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Heat Pipes & Vapor Chambers Design Guidelines

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Heat Pipes & Vapor Chambers Design Guidelines --- Heat Pipes & Vapor Chambers Design Guidelines 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 2 • Every penny counts! • Nominal power densities <25 w/cm 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 VS 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.
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