An Introduction to Thermal Management in electronic system packaging

1
An Introduction to Thermal Management in
electronic system packaging

• Dr. David W Shao
• Ericsson AB
• Stockholm, Sweden
• Dr.Li-Rong Zheng and Prof H. Tenhunen
• Royal Institute of Technology (KTH),
  Kista-Stockholm

2
Heat flux- from your experience
3
Where heat comes from?- an example

• Thinking
• Many components on a PCB
• Many PCB in a system
• etc.

4
Intel microprocessor power dissipation

• Ref. CPU power challenges, ISPLED99

5
Telecommunication and computing systems

• Heat generation per system footprint area
• Ref. electronics cooling, Dec. 2000

6
Maximum chip temperatures
7
Temperature vs. power dissipation

• Example 14-pin DIP with wire resistor and
  temperature sensor

8
Why low temperatures?

• Arrhenius equation decreasing every 10C reduces
  the failure rate by a factor of 2 within
  20C-140C for die temperature.
• US Dept. Of Defense Handbook 217 every 10C
  drop doubles the life.
• Every 10C lower transistor temperature gives 1-3
  performance improvement, depending on
  constrution.
• Ref. Electronics cooling, sept. 2000

9
Failure rates depend on temperatures
10
Temperature dependent CMOS gate delay
11
Thermal management objectives

• Preventing catastrophic failures
• softening and/or vaporisation of organic
  materials
• softening or melting of solders
• thermal stress fracture of leads, joints, and
  seals
• fatigue-induced fracture or creep-induced
  deformation of encapsulants, adhesives, and
  laminates
• electronic failures due to device and
  metallization related errors
• Reducing failure rates due to temperature
  enhanced wearing and differential thermal
  expansion (Arrhenius activated processes)
• Maintain specified performance (e.g.. system
  clock rate) on all operational conditions
• Increased chip power density 10 W_at_5mmx5mm chip
  size 4x105 W/m2 (1/100) heat flux across
  surface of Sun _at_6000 C

12
Thermal management at different levels
13
Contents of thermal management

• Heat transfer theory
• Component/board modelling
• System modelling
• Cooling methods
• Experiments and Simulations
• Etc.

14
Heat transfer basic heat conduction

• Heat conduction diffusion phenomenon in the solid

15
Heat transfer basic convection

• Convection created by fluid movement

16
Heat transfer basic radiation

• Radiation is viewed as the propagation of
  electromagnetic waves.
• Thermal radiation requires no matter

17
An overview of heat transfer coefficient
18
Thermal resistance concept

• Steady-state transport (Qheat flow, Across
  sectional area, Lthickness, k thermal
  conductivity of the material)
• Heat transfer equation
• (T1-T2 ) (1/(h1A)L/kA1/(h2A))Q RQ, comparing
  to UIR
• Thermal resistance RL/(kA) for heat conduction,
• R1/(hA) for convection, (unit K/W)

h210 W/m2K 0.1 m2K/W
h15000 W/m2K 0.0002 m2K/W
200 W/mK 0.00001 m2K/W
19
Thermal conductivity- an important property
W/cm C
20
A typical package thermal resistance

• Package thermal resistance can be divided into
• chip-to-package(junction-to-case) thermal
  resistance (Rcp or Rint)
• depends on used technologies
• package-to-ambient thermal resistance (Rpa or
  Rext)
• can be controlled by mechanical/thermal design
• Junction-to-ambient thermal resistance Rja Rcp
  Rpa
• Chip temperature Tj Tambient P(consumed)xRja

21
Is thermal design important?
22
Examples of heath paths
23
Which path does heat flow favour?

• Cavity down favour heat sink on the top
• Cavity up favour cooling in PCB

24
Packaging resistance depends on many factors
25
Package types

• Various leadframe and BGA packages
• Leads contact PCB
• Leadframe Dual-sided and quad packages
• BGA laminate package
• Both are encapsulated in plastic cap
• Ref. Electronics cooling, may, 1997

26
Junction-to-case thermal resistance an example
27
Example 20 W power consumption in package
28
Package thermal resistance Rjc and Rca
29
Calculating thermal resistances
30
Thermal resistance Spreading factor H
31
Thermal resistance spreading factor H
32
Example Alumina substrate thermal resistance

• 25mmx25mmx2mm alumina substrate
• k21 W/mK
• R x/(kA) 2x10-3/(21 (25x10-3)2) 0.15 C/W

33
Example chip with heat spreader
34
Example C-4 solder bump
35
Example Cooling piston in contact with chip
36
IBM Thermal Conduction Module
37
Example PGA single chip module
38
Example PGA single chip module
Rtotal13.2 K/W.Junction temperature is given
by TjQchipRtotalTairIf Tair25 C and
Qchip4 WW/o grease Tj83.4 CW grease
Tj77.8 C
39
Component models

• Possible heat flow in any directions
• Physical model is real but not practical
• Simplified (logical) model is usually used in the
  calculation.

40
Component models

• 2-parameter model
• 3-parameter model

41
Thermal analysis for a leadframe package mounted
to a PCB

• Heat conduction, convection and radiation
  combined
• 70-90 heat to ambient by board
• Thermal performance
• R JA (TJ - TA)/P

42
Parameter analysis pad/inner lead tip gap

• Simulation object 160 lead, 28 mm QFP
• Ref. 42th ECTC symposium, 1992

43
Parameter analysis leadframe thermal conductivity

• Test 176 lead, 24 mm TQFP
• Less sensative
• Ref. electronics cooling, may 1997

44
Parameter analysis pad size

• Test 208 lead, 28 mmTQFP
• Pad size rely on die size and bond wire length
• Ref. electronics cooling, may 1997

45
Thermal performance enhancement leadframe

• Base 304 lead, 40 mm MQFP
• Key decrease thermal resistance between leads
  and pad
• Ref. electronics cooling, may 1997

46
Reducing control resistance gets biggest benefit
Temp.

• Current overall K9.979 W/m2K
• If 200 W/mK increased to 400 W/mK, K9.9795 W/m2K
• If 10 W/m2K increased to 20 W/m2K, K19.9 W/m2K
• In conclusion, always reduce the biggest thermal
  resistance!

10 W/m2K 0.1 m2K/W
5000 W/m2K 0.0002 m2K/W
200 W/mK 0.00001 m2K/W
47
Thermal performance enhancement BGA package

• Add thermal vias and thermal balls under die
• Add metal planes in package laminate
• Base 35 mm package
• Ball matrix a)352 lead, b)388 lead
• SBAG, SuperBAG, trademark
• Ref. electronics cooling, may 1997

48
Contact thermal resistance

• Ref. electronics cooling, may 1997

49
Thermal greases a must for high local heat flux

• Silicone oils with thermal conductive filler
• Thermal conductivity, up tp 7 W/mK
• Thermal resistance, 0.25-0.6 C/W, gt50 reduction
  compared to the dry interface
• Excellent surface wetting and flowing ability
• Very thin thermal interface even at low pressures
• Used for discrete devices
• Low cost
• Difficult to handle manually

50
Interface materials

• Thermally conductive adhesive tapes
• Gap fillers
• Pad
• Film
• Phase change materials

51
Influence of cooling to total Rja
52
Thermal spreading

• Thermal resistance is related inversely to the
  area. Reduction possible by increasing the
  effective cross-section area
• Typical area is package footprint
• Fins can add area by factor 20
• If chip has a power flux 50W/cm2 (alpha-processor
  or ECL gate array). With direct air cooling of
  chip area the temperature rise would be 1600 C -gt
  thermal flux must be distributed! If 50 C
  temperature increase allowed, 2 W/cm2 power flux
  is required, by spreading power to larger area

53
Convection heat transfer (Rca)
54
Air cooling

• Natural convection driven by density difference
  (Temp diff.)
• Forced convection driven by mechanical means,
  fan or blower etc.

55
Natural convectionhorizontal surfaces

• Heated surface facing upwards
• HTC decrease by 50 if heated surface facing
  downwards

characteristic length average side length
56
Natural convection vertical surfaces

• Velocity and temp profiles
• Laminar or turbulent depending on temp difference
• Plate height characteristic length
• Dimensionless numbers Grashof and Rayleigh

57
Natural convection parallel plates

• Velocity profiles for two extreme cases
• Wall to wall distance is characteristic length
• Bar-Cohen correlation

58
Natural convection with heat sinks

• Combine parallel plate with external natural
  convection models
• Ref. electronics cooling, sept.2000

59
Natural convection vs Radiation

• For a horizontal board
• Air temperature 50 C

.                                               
                                                  
                                                  
                                   
60
Example forced convection
61
Forced convection in a parallel plate flow

• Parallel plates flow is a typical case, e.g. PCB,
  heat sink
• Around Re2400 is transition point between
  laminar and turbulent. For PCBs, turbulent may
  start earlier.
• Different correlations were developed, with
  considering entrance effect

62
Forced convective cooling of PC
63
Forced convection RISC workstation
64
Forced convection Heat sink application

• Heat sink dimension 306180, 1.0/3.1,
• heat dissipation 340W, front w3.6 m/s, R0.088
  C/W, pressure drop74 Pa.

65
3rd generation IBM MCM with heat sinks
66
Why heat sinks?
q h A (Theat sink Tair)  h heat
transfer coefficient T temperature
67
Casting heat sinks

• For high volume products
• Lower thermal conductivity
• Limited geometry dimensions
• Lower aspect ratio 41

68
Bonded heat sinks

• Epoxy bonding of fins into a heat spreading base
• Brazed assemblies
• Cold formed or swaged
• Welded ultrasonic or resistance
• Stacked fins (fin and base extruded individually
  and formed together)    

69
Folded heat sinks

• Larger area, little weight
• Bonded, brazed or soldered

70
Aluminum extrusions

• Easy manufacture, low cost
• High thermal conductivity, 210 W/m K
• The aspect ratio of fin height to spacing has
  increased over the past decade from 61 up to as
  much as 151

71
Improve heat transfer in heat sinks

• Increasing airflow velocity
• pressure drop and the acoustic noise.
• Crosscutting of flat fins into multiple short
  sections. This improves the heat transfer
  coefficient at the fin surface. The drawback of
  this method is the resultant additional pressure
  drop.
• Augmentation of fins is similar to cross cutting
  but adds a separate "twist" in the leading and
  trailing edge of a fin.
• Impingement (jet) cooling of heat sinks is
  achieved using a high-speed airflow directed at
  the fin tip toward the base.

72
Cutting effect in heat sink

• Break-down in boundary layer, higher heat
  transfer coefficient
• May be higher pressure drop

73
Enhancement heat sink

• Thiner boundary layer without significant
  increase in pressure drop
• Best gt3m/s
• Ref. PCIM Magazine, nov 1997

74
Enhancement heat sinks
Bent Fin Case
Tuning Fork Case
Thermal performance comparison

• Pressure drop is increased a little
• Ref Aavid

75
Heat sink selection

• Total Rja Rjc Rcs Rsa (Tj - Ta)/Q
• Heat sink Rsa ((Ts - Ta)/Q) - Rjc - Rcs

76
Is it difficult to select a heat sink?

• Selection of heat sink is difficult, many
  parameters (geometry, thermodynamic properties
  etc.), external factors (cost, space, fan, noise
  etc.), compromise is usually your choice.
• Comparing different heat sinks is actually in
  some ways to evaluate the thermal and hydraulic
  performances under certain assumptions. These
  assumptions construct a base for comparison,
  which is strongly related to results.

77
An example heat sources layout

• A PCB board attached to a heat sink, 200230 mm
• Two big transistors, 100 W each, with a footprint
  of 489 mm

Transistors
Air flow
78
Three heat sinks
79
General evaluations temperature rises
Approaching velocity 3.4 m/s
Calculated is in a rough agreement with measured
80
General evaluation thermal resistance

• Resistance

81
General evaluation pressure drop

• For a long heat sink, pressure drop due to
  friction is dominant
• The cutting effect in the skived heat sink has
  not been considered

82
What is special with the skived heat sink?
Extruded
Skived
83
Is it good with cuttings in the skived heat sink?

• Heat transfer increased owing to the breakdown
  in the boundary layer
• Experimental measurements
• Optimal design
• Further investigation
• Pressure drop unavoidably increased
• how much?

84
Pressure drop calculated vs experimental

• A good agreement in the extrusion heat sink
• A large difference, up to 65, in the skived heat
  sink due to the cuttings

85
Thermal resistance vs pressure drop

• Pressure drop based on measurement
• Comparison is made at the same pressure drop
• Not available conditions in practice

86
Fan and system performances

• A larger pressure drop results in a smaller flow
  rate, vice versa

87
Is it good with Volumetric heat transfer
efficiency?
Heat dissipation Air mass flow rate through
the heat sink Specific heat capacity of air
Temperature difference between the heat sink
and the ambient air
88
System examination

• Operating points are determined according to fan
  and system (duct, heat sink etc.) performance
• Changing any parts will change to a new operating
  point
• Always available (in theory), a rational base for
  comparison

89
Performances at the operating point
90
Comparing performances at the operating point
(Thermal resistances, C/W)

• 8-10 larger Rth with the skived at lower flow
  rates (speed 1)
• Similar at higher flow rates (speed 2)

91
Performance difference due to comparison base
How much is the skived worse than the extruded?
92
Remarks

• A simple general evaluation is most popular but
  the results may not reflect the fact.
• The examination of overall system that includes
  the characteristic of the fan, duct and heat sink
  etc. is strongly recommended, especially for the
  heat sinks that have a large variation in
  pressure drop.

93
Trends in heat sink technology

• Extrusion with a higher aspect ratio, up to 25
• Sintered heat sinks
• Metal injection molding
• Increasing heat spread in base plate
• Copper as a base
• Embedded heat pipes, vapor chambers
• Copper or carbon based materials (copper graphite
  350 W/m K)

94
Heat sink with embedded heat pipe

• Reduce thermal resistance 50
• Difficult to place components?

95
Fan types

• Usually used with forced flow
• Axial fan, centrifugal radial blower
• System performance relates to various types

96
Fan selection

• Measured at 1 m from intake side

97
Air cooling limit
Fan power
Cooling capacity

• High velocity give a higher power consumation and
  preesure drop
• Small fin distance give more area, but limited.

98
Why cooling capacity limit ?

• Coolin capacity avaiable temperature difference
  and mass flow
• Limit is independent of fin shape

99
Liquid cooling types

• Dissipates more heat with considerably less flow
  volume
• Less acoustic noise
• Direct and indirect cooling
• Direct system coolants are chemical
  compatibility with the chips and other packaging
  materials, e.g. water
• Humidity and leakage for indirect system?

100
Liquid cooling examples

• IBM (1970s) Liquid Encapsulated Module with pool
  boiling
• Coolant FC-72 with 1700 to 5700 w/m2-K
• Cool 4 W chips (4.6 mm x 4.6 mm) and module
  powers up to 300 W
• Ref. Electronic Packaging and Production, July
  1986

101
Liquid cooling examples

• CRAY-2 supercomputer direct cooling
• Forced convection with FC-77
• 600-700 W/module(8 PCBs), total 24 modules
• Ref. Electronic Packaging and Production, July
  1986

102
Liquid-cooled cold plates

• Copper tube in direct contact the heated surface
• Single tube with no joints eliminates leak
  potential
• Ref Aavid

103
Cooling single phase or two-phases?

• What do we want?
• Higher heat capacity
• Higher heat transfer coefficient

104
Cooling mechanisms

• Single phase by aid of sensible heat, i.e.
  temperature rise.
• Two-phase boiling by aid of latent heat, i.e.
  phase change.

105
Sensible and latent heat in a h-p graph
106
Sensible heat

• m1 kg/s, tout-tin10 C
• Air Q10 kW
• Water Q41.8 kW

107
Latent heat

• m 1kg/s
• Water Qmr2200 kW

108
A high local heat flux requires a high local heat
transfer coefficient

• Heat flux qQ/A (W/m2)

109
Heat transfer coefficients in single phase

• Air 2-100 (W/m2K)
• Water 1000-15000 (W/m2K)

110
Heat transfer coefficients in two-phase boiling
with water

• Conventional (gt2-3 mm)
• 2500-25000 (W/m2K)
• Micro-space
• Micro-channel (um-2mm)
• up to 50000 (W/m2K)
• Wick structure (um)
• up to 50000 (W/m2K), enlarge area

111
Heat pipe

• Evaporation and condensation
• Self circulation without driving equipment, e.g.
  Compressor
• High heat transfer coefficient owing to
  two-phases
• Any orientations

.
112
Heat pipe

• Temperatures from lt -243C ( titanium
  alloy/nitrogen) , to gt2000C (tungsten/silver).
• Electronic cooling, copper/water is typically
  used. Copper/methanol is used forlt 0C.
• Factors compatibility of materials, operating
  temperature range, diameter, power limitations,
  thermal resistances, and operating orientation
• Power limitations, thermal resistances are main
  factors for electronics cooling applications

113
Heat pipe design total thermal resistance
  dT qevap Revap qaxial Raxial qcond
Rcond

• Sum conduction through the wall, conduction
  through the wick, evaporation, axial vapor flow,
  condensation, and conduction losses back through
  the condenser section wick and wall.
• Facotrs geometry, evaporator length, condenser
  length, wick structure, and working fluid etc.
• E.g. copper/water with powder metal 0.2 cm2-C/W
  for evaporator/condenser, 0.02 cm2-C/W for axial
  resistance

114
Heat pipe application

• 75-100 W
• 0.2-04 C/W
• Thermacore products

115
Heat pipe application

• Silicon Controlled Rectifiers (SCR's), Insulated
  Gate Bipolar Transistors (IGBT's) and Thyristors
• Up to 5 kW
• 0.05-0.1 C/W
• Ref. Thermacore

116
Natural convection around a cellullar phone

• 1 W PA, 1 W others
• External natural convection, air velocity up to
  0.2 m/s
• Heat conduction dominant inside (Grashof nr.
  100-300), no radiation
• Maxi. Temp 98 C. at room temp.

117
Heat pipe application in a cellullar phone

• MCM soldered to system card with a micro heat
  pipe (2 mm) which connect antenna
• Ref. electronics cooling May 2000

118
Cellullar phone w and w/o heat pipe

• Heat dissipation 2.5W
• Temperature rise is used
• Junction temp reduction by 6.7C

119
Two-phase thermosiphon (heat pipe)

• Same principle as heat pipe
• Condenser must be above evaporator

120
Example thermosiphon
121
Spray cooling principle
122
Spray cooling parameters

• Dielectric fluids, FC72, water
• Atomization effect
• Critical heat flux, 100 W/cm2

123
Spray cooling an example Cellular Base Station

• Increase HTC
• R Reduce noise level
• F Flexible EMC Shielding
• L Leakage?

124
Vapor compression refrigeration system

• Reach low temperatures
• COP2-3
• Reliable system
• E.g. Outdoor base station cabinet
• Investment and operating cost
• Enviroment issue, e.g. CFC

125
Thermoelectric cooling principle

• Peltier Effect, discovered in 1834, DC current
  applied across two dissimilar materials causes a
  temperature differential.
• Equations
• Qh Qc Pin
• Th Tamb (R) (Qh)
• dT Th - Tc

126
Thermoelectric cooling performance

• dT depends on parameters
• E.g. I3.6A, U10V, Qc22W, P36W, COP0.6
• Materials selection
• Available multi-stages, usually for dTgt55 C
• Ref. electronics cooling, sept 1996

127
Simulation tools PCB optimization

• 22 components
• 1148 nodes

128
Simulation tools Component-Board level
129
Simulation tools Subrack-Room level