Heat Pipes & Vapor Chambers Design Guidelines

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Presenter

George Meyer

CEO, Celsia Inc. We’ll Answer these Questions

• Benefits and consequences of solid materials Do I Need 2-Phase? • Some basic rules of thumb

Are Vapor Chambers Just • Similarities: design, wicks & performance limits Flat Heat Pipes? • Differences: for moving or spreading heat

• Heat pipes: diameter, quantity, and shape What Size Do I Need? • Vapor chambers: sizing

• Attaching it to the condenser How Do I Integrate Them? • Mounting it to the heat source/ PCB

What Should the Heat • Types of condensers Exchanger Look Like? • Pros and cons

How Do I Model • Heat sink ballpark • CFD analysis Thermal Performance? • Excel Model • Proto testing

2 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.

Aluminum Copper Two Phase (baselined at 1X)

Base thk: 1X Base thk: 0.5X Base thk: 0.5X Weight: 1X Weight: 3X Weight: 2X Cost: 1X Cost: 1.6X Cost: 1.8X

Conduction Loss in the Base: Conduction Loss in the Base: Conduction Loss in the Base: 22o Celsius 17o Celsius. If base same thickness 4o Celsius as Aluminum then 11o C If conduction loss through the base is greater than 10o C, it’s a good sign that you could be conduction limited

3 2-Phase Rules of Thumb 2-phase devices are incredible heat conductors. • 5 to 50 times better conductivity than aluminum or copper using 1 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

Ideal when heat needs to be moved more than 30-50mm • Remote fin stacks (heat exchangers) are a perfect example 2

If you’re interested in spreading heat to reduce hot spot and/or attach to a local heat exchanger 3 • The ratio of heat spreader to heat source should be on the order of 20:1 greater area

As with any heat pipe heat sink design, size the heat pipe with an additional 25% thermal headroom 4

4 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

5 2-Phase Similarity: Wick Structures

Power Density Resistance Orientation 2 Sintered <500w/cm 0.15-0.03 Good for freeze/thaw and bent shapes +90o to -90o o 2 Powder Small heat sources up to 1,000 w/cm2 c/w/cm <30 w/cm2 Screen 0.25-0.15 Main use is for very thin heat sinks due to +90o to -5o high evaporator resistance. Limited oc/w/cm2 bending. Grooved <20w/cm2 0.35-0.22 o o Entry level price / performance must be o 2 +90 to 0 gravity aided/neutral. c/w/cm Evaporator Resistance (Typical Two Phase) Grooved +90o -90o

2 Screen C/w/cm o

Sintered Powder

Power Density w/cm2

6 2-Phase Device Similarity: Performance Limits

Capillary Vapor Sonic Entrainment Boiling Limit ‘the limiting factor’ Pressure Device above Cause Operating well Start-up power, designed power Input power Radial heat flux below design temp low temp combo input or at low exceeds design exceeds design temp Vapor flow Large internal Condenser flooded Capillary pump Problem Wick dries out prevented pressure drop with excess fluid breaks down

Increase vapor Increase vapor Solution Change working Modify wick Increase wick heat space or operating space or operating fluid structure flux capacity temperature temp Sonic Capillary limit is the ability of a Entrainment Boiling particular wick structure to provide Cap Limit adequate circulation for a given working fluid. It is usually the limiting factor for terrestrial applications.

7 Wick and Vapor Limits Are What Matter

Heat Pipe Vapor Chamber

• 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

8 2-Phase Device Similarity: Customization

Porous Wick “Standard” Wick

• 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 • 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

9 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.

10 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

11 2-Phase Differences: Overview Hybrid 1-Piece Traditional 2-Piece Heat Pipe Vapor Chamber Vapor Chamber

Initial Form Small diameter tube 3-10mm Very large diameter tube Upper and lower stamped Factor 20-75mm plates Shapes Round, flattened and/or bent Flattened rectangle, surface Complex shapes in x and y in any direction embossing & z-direction bendable direction, surface embossing Typical 3-8mm diameter or flattened 1.5-4mm thick, up to 100mm W 2.5-4mm thick, up to 100mm Dimensions to 1.5-2.5mm. Length 500mm+ by 400mm L W by 400mm L

Mounting to Indirect contact though base Direct contact. Mounting pressure Direct contact. Mounting Heat Source plate unless flat & machined up to 90 PSI pressure up to 90 PSI

Relative Cost Very cost effective, but Comparable to 2-4 heat pipes in More expensive than 1-piece increases quickly with large higher power and/or high heat design due to additional diameter, custom wick flux applications tooling cost and labor time, structure, secondary ops but large scale production closes the gap

12 2-Phase Differences: Moving or Spreading Heat?

While there’s no hard line of distinction between the two, think of the difference like this…

Moving Spreading

Local Remote Condenser Condenser

Heat Direct Mount Heat Pipes Vapor Chamber

Heat Mounting Plate(s)

• Linear heat flow • Multidirectional heat flow • Remote condenser, usually • Local condenser, almost always

13 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

Remote Condenser

Heat Pipes

Mounting Vapor Plate(s) Chamber Heat Heat

Outer heat pipes will perform poorly as they Solution: Use vapor chamber alone or in are not directly beneath the heat source combination with heat pipes (to move the heat)

14 When Moving Heat to a Remote Sink

99% of All Applications Use Heat Pipes

• Complex shapes often required • Readily available in volume • Easily bendable in any direction • 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.

15 When Spreading Heat to a Local Sink

Heat Pipes are a Good Choice If

• Plenty of air flow • Normal ambient • Lots of room for fins • • Nominal power densities <25 w/cm2 Every penny counts!

Example – Telecom Equipment Application

• 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.

16 When Spreading Heat to a Local Sink

Vapor Chambers may be the Best choice if

• Z direction (height) is limited • High ambient or low air flow • Power densities are high • Every degree counts!

Example – Higher GPU power & density required design modification 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

6 oC Cooler

17 When Moving Heat to Remote Sink

Vapor Chambers are a Good Choice If • Thermal performance is critical • Ultra-thin VC can allow for more fin area

Example #1 – Three 450W RGB Laser Diodes for 3D Projector 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

18 Spreading & Moving – Some Oddballs

Example #1 – Flattened / machined HPs are sometimes used to mimic a vapor chamber Gaming desktop cooling overclocked processors

• Heat pipes make direct contact with the heat Limited source, eliminating 2 interface layers Direct ContactSpreading Heat Pipes • If implemented correctly performance can be X Direction good, but cost rises quickly and heat spreading in X direction is still limited

Example #2 – Heat pipes and vapor chamber used in combination

Small form factor desktop PC cooling Core I7 chip Vapor • VC replaced copper mounting plate Chamber • 5 degree better performance than original design with reduced hot spots

19 Bending & Shaping

Heat Pipe 1-PieceHeat Pipes Vapor Chamber 2-Piece Vapor Chamber • 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

20 Sizing Two-Phase Devices (copper, sintered wick, water)

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

* Horizontal Orientation **Thicker wick than 3-6mm examples

Challenge – Cool an 70W ASIC size 20x20mm with one-90o bend

Options Three 6mm Two 8mm Round Heat Flat Heat Pipes Pipes HP Width 20mm = 18mm + (2x1mm) 22.3mm = 2 x 11.14mm baseplate gap 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 81 watts 74 watts Account for 90o Bend

21 Conclusion: Both configurations will move the heat to the condenser Most Used PCB Mounting Options

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

22 Heat Exchanger Design (Fins)

Condenser Cost Typical Benefits Potential Drawbacks Type Extruded $ ∙ Readily available ∙ Dimensions are limited ∙ Easy to manufacture to custom ∙ Fin height limited ~20x fin width specifications, including grooves for ∙ Base & fins are same material, usually heat pipes aluminum Die Cast $ ∙ Net shape ∙ Lower thermal conductivity ∙ Low weight ∙ Potential for porosity ∙ Easily customizable ∙ Not generally used with heat pipes Bonded $$ ∙ Large heat sink sizes ∙ If fins are epoxied in place, increasing ∙ Base and fins can be of different the thermal resistance materials Skived $$ ∙ Fin and base from solid piece of ∙ Base may be thicker than needed = metal, usually copper higher weight ∙ High density fins possible ∙ Fins damage easily ∙ More design flexibility than extrusion Fin Pack & $$ ∙ Low-high fin density ∙ Generally, for fins less than 1mm Zipper Fins ∙ Low weight thick ∙ High design options, including center mounted heat pipes Forged $$$ ∙ Fin design in many shapes (pin, ∙ Usually reserved for higher volume square, oval, etc) products as tooling is expensive Machined $$$$ ∙ High thermal conductivity ∙ None, other can cost. ∙ Complicated designs OK ∙ Not good for high volume due to production time 23 Assembly Attachment

Soldering Thermal Epoxy 90% of the time 10% of the time Direct copper to copper bonds or Only for very large parts. Test to nickel plating over aluminum ensure minimal thermal impact

Direct contact VC VC soldered to a epoxied to an machined forced aluminum heat sink convection heat sink

Steel mounting HBLED vapor plate soldered to a chamber – natural vapor chamber convection

Heat pipes soldered 1.5mm VC memory to nickel plated module copper blocks

24 Thermal Solution Design Process Estimate the size and number of two-phase devices to spread and/or move heat 1 • Generally a good idea to spec heat pipes to 75% of their Qmax Estimate the heat sink size necessary to dissipate the heat • There’s a very simple equation that will get you close. 2

Model in Excel to include more variables • Additional Typical variables used: fin variables (thickness, pitch,

3 height), TIM, base thermal conductivity

CFD Modeling to optimize design & understand performance factors in a more dynamic way 4 • Factors in air flow dynamics, radiation effect, conduction nuances, and unique component shapes – among others

Prototype solution and conduct system tests • Real data – there’s no substitute! 5 25 Heat Sink Volumetric Calculation Benefits: Quick & easy ballpark of required heat sink size Application Drawbacks: Doesn’t account for fin or base variables, nor specific XYZ Four 200 Watt Heat dimensions – only overall volume Sources = 800W

2.5 m/s Air Flow Equation:

• V = (Q*Rv)/Delta T

------12”------• V = heat sink volume based on outside dimensions 200W • Q = heat source power, Rv = volumetric thermal resistance • Delta T = difference between T-case and max ambient temperature. o • Example Application: Q = 800W, Delta T = 40 C (80 Tcase - 40 Ambient) 200W • Moderate Forced Air Flow at 2.5m/s • Rv based on the below table (well verified data)

3 Air Flow (m/s) Rv (cm – C/W) 200W Natural Convection 500-800 1 m/s (gentle air) 150-250 2.5 m/s (moderate air) 80-150 Midpoint = 115 5 m/s (high air) 50-80 • Estimate based on above: 2,300cm3 = (800*115)/40 = 140in3 200W • How close is this to reality? 4.0” • 3 Actual dimensions after further modeling are 120in – the volumetric ------2.5”------ballpark is within 15% of the actual

26 Excel Analysis

• 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

Aluminum Base Heat Sink Delta-T Analysis (Deg. C)

2-Phase Heat Sink Delta-T Analysis (Deg. C)

27 CFD Analysis and Prototyping

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)

MentorFloTherm Graphics

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

28 Q&A --- Thanks for Attending!