on convection, radiation and
phase change
Required input: Geometry, flow conditions, material properties
including fluid
Mesh dependent on the chosed model
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Modeling
Finite Difference Method (FDM ) Example
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Packaging thermal parameters
Mount package on a standard test board
Mount thermocouple on top of the package center
Mount thermacouple on board at the edge of package
Put package in a standard test environment
Wind tunnel to vary the air speed
Apply known amount of power
Measure temperature of Tj, Ta, Tb, Tt
Calculate Rja, Ψjt, Ψjb
Ta
Tt
Chip
Tj
Tb
Package
PCB test board
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Packaging thermal parameters
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Packaging thermal resistance Rjc
All heat is removed from top of the package
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Thermal interface material resistance Rcs
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Thermal interface material resistance Rcs
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
p
et
tl
T
ou
p
ir
flow
A

t
le
in
T
2

4
T
Heat sink thermal resistance Rsa
wer
lo
B
le
zz
No
er
b
m
ha
stc
te
low
irf
A
rs
te
ea
Rsa 
Ts  Tinlet
Q
Q  k s As
d
le
a
c
ts
o
n
re
a
s
ion
ns
e
im
D
tion
sula
In
H
k
loc
b
r
pe
o
C
3
T
al
ri
ate
em
fac
2
T
al
rm
er
he
T
int
s
T
Heat sink
T3  T2
L
Heat source size: 25 mm x 25 mm
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Measurement
Heat sink thermal resistance Rsa
The rmal Re s is tanc e , ( o C/ W )
0.600
Test data
Present method (Eqs. 6-8)
Teertstra [1]
Copeland [2]
0.500
0.400
0.300
0.200
0.100
0.000
0
0.01
0.02
0.03
3
Flow rate (m /s )
0.04
0.05
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CSE291: Interconnect and Packaging, UCSD, Winter 2006
Q&A
Page 38
Guoping Xu
Guoping.xu@sun.com