heat pipes & vapor chambers design...
TRANSCRIPT
Heat Pipes & Vapor Chambers Design Guidelines
Presenter George MeyerCEO, Celsia Inc.
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We’ll Answer these Questions
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Do I Need 2-Phase?
Are Vapor Chambers Just Flat Heat Pipes?
What Size Do I Need?
How Do I Integrate Them?
What Should the Heat Exchanger Look Like?
How Do I Model Thermal Performance?
• Heat sink ballpark• Excel Model
• Types of condensers• Pros and cons
• CFD analysis• Proto testing
• Benefits and consequences of solid materials• Some basic rules of thumb
• Similarities: design, wicks & performance limits• Differences: for moving or spreading heat
• Heat pipes: diameter, quantity, and shape• Vapor chambers: sizing
• Attaching it to the condenser• Mounting it to the heat source/ PCB
If conduction loss through the base is greater than 10o C, it’s a good sign that you could be conduction limited
When to Use 2-Phase Devices
The Short Answer• Only when the design is conduction limited or
• Non-thermal goals such as weight or size can’t be achieved with other materials such as solid aluminum and/or copper.
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Two PhaseAluminum(baselined at 1X)
Copper
Base thk: 1XWeight: 1XCost: 1X
Base thk: 0.5XWeight: 3XCost: 1.6X
Base thk: 0.5XWeight: 2XCost: 1.8X
Conduction Loss in the Base:22o Celsius
Conduction Loss in the Base:4o Celsius
Conduction Loss in the Base:17o Celsius. If base same thickness
as Aluminum then 11o C
2-Phase Rules of Thumb
2-phase devices are incredible heat conductors. • 5 to 50 times better conductivity than aluminum or copper using
copper/water 2-phase• 1,000 to >50,000 w/mK. Exact figure is primarily dependent on the
distance the heat is transported. Important as input for CFD modeling
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Ideal when heat needs to be moved more than 30-50mm• Remote fin stacks (heat exchangers) are a perfect example
As with any heat pipe heat sink design, size the heat pipe with an additional 25% thermal headroom
If you’re interested in spreading heat to reduce hot spot and/or attach to a local heat exchanger
• The ratio of heat spreader to heat source should be on the order of 20:1 greater area
2-Phase Similarity: Inner Workings
Heat pipes & vapor chambers transfer heat through the phase change of liquid to vapor and back to liquid
Liquid is passively pumped from condenser to evaporator by capillary action
Used for very efficient heat transport & spreading
No noise or moving parts with very high reliability
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+90o -90o
oC
/w/c
m2
Power Density w/cm2
Sintered Powder
Grooved
Screen
Evaporator Resistance (Typical Two Phase)
2-Phase Similarity: Wick Structures
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Sintered Powder
Screen
Grooved
Orientation Power Density Resistance
<500w/cm2
Good for freeze/thaw and bent shapesSmall heat sources up to 1,000 w/cm2
0.15-0.03 oc/w/cm2 +90o to -90o
<30 w/cm2
Main use is for very thin heat sinks due to high evaporator resistance. Limited
bending.
0.25-0.15 oc/w/cm2 +90o to -5o
<20w/cm2
Entry level price / performance must be gravity aided/neutral.
0.35-0.22 oc/w/cm2 +90o to 0o
2-Phase Device Similarity: Performance Limits
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Vapor Pressure
Sonic Entrainment Capillary‘the limiting factor’
BoilingLimit
Problem
Solution
Cause Operating well below design temp
Start-up power, low temp combo
Device above designed power input or at low
temp
Input power exceeds design
Radial heat flux exceeds design
Vapor flow prevented
Large internal pressure drop
Condenser flooded with excess fluid
Capillary pump breaks down
Wick dries out
Change working fluid
Increase vapor space or operating
temperature
Increase vapor space or operating
temp
Modify wick structure
Increase wick heat flux capacity
Capillary limit is the ability of a particular wick structure to provide
adequate circulation for a given working fluid.
It is usually the limiting factor for terrestrial applications.
Cap Limit
Boiling
Sonic
Entrainment
Wick and Vapor Limits Are What Matter
• Qmax is determined by the lower of the wick and vapor limits • Wick limit = capillary limit
• Vapor limit = combination of sonic and entrainment limits
• For heat pipes the wick limit is usually the limiting factor, unless flattened
• For vapor chambers the vapor limit is the usually the limiting factor
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Heat Pipe Vapor Chamber
2-Phase Device Similarity: Customization
• Don’t simply rely on published data from one heat pipe manufacturer
• Changes to wick thickness & porosity as well as the amount of working fluid can greatly effect performance
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“Standard” Wick
• Pore radius, for instance, is increased when a higher Qmax is needed - while shrinking it improves capillary action for applications where the condenser is below the evaporator
Porous Wick
Orientation Affects Qmax
• With standard heat pipes, Qmax drops by 90-95% depending on orientation• Both extremes can be optimized but not in tandem• Increasing the Qmax to work against gravity decreases the Qmax when
working with gravity.
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2-Phase: Effective Thermal Conductivity
• Two phase device thermal conductivity increases with the length of the device.
• Important to know for CFD modeling.
• For industrial applications where powers are higher and the distances may be longer, the numbers are typically from 15,000 to as high as 50,000+, but rarely reach the often cited 100,000 w/mK figure
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2-Phase Differences: OverviewTraditional 2-Piece
Vapor ChamberHybrid 1-Piece
Vapor ChamberHeat Pipe
Initial Form Factor
Small diameter tube 3-10mm Very large diameter tube 20-75mm
Upper and lower stamped plates
Shapes Round, flattened and/or bent in any direction
Flattened rectangle, surface embossing & z-direction bendable
Complex shapes in x and y direction, surface embossing
Typical Dimensions
3-8mm diameter or flattened to 1.5-2.5mm. Length 500mm+
1.5-4mm thick, up to 100mm W by 400mm L
2.5-4mm thick, up to 100mm W by 400mm L
Mounting to Heat Source
Indirect contact though base plate unless flat & machined
Direct contact. Mounting pressure up to 90 PSI
Direct contact. Mounting pressure up to 90 PSI
Relative Cost Very cost effective, but increases quickly with large diameter, custom wick structure, secondary ops
Comparable to 2-4 heat pipes in higher power and/or high heat flux applications
More expensive than 1-piece design due to additional tooling cost and labor time, but large scale production closes the gap
2-Phase Differences: Moving or Spreading Heat?
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While there’s no hard line of distinction between the two, think of the difference like this…
Moving Spreading
• Linear heat flow
• Remote condenser, usually
• Multidirectional heat flow
• Local condenser, almost always
Local CondenserRemote
Condenser
Heat
Heat
Heat Pipes
Mounting Plate(s)
Vapor Chamber
Direct Mount
What if BOTH Spreading & Moving Are Important
• You’ll typically encounter this problem with challenging applications
• High-heat flux
• Enclosures tightly packed with electronics
• Limited or no air flow
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Remote Condenser
Heat Pipes
Mounting Plate(s)
Heat
Heat
Vapor Chamber
Outer heat pipes will perform poorly as they are not directly beneath the heat source
Solution: Use vapor chamber alone or in combination with heat pipes (to move the heat)
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When Moving Heat to a Remote Sink
99% of All Applications Use Heat Pipes
• Complex shapes often required
• Easily bendable in any direction• Readily available in volume
• Will work against gravity ± 45°
Example #1 – Notebook computer
2 flattened heat pipes cool 3 heat sources
• With the right thermal modeling, heat sources can be daisy chained onto one device.
• Good example of heat pipe design flexibility.
When Spreading Heat to a Local Sink
Heat Pipes are a Good Choice If• Plenty of air flow• Lots of room for fins• Nominal power densities <25 w/cm2
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Example – Telecom Equipment Application
• Normal ambient
• Every penny counts!
• For moderate performance applications where spreading needs to be augmented, the use of several heat pipes embedded into the base may be sufficient
• The use of heat pipes in the base does not eliminate the conductive losses but will help to reduce them.
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One 3mm thick vapor chamber replaced two 8mm heat pipes. 6 degree C better performance and more even heat distribution across heat source surface
• Direct contact to the VC means one less interface and better spreading
• Flat design allows for additional fin area
• Vapor chamber ideal when several heat sources need to be isothermalized
When Spreading Heat to a Local Sink
Vapor Chambers may be the Best choice if
• Z direction (height) is limited
• Power densities are high
• High ambient or low air flow
• Every degree counts!
Example – Higher GPU power & density required design modification
6 oC Cooler
VS
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Three vapor chambers each 70mm W x 300mm L move heat to a common fin stack
• Vapor chambers were used in place of heat pipes to reduce conduction loss
When Moving Heat to Remote Sink
Example #1 – Three 450W RGB Laser Diodes for 3D Projector
Vapor Chambers are a Good Choice If
• Thermal performance is critical• Ultra-thin VC can allow for more fin area
Spreading & Moving – Some Oddballs
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Example #2 – Heat pipes and vapor chamber used in combination
Small form factor desktop PC cooling Core I7 chip
• VC replaced copper mounting plate
• 5 degree better performance than original design with reduced hot spots
Example #1 – Flattened / machined HPs are sometimes used to mimic a vapor chamberGaming desktop cooling overclocked processors
• Heat pipes make direct contact with the heat source, eliminating 2 interface layers
• If implemented correctly performance can be good, but cost rises quickly and heat spreading in X direction is still limited
X Direction Spreading Limited
Direct Contact Heat Pipes
Vapor Chamber
Bending & Shaping
2-Piece Vapor Chamber1-Piece Vapor ChamberHeat Pipe
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Heat Pipes• Bend radius 3X diameter of heat pipe. Each 45 degree bend
reduces Qmax by ~2.5%
• Flatten to 1/3 diameter of original pipe (typical)
• Machining if pipe wall thickness permits. Allows direct contact with heat source
1-Piece Vapor Chamber• 10mm bend radius along narrow plane
• Flattened to 1/10 – 1/20 diameter of original pipe (typical)• Typical thicknesses between 1-4mm
• Surface pedestals of 0.5-1.0mm high available for recessed heat sources
2-Piece Vapor Chamber• Stamped bend to 1x thickness of the sheet metal, typically
done as a ‘step’. Note – steep bends increase vapor pressure drop significantly
• Upper and lower plates are stamped flat.
• Stamped surface pedestals of 3-5mm high available for recessed heat sources
Sizing Two-Phase Devices (copper, sintered wick, water)
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* Horizontal Orientation **Thicker wick than 3-6mm examples
Challenge – Cool an 70W ASIC size 20x20mm with one-90o bend
Diameter 3mm 4mm 5mm 6mm 8mm**Max Power (Qmax watts)* 15 watts 22 watts 30 watts 38 watts 63 watts
Typical Flattening Height 2.0mm 2.0mm 2.0mm 2.0mm 2.5mm
Resulting Width 3.57mm 5.14mm 6.71mm 8.28mm 11.14mm
Flattened Max Power* 10.5 watts 18.0 watts 25.5 watts 33.0 watts 52.0 watts
Options Three 6mmRound HeatPipes
Two 8mmFlat HeatPipes
HP Width 20mm = 18mm + (2x1mm) baseplate gap
22.3mm = 2 x 11.14mm
Qmax per HP 38 watts 52 watts (flattened)
Qmax Total 114 watts 104 watts
Run at 75% Qmax 85.5 watts 78 watts
Less 5% Qmax to Account for 90o Bend
81 watts 74 watts
Conclusion: Both configurations will move the heat to the condenser
Most Used PCB Mounting Options
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Stamped & Clip Style Attachment
• Pros: $ low cost
• Cons: Low pressure- higher TIM resistance
• Best Use: small, lower power applications
Spring Loaded Screws
• Pros: Higher pressure for better TIM performance
• Cons: hardware can get pricy
• Best Use: Heavier heat sinks and/or higher shock and vibe requirements
Heat Exchanger Design (Fins)
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Condenser Type
Cost Typical Benefits Potential Drawbacks
Extruded
$ ∙ Readily available∙ Easy to manufacture to custom
specifications, including grooves for heat pipes
∙ Dimensions are limited∙ Fin height limited ~20x fin width∙ Base & fins are same material, usually
aluminum
Die Cast $ ∙ Net shape∙ Low weight∙ Easily customizable
∙ Lower thermal conductivity∙ Potential for porosity∙ Not generally used with heat pipes
Bonded $$ ∙ Large heat sink sizes∙ Base and fins can be of different
materials
∙ If fins are epoxied in place, increasing the thermal resistance
Skived $$ ∙ Fin and base from solid piece of metal, usually copper
∙ High density fins possible∙ More design flexibility than extrusion
∙ Base may be thicker than needed = higher weight
∙ Fins damage easily
Fin Pack & Zipper Fins
$$ ∙ Low-high fin density∙ Low weight∙ High design options, including center
mounted heat pipes
∙ Generally, for fins less than 1mm thick
Forged $$$ ∙ Fin design in many shapes (pin, square, oval, etc)
∙ Usually reserved for higher volume products as tooling is expensive
Machined $$$$ ∙ High thermal conductivity∙ Complicated designs OK
∙ None, other can cost.∙ Not good for high volume due to
production time
Assembly Attachment
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Direct contact VC epoxied to an
aluminum heat sink
Steel mounting plate soldered to a
vapor chamber
VC soldered to a machined forced
convection heat sink
Soldering 90% of the time
Thermal Epoxy 10% of the time
Direct copper to copper bonds or nickel plating over aluminum
Only for very large parts. Test to ensure minimal thermal impact
HBLED vapor chamber – natural
convection
Heat pipes soldered to nickel plated copper blocks
1.5mm VC memory module
Thermal Solution Design Process
Estimate the size and number of two-phase devices to spread and/or move heat
• Generally a good idea to spec heat pipes to 75% of their Qmax
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Estimate the heat sink size necessary to dissipate the heat• There’s a very simple equation that will get you close.
CFD Modeling to optimize design & understand performance factors in a more dynamic way
• Factors in air flow dynamics, radiation effect, conduction nuances, and unique component shapes – among others
Model in Excel to include more variables• Additional Typical variables used: fin variables (thickness, pitch,
height), TIM, base thermal conductivity
5Prototype solution and conduct system tests
• Real data – there’s no substitute!
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Heat Sink Volumetric Calculation Benefits: Quick & easy ballpark of required heat sink size
Drawbacks: Doesn’t account for fin or base variables, nor specific XYZ dimensions – only overall volume
Equation:
• V = (Q*Rv)/Delta T• V = heat sink volume based on outside dimensions
• Q = heat source power, Rv = volumetric thermal resistance
• Delta T = difference between T-case and max ambient temperature.
• Example Application: Q = 800W, Delta T = 40oC (80 Tcase - 40 Ambient)
• Moderate Forced Air Flow at 2.5m/s
• Rv based on the below table (well verified data)
• Estimate based on above: 2,300cm3 = (800*115)/40 = 140in3
• How close is this to reality? • Actual dimensions after further modeling are 120in3 – the volumetric
ballpark is within 15% of the actual
Air Flow (m/s) Rv (cm3 – C/W)Natural Convection 500-800
1 m/s (gentle air) 150-250
2.5 m/s (moderate air) 80-150
5 m/s (high air) 50-80
ApplicationFour 200 Watt Heat
Sources = 800W
2.5 m/s Air Flow
200W
200W
200W
200W
------------------------------12”--------------------------------
--------2.5”--------4.0”
Midpoint = 115
Excel Analysis
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• Using Excel provides a more granular level of understanding of both the heat sink system as well as a comparison between a solid aluminum base and a two-phase base
• The goal Delta-T total is 40oC with 100 CFM air flow
2-Phase Heat Sink Delta-T Analysis (Deg. C)
Aluminum Base Heat Sink Delta-T Analysis (Deg. C)
CFD Analysis and Prototyping
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CFD Benefits• Multivariate optimization allows focus on key design goals• Fully understand which variables most affect performance• Reduces the number of prototype iterations
CFD Potential Drawbacks• Software and experienced personnel are expensive• Output is only as good as the input (garbage in, garbage out)
Mento
r Gra
phics
FloTherm
Prototype Benefits
• Heat sink and system validation to account for un-modeled or incorrectly modeled CFD variables
• Test data can be used to refine future CFD models
Prototype Potential Drawbacks
• Cost and time
