1. OVERVIEW OF ADVANCED THERMAL MATERIALS
Carl Zweben, PhD
Life Fellow ASME
Fellow SAMPE and ASM
Associate Fellow, AIAA
Advanced Thermal Materials Consultant
62 Arlington Road
Devon, PA 19333-1538
Phone: 610-688-1772
E-mail: c.h.zweben@usa.net
http://sites.google.com/site/zwebenconsulting
Copyright Carl Zweben 2010 1
2. The information in these slides is part of a short
course on composite materials that is presented
publicly and in-house
Contact author for information
Copyright Carl Zweben 2010 2
3. OUTLINE
• Introduction
• Semiconductors, ceramic substrates and
traditional thermal materials
• Advanced thermal materials
• Applications
• Summary and conclusions
• Appendix (terminology and abbreviations)
Copyright Carl Zweben 2010 3
5. INTRODUCTION
• Critical thermal management problems:
– Heat dissipation
– Thermal stresses cause
• Warping, fracture, fatigue, solder creep
• Primarily due to CTE mismatch
• An issue for all cooling methods
• Problems similar for
– Microprocessors, power modules, RF
– Diode lasers
– Light-emitting diodes (LEDs)
– Plasma and LCD displays
– Photovoltaics
– Thermoelectric coolers (TECs)
Copyright Carl Zweben 2010 5
6. INTRODUCTION (cont)
• Microelectronic thermal problems well known
– Xbox 360 $1 billion “Red Ring of Death” failure
widely cited as thermal issue
– Nvidia $150-200 million GPU thermal problem
– “Burned groin blamed on laptop” (BBC 11//02)
• Solder thermal fatigue limits laser pulsing
• Higher process temperatures for lead-free solders
– Increased thermal stresses & warping
• Higher ambient temperatures
– E.g. automotive under hood
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7. INTRODUCTION (cont)
• Weight (mass) important
– Portable systems
– Vibration and shock loads
• Volume and thickness decreasing
• Cooling significant part of total cost of ownership
– System
– Building, data center
• System cooling power increases building cooling
load
• Low-CTE “Thermount” PCB withdrawn from
market in 2006
– No current thin-ply replacement
Copyright Carl Zweben 2010 7
8. INTRODUCTION (cont)
• Traditional thermal materials inadequate
– Decades old: mid 20th Century
– Impose major design limitations (see later)
• In response to critical needs, an increasing number
of advanced materials have been developed
• Many with ultrahigh-thermal-conductivity
– k = 400 to 1700 W/m-K
– Low CTEs
– Low densities
– R&D to high-volume production
Copyright Carl Zweben 2010 8
9. INTRODUCTION (cont)
• Can now match CTEs of chips, lids, heat sinks, and
PCBs
– Reduces thermal stresses and warping
– Possibly eliminates need for underfill
– Enables use of hard solder attach
• Low thermal resistance
– Low-CTE solders under development
• Thermally conductive PCBs provide heat path
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15. WHAT’S WRONG WITH TRADITIONAL THERMAL
MATERIALS?
• Copper and aluminum
– High CTEs
• Thermal stresses, warping
• Require compliant polymeric and solder
thermal interface materials (TIMs)
– Higher thermal conductivities desirable
– Copper has high density
• What’s wrong with compliant polymeric TIMs?
– Pump-out and dry-out for greases
– High thermal resistance for most
– Increasingly, the key contributor to total thermal
resistance
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16. WHAT’S WRONG WITH TRADITIONAL THERMAL
MATERIALS? (cont)
• What’s wrong with compliant solders?
– E.g. indium alloys
– Process problems (voiding, poor wetting)
– Poor fatigue life (low yield stress)
– Creep
– Intermetallics
– Corrosion
– Electromigration
– Relatively low melting point
– Cost higher than many solders
DIRECT ATTACH WITH HARD SOLDERS DESIRABLE
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17. WHAT’S WRONG WITH TRADITIONAL THERMAL
MATERIALS? (cont)
• Low-CTE materials seriously deficient
– E.g. alloy 42, Kovar, tungsten/copper,
molybdenum/copper, copper-Invar-copper, etc.
– Conductivities < aluminum (200 W/m-K)
– High densities
– High cost
• CVD diamond
– High thermal conductivity
– Low CTE
– Expensive
– Thin flat plates only (i.e. CVD diamond films)
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19. NEW THERMAL MANAGEMENT MATERIALS
• Many advanced materials
– Various stages of development
– R&D to large scale production
– New ones continuously emerging
• Monolithic materials
– Primarily carbonaceous (graphitic)
• Composites
– Polymer matrix
– Metal matrix
– Metal/metal alloys-composites
– Carbon matrix (e.g. carbon/carbon)
– Ceramic matrix
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20. NEW THERMAL MANAGEMENT MATERIALS (cont)
• Al/SiC first, and most successful advanced thermal
material
– First used by speaker and colleagues at GE for
electronics and optoelectronics in early 1980s
– New processes developed
– Millions of piece parts produced annually
– Part cost dropped by orders of magnitude
– Microprocessor lids now $1-5 in high volume
– CVD diamond and highly-oriented pyrolytic
graphite inserts increase heat spreading
• “Hybrid materials” approach
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22. ADVANCED MATERIALS PAYOFFS
• Lower junction temperatures
• Reduced thermal stresses and warpage
• Simplified thermal design
– Possible elimination of fans, heat pipes, TECs,
liquid cooling, refrigeration
• Increased reliability
• Improved performance
• Weight savings up to 90%
• Size reductions up to 65%
• Dimensional stability
• Improved optical alignment
Copyright Carl Zweben 2010 22
23. ADVANCED MATERIALS PAYOFFS (cont)
• Possible elimination of underfill
• Increased manufacturing yield
• Reduced electromagnetic emission
• Reduced power consumption
• Longer battery life
• Reduced number of devices (e.g. power modules,
LEDs)
• Low cost potential
– Component
– System
– Total cost of ownership (TCO)
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24. DISADVANTAGES OF SOME ADVANCED MATERIALS
• Higher cost (low volumes, reinforcements)
• Limited service experience
• Low fracture toughness
• Possible hysteresis
• Ceramic materials hard to machine
• Some particulate materials hard to metallize
• Surface roughness and flatness
• Edge sharpness (laser diodes)
• Direct attach during infiltration complicates rework
• Galvanic corrosion potential
• Porosity (not hermetic)
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25. COMPOSITE MATERIAL REINFORCEMENTS
Discontinuous Fibers,
Continuous Fibers Whiskers
Particles Fabrics, Braids, etc.
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26. CTE OF SILICON-CARBIDE-PARTICLE-REINFORCED
ALUMINUM (Al/SiC) vs PARTICLE VOLUME FRACTION
25 Aluminum
COEFFICIENT OF THERMAL
Powder Metallurgy
EXPANSION (ppm/K)
20 Infiltration
Copper
E-glass PCB
15 Beryllium
10 NEW MATERIAL
Titanium, Steel
Alumina
5
Silicon
0
0 20 40 60 80 100
PARTICLE VOLUME FRACTION (%)
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28. SPECIFIC PROPERTIES
• Specific property is absolute property divided by
density
• Figure of merit when weight is important
• If specific gravity (S.G.) is used for density,
absolute and specific properties have same units,
e.g.
– Thermal conductivity, k = W/m-K
– Specific thermal conductivity, k/S.G = W/m-K
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29. SPECIFIC THERM. COND. vs CTE FOR PACKAGING MATERIALS
670
350
SPECIFIC THERMAL CONDUCTIVITY
Si, GaAs, Silica, Alumina, Beryllia,
Aluminum Nitride, LTCC
HOPG (740)
300
Diamond-Particle-Reinforced Metals
and Ceramics
250
C/C
(W/mK)
200
C/Ep SiC/Al (Al/SiC)
150
C/Al
Aluminum
100 C/Cu
Si-Al
50 Copper
Invar Kovar
Cu/W
0
-5 0 5 10 15 20 25
COEFFICIENT OF THERMAL EXPANSION (ppm/K)
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39. SUMMARY AND CONCLUSIONS
• Thermal management now critical problem for
microelectronics and optoelectronics
• Traditional thermal materials inadequate
– Mid-20th century
• Low-CTE, low-density materials with thermal
conductivities up to 1700 W/m-K available
• Can now match CTEs of chips, lids, heat sinks,
and PCBs
– Reduces thermal stresses and warping
– Possibly eliminates need for underfill
– Enables use of hard solder attach
• Low thermal resistance
Copyright Carl Zweben 2010 39
40. SUMMARY AND CONCLUSIONS (cont)
• Several advanced materials well established
– SiC particle/aluminum
– Silicon-aluminum
– Carbon fiber/polymer
– Natural graphite
– Pyrolytic graphite sheet
– Highly-oriented pyrolytic graphite
• Diamond composites used in production
microelectronic and optoelectronic systems
• Short (2-3 year) cycle from introduction to
production demonstrated
• Applications increasing steadily
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41. WE ARE THE INFANCY OF A
PACKAGING MATERIALS REVOLUTION
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43. TERMINOLOGY
• Homogeneous
– Properties constant throughout material
• Heterogeneous
– Properties vary throughout material
– E.g. different in matrix and reinforcement
– Composites always heterogeneous
• Isotropic
– Properties the same in every direction
• Anisotropic
– Properties vary with direction
• Inplane isotropic (transversely isotropic)
– Properties the same for every direction in a
plane (different perpendicular to the plane)
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