Portable Power Challenges: Spreading the Heat

1
Portable Power Challenges Spreading the Heat

• Darvin Edwards
• TI Fellow
• Manager Advanced Package Modeling and
  Characterization
• Texas Instruments
• rvin_at_ti.com

2
Outline

• Trends driving portable power concerns
• Best Design Practices
• stacked die packages
• stacked packages
• hot spots
• Modeling Issues
• compact models
• the special handheld environment
• Compact System Optimizations
• Summary

3
Trends

• Whats driving thermal concerns in portable
  electronics? Teenagers!
• Teenagers what phones with
• Cameras
• Video cameras
• Games
• Downloadable music
• GPS
• TV
• Email PDA functions
• Internet access
• Bluetooth
• USBII
• 4-8GB hard drives
• And the phone functions

4
Trends

• Portable devices moving towards ever more compact
  electronics
• stacked die
• stacked packages
• embedded die
• Concentrates heat locally, drives up power
  density
• Is this an issue?
• phones run 1-3 watts
• could compress all circuits into one module
• cooling 2-3 watt package without heat sink is
  usually challenging

Spacer
5
Trends

• Low cost package types dont offer many routes
  for heat spreading away from die
• 1 or 2 layers of routing only
• Stacked packages have difficulty getting heat to
  PCB from top package
• Thin die have more hot spot issues than thicker
  die
• WSPs provide very localized hot spots on PCBs
• Dies of highest concern
• Battery charger die
• RF power amplifiers
• SOCs

6
Best Design Practices

• To optimize stacked die thermal performance,
  conduction paths to the PCB must be optimized
• Largest die must go on bottom of package for best
  thermal performance
• Large die acts as a heat spreader to get heat to
  solder balls and PCB
• With good die spacer thermal conductivity, high
  power die can be on top or bottom
• When the largest die is on the bottom, package
  thermal performance will be nearly the same as a
  single die package with the same large die
• Theta-ja and Theta-jb will be barely changed
• Theta-jc might go either way, depending on die
  thickness and die sizes in the package
• Theta-jc is a dont care for most portable
  applications
• Stacking adhesives should be void free for
  optimal thermal conduction
• System problem Stacking die results in higher
  power density than single die package
• Higher junction temperature will result
• Need better thermal spreading in PCB
• Moving thermal balls to beneath edge of
• chip improves thermal performance
• over a center-only array

7
Best Design Practices

• To optimize stacked package thermal performance,
  thermal conduction through the stack must be
  maintained
• The top packages will not conduct heat into the
  PCB as effectively as bottom package
• Thermal balls not possible for top packages
• Thermal performance of top packages will degrade
• Adhesive between packages can provide relief
• The thermal performance of the bottom most
  package will look largely the same as if attached
  to PCB alone
• This generalization assumes heat sinking was not
  the plan
• Assumes zero heat generation in packages on top
• The best system design is to put high power die
  in the bottom package for best thermal conduction
  into PCB
• Thermal performance of stacked packages is
  somewhat worse than for a stacked die approach
  with the same die compliment
• Best practice modeling should include Cu
  conductor details of package
• true of almost every package type

8
Cooling Ideas for Stacked Packages
Al or Cu Spreader
Soldered Connection
Top Hat Spreader for Stacked Packages
Plastic Case
Gap Filler
Metallized Case
Gap Filler Conduction to Chassis
9
The Hotspot Issue

• Thinner die increase spreading resistance
• hot spots get hotter
• Can reduce impact of hotspot by attaching die
  directly to high thermal conductivity spreader
  such as Cu plate
• cost issue
• possible reliability issue

10
Hotspot Best Design Practices

• Hotspots thin die imply need for package/chip
  codesign
• In stacked die packages, hot spots on one die can
  create hotspots on another
• die thermal design tools need to comprehend
  die-to-die interactions
• Dont cluster hotspots
• spreading hotspots apart allows heat spreading
  from individual hotspots over wider areas,
  increasing conduction from each hot spot
• applies to die and PCB design
• If only one hotspot, center of die is better than
  corner of die
• allows maximum heat spreading
• Keep hotspots out of corners
• a hotspot in a corner blocks heat flow in 3
  directions

11
Hotspot Best Design Practices

• Center is better than corner for single hotspot
• 0.5x0.5 mm hotspot 2 design placements
• 38 difference

Center Spot Max. Temp. 62C Theta-ja 37C
Corner Spot Max. Temp. 76C Theta-ja 51C
12
Hotspot Best Design Practices
With multiple hotspots, sweet location is
somewhere in between
1
2
2
1
1
2
3
4
4
3
3
4
Best Design
Worst Design
Better Design
Sweet Spot
13
Hotspot Reliability Implications

• Device reliability is a linear function of the
  active die area and is exponential with
  temperature
• small hotspots will have minor impact on
  reliability
• large hotspots will have major impact on
  reliability
• higher temperature spots will have large impact
  on reliability, regardless of size
• Typical reliability issues
• electromigration
• hot electron effects
• Kirkendall voiding in Au/Al wire bonds
• Reliability and functionality need to be
  characterized at the temperatures the hotspots
  will reach

14
Modeling Challenges for Portable Die

• No standard compact model topologies for stacked
  die or stacked packages
• present compact models dont include hot spots
• In suggested model, compact model topology
  changes dependent on which die is largest
• Inter-die resistance represented

• Potential Stacked Die Topology
• modification of standard QFP
• compact model

Inter Die Resistance
15
Modeling Challenges for Portable Die

• Stacked packages may be represented with stacked
  compact models

Standard package compact model
Stacking of compact models
Thermal Shunts (Shorts)
16
Modeling Issues Handheld Environments

• When modeling portable devices, multiple thermal
  boundary conditions must be included
• Handheld
• Operating in case
• Operating on a table
• Example typical CSP device in cell phone
• When held in the hand, a portable device is
  liquid cooled
• There is a limit to the power which can be
  comfortably dissipated by a portable device into
  the hand
• To extend beyond this limit, rejection of heat to
  the air is required

17
Modeling Application Including the Hand

• Hand is modeled as block of fat
• Exterior of fat held at body temperature, 37.1C

18
Modeling Application Including the Hand

• Good correlation to experiment achieved

Still air box measurement
Operation mode measurement
Courtesy M. Naeshiro (TTC)
19
Compact System Optimizations

• Optimizations
• vents even on portable devices
• heat spreaders in plastic chassis boxes
• conduction enhancements from PCB to box
• screws, gap fillers
• phase change materials to absorb transient
  thermal events
• low speed fans for portable devices
• in hot PCBs, thermally partition PCBs to
  isolate devices with lower allowed operating
  temperatures from those with higher

low speed fan
Heat Spreader
Gap Filler or Phase Change
20
Compact System Optimizations
Piezoelectric Fans
Heat Pipe Applications
21
Offsetting Trends

• Get Hotter Higher maximum junction temperature
  ratings allow easier thermal design
• Challenge high temperature with low leakage
• Get Smarter Understanding the life usage thermal
  profile allows tuning of thermal solution vs.
  reliability
• A system specified at 125C continuous operation
  may allow 300 hrs at 150C with good reliability
• Design system for reliability goals, not just
  maximum temperature
• Must insure system functions at hottest
  temperature

Product Specific
22
Summary

• Portable devices are offering thermal challenges
• stacked die and stacked packages increase power
  density
• thinned die have potential hot spot issues
• Hotspots may need to be analyzed for thin die,
  even in portable applications
• multiple environments must be considered when
  performing thermal design of a system
• New cooling schemes are on the way to handle
  higher power densities in portable systems
• Thermal lifetime application profiles should be
  used to estimate device reliability rather than
  maximum operating temperatures