How Do Heat Pipes Work | Heat Pipes 101

How Do Heat Pipes Work | Heat Pipes 101

by | Dec 8, 2019 | Heat Pipe

How Do Heat Pipes Work | Heat Pipes 101

 

This article covers how heat pipes and vapor chambers work along with typical uses and configuration options.  Further, it is designed to be a quick read with links to detailed information throughout the text.

 

How Do Heat Pipes Work?

There are three ‘parts’ to a heat pipe that allow it to work: a vacuum sealed enclosure, a wicking structure, and a working fluid. By a large margin, the most common type is a copper enclosure, a sintered copper wick structure that bonds to the interior surface, and de-ionized water as the working fluid. This configuration generally translates to non-space environments with required Max Ambient of less than 80 oC and will be the configuration presented throughout this article.

The graphic below illustrates heat pipe working principles. As heat is applied, some of the liquid turns to vapor and travels to an area of lower pressure toward the cooling fins. This allows the vapor to cool and return to liquid form where it is absorbed by the porous wick structure and transported back to the heat source via capillary action – the same principle that will soak the entirety of a paper towel if only one corner is exposed to water.

 

Heat Pipe Working Principles

 

Heat pipes are typically available in sizes from 2-12mm diameter and can be flattened and bent. Moreover, wick properties such as thickness and porosity can be changed to tune thermal performance (Qmax or maximum power carrying capacity in watts). Click here to use the online heat pipe calculator to understand Qmax by pipe size and angle of orientation. A few points:

  • Larger diameter heat pipes have higher Qmax.
  • Qmax is additive. If one pipe can carry 20W, two can carry 40W and so on.
  • Qmax is reduced when the heat pipe is bent, the capillary action is against gravity, required operating altitude above sea level increases, and often when the pipe is flattened (a small amount of flattening typically won’t affect it).

The working principles of vapor chambers are identical to heat pipes. In fact, vapor chambers are often referred to as planar heat pipes. The distinction really comes down to the width to height aspect ratio. A flattened heat pipe typically won’t exceed 4:1, whereas a vapor chamber can go up to around 60:1.

 

Importance of Heat Pipe Technology

You already know that heat pipes and vapor chambers are two-phase heat transfer devices used to increase the thermal performance of heat sinks that would otherwise use only a solid metal base and fins. But, what drove their mass adoption?

Put simply, heat pipes are widely used because modern electronic components have increased in thermal design power (watts of dissipated heat) and, perhaps more importantly, power density (W/cm2). With these increases, engineers realized they needed to reduce the conduction limits of solid metal. Vapor chambers and heat pipes have, in most cases, dramatically higher thermal conductivities than do solid aluminum or copper. For reference, the thermal conductivity of aluminum is ~200 W/(mK), copper is ~400 W/(mK), and two-phase devices are typically upwards of 6,000 W/(mK) – often dramatically higher.

Unlike solid metal, the effective thermal conductivity of two-phase devices changes depending on a host of variables, but mainly with the distance the heat is transferred. The longer the distance, within reason, the higher the thermal conductivity – all else being held constant. See online heat pipe performance calculator for the precise heat pipe thermal conductivity for your application. The chart below illustrates how quickly thermal conductivity increases with heat pipe length.

 

Heat Pipe Effective Thermal Conductivity

Heat Pipe Effective Thermal Conductivity as Function of Length

 

 

Typical Configuration & Uses

Rules of Thumb

  • Use vapor chambers to spread heat across the base of a local fin array (condenser).
  • Use heat pipes to move heat to a remote fin array or enclosure wall.

 

Typical usage scenario for vapor chamber and heat pipes

Vapor Chambers Spread Heat | Heat Pipes Move Heat

 

There are always exceptions, but here are the reasons. Heat pipes can be bent in any direction making them ideal for snaking around PCB components. This makes them well suited for moving heat to a remote condenser which most often requires some maneuvering. Conversely, vapor chambers have a continuous internal vapor space. This allows heat to be distributed in every direction to the remote corners and edges of the fin array, maximizing total fin efficiency.

 

Telltale Signs You Might Need a Heat Pipe or Vapor Chamber Device

Here’s a list of conditions where two-phase devices might be considered:

  • Having to move heat more than 50mm from heat source to remote condenser. Below this, solid copper bar or rods will be almost as effective.
  • When the bottom area (base) of a local fin array is greater than 10X the area of the heat source. Remember, lower air flow means larger fin area for a given heat source. This often translates to a larger base footprint as you might not have the vertical space (Z-height) and you certainly won’t have the fin efficiency to increase fin height indefinitely. See our online heat sink size calculator to get a quick estimate of the required heat sink size for your application.
  • If a solid copper heat sink (fins and base) meets thermal requirements, but not weight/shock and vibe requirements. A solid copper base is considerably heavier than a comparable vapor chamber base. Additionally, using a two-phase base may allow the use of aluminum fins, further reducing weight.
  • When thermal budgets are below 40 oC, especially when that’s coupled with low/no airflow. To calculate thermal budget, subtract the max operating temperature at which the finished device is designed to operate (Max Ambient) from the max case temperature (Tcase) of the IC – or junction temperature for bare die ICs (Tjunction). This second figure will be provided by the IC manufacturer.  You can use our online heat sink calculator to determine the total delta-T of your heat sink and compare that to your thermal budget.

 

Types of Heat Sinks Used with Two-Phase Devices

Lower Unit Cost – Extruded heat sinks are the most cost effective but have limited design flexibility. Die cast heat sinks are generally used as the enclosure lid with the fins exposed to the environment but high up-front tooling cost limits these to higher-volume applications.

Unique Fin Requirements – Thermal engineers sometimes need heat sinks with either very tall fins or very thin fins that are tightly spaced. Respectively, bonded fin heat sinks and skived fin heat sinks suit these requirements nicely. An advantage of bonded fin designs is that the heat sink base and the fins can be of different metals.

Most Used – What you’ll see most often paired with heat pipe or vapor chamber designs are zipper fins (also called fin packs). They are low weight and can achieve a very high fin density. Vapor chambers can be soldered to the bottom of these or heat pipes can be run through the center of the fins.  For low volume, very complicated designs where performance matters, machined heat sinks are the usual solution.

 

Types of Heat Sinks Used with Vapor Chambers & Heat Pipes

 

Related Links

Heat Pipe Design Guide

Heat Pipe Design Guide

by | Nov 24, 2019 | Heat Pipe

Heat Pipe Design Guide

 

The focus of this heat pipe design guide is on sintered copper heat pipes (w/water) for electronics cooling applications. This typically translates to dissipated heat of between 20-200 watts (less if power density is high) and power density up to around 25 W/cm2. 

This heat pipe design guide will cover the following topics:

  1. Typical Uses of Heat Pipes
  2. Heat Pipe Specifications & Tolerances
  3. Heat Pipe Performance: Sintered Copper Wick & Heat Pipe Carrying Capacity
  4. Secondary Operations Performed on Heat Pipe Designs
  5. Heat Pipe Selection Example
  6. Heat Pipe Design Guidance for Heat Sink Integration
  7. Heat Pipe Modeling Tips

 

Typical Uses of Heat Pipes

Used properly, and under the right conditions, heat pipes dramatically improve heat sink performance. This design reality is due to the very high thermal conductivity of heat pipes; generally between 10-100 times that of solid copper. Unlike solid metal, heat pipe thermal conductivity changes with several variables – length being the most notable. Consequently, very short heat pipes of 50mm or less have thermal properties that might be better served by using solid copper or aluminum. Here are the most common usage configurations for heat pipes as part of a heat sink assembly:

Moving Heat to a Remote Heat Sink

Heat pipes are used to move heat, in any direction or orientation, from the heat source (evaporator) to the heat sink (condenser). Pictured below are a couple of examples.

 

Heat Pipes Used to Move Heat to a Remote Heat Sink

 

 

Spreading Heat to a Local Heat Sink

When a two-phase device is needed yet cost is a driving factor, heat pipes can be used to spread heat to a local heat sink. A vapor chamber in either of these two applications will reduce the total heat sink delta-T by 4-9 oC . The improvement is due to the lower thermal resistance of a vapor chamber as well as the way it interfaces with the heat source (direct contact). Note that both these examples use a solid copper spreader that attaches to the heat source, then heat moves to the heat pipes (indirect contact).

 

Flat Heat Pipes Used to Spread Heat to a Local Heat Sink

 

Heat Pipe Specifications & Tolerances

The theoretical operating temperature limits of sintered copper water heat pipes are 0-250 oC, although in practice heat pipes don’t really start to operate until around 20 oC. Below 0 oC, the water freezes within the sintered wick structure but causes no damage due to expansion as the amount of liquid is so small. For example, a typical 6mm heat pipe that’s 150mm in length contains about 1cc of water.

A quick note on heat pipe reliability. Heat pipes have been extensively tested for decades. Their typical lifespan is at least 20 years and can go through thousands of freeze-thaw cycles without damage. Heat pipe failure is most likely to occur A) due to poor manufacturing processes and B) as a result of exposure to unplanned conditions: corrosive substances and unintended physical damage are the most common. Celsia mitigates the first cause of failure by helium testing every heat pipe for leakage and Qmax performance. The second cause of failure can be addressed by nickel plating the heat pipe.

 

List of tests performed on heat pipes and heat sink assemblies

Celsia Heat Pipe & Heat Sink Testing

 

The below chart provides heat pipe specifications and tolerances. Please contact us with any additional questions.

 

Heat Pipe Specifications & Tolerances

 

Heat Pipe Performance

Heat pipe carrying capacity (Qmax) is a measure of the amount of heat in watts the device can carry. It is governed, mainly, by the capillary limit of the sintered wick material whose performance can be modified by varying the thickness and/or the porosity/permeability of the wick. However, there is no one ideal wick design. It changes depending on the application requirements.

Celsia’s online Heat Pipe Calculator provides performance information based on two wick designs: standard and performance. However, we regularly design custom wick structures to precisely match customer requirements. These include the ability to vary the wick structure from one portion of the heat pipe to the next. Please contact us if you require performance figures not presented here.

The charts below show output from the heat pipe calculator using the following user-selected parameters:

  • Heat Pipe Length: 200mm
  • Evaporator Length: 25mm
  • Condenser Length: 75mm
  • Wick Type: Standard
  • Operating Temperature: 60 oC

The first chart shows heat pipe carrying capacity (Qmax) vs Angle of Operation. At +90 degrees the evaporator is directly below the condenser, at –90 the reverse is true.

 

Chart showing how heat pipe carrying capacity decreases as the heat pipe is required to work against gravity

Heat Pipe Carrying Capacity is Reduced When Required to Work Against Gravity

 

While this chart shows a nearly 90% drop-off in Qmax from +90 to -90 (standard wick), the accompanying table (not shown) gives the precise Qmax by angle. For instance, if the application requires the heat pipe to operate a no less than horizontal (0 degrees), an 8mm heat pipe will carry 62 watts of power from the heat source given the input parameters shown earlier.

The next chart (not shown) and associated table (shown) in the calculator has to do with the change in temperature (delta-T) from one end of the heat pipe to the other. This measurement is not actual length but effective length which is the heat pipe distance from the midpoint of the evaporator to the midpoint of the condenser.

Heat Pipe Thermal Resistance Calculation

Chart Used in Calculating Heat Pipe Thermal Resistance

 

To calculate heat pipe thermal resistance, divide its delta-T by the power input. By choosing an  8 mm heat pipe with input power of 40 oC the thermal resistance is 4.3/40 = 0.11 oC/W. Additionally, the heat pipe calculator provides the thermal conductivity for use as a necessary input to CFD programs like FloTherm. Visit this link for additional information on how to use the heat pipe calculator.

 

Secondary Operations to Heat Pipe Designs

Before heat pipes are integrated into a heat sink, engineers have several secondary operations to choose from.

Flattening the Heat Pipe

Typically, sintered copper heat pipes can be flattened to a maximum of between 30% to 65% of their original diameter.  However, the heat pipe carrying capacity is often negatively affected. The table below shows the Qmax for the most common heat pipe sizes which are round vs flattened.  For example, a 3mm heat pipe fattened to 2mm will have heat carrying capacity 30% less even though the pipe has only been flattened by 33%. Compare that to a 6mm heat pipe flattened to 2mm. Its Qmax is reduced by 13% even though it’s 66% flatter.

 

Flat Heat Pipe Power Carrying Capacity

* Horizontal Orientation

** Thicker Wall and Wick Structure

Why does flattening smaller heat pipes have more of a negative effect on Qmax?  Simply put, there are two heat pipe performance limits important for terrestrial applications: the wick limit and the vapor limit.  The wick limit is the ability of the wick to transport water from the condenser back to the evaporator.  As mentioned, the porosity and thickness of the wick can be tuned to specific applications, allowing for changes to Qmax and/or ability to work against gravity.  The vapor limit for a particular application is driven by how much space is available for the vapor to move from the evaporator to the condenser.  It is the lower of these two limits, for heat pipes that have been designed to meet application requirements, that determines Qmax.

 

Heat Pipe QMax is the Lesser of the Wick and Vapor Limits

 

The above chart illustrates this dynamic. The round 3mm heat pipe (blue and orange lines) has vapor and wick limits that are almost identical. Flatting it to 2mm results in the vapor limit below the wick limit. For a 6mm round heat pipe, there is a lot of excess vapor limit so the Qmax won’t be diminished until the pipe is reduced considerably.

Bending Heat Pipes

Bending the heat pipe will also affect the maximum power handling capacity, for which the following rules of thumb should be kept in mind.

  • First, minimum bend radius is three times the diameter of the heat pipe.
  • Second, every 45 degree bend will reduce Qmax by about 2.5%.  From Table 1, an 8 mm heat pipe, when flattened to 2.5mm, has a Qmax of 52 W.  Bending it 90 degrees would result in a further 5% reduction.  The new Qmax would be 52 – 2.55 = 49.45 W.
 
Heat Pipe Plating

Nickel plating heat pipes is done to protect against corrosion in situations where the parts are exposed to the environment. It can also be done purely for aesthetic reasons.

Heat Pipe Selection Example

Assume a 20 x 20mm heat source dissipating 70 watts of power with a single 90 degree bend – what are the appropriate heat pipe options?

 

Example: Choosing the Correct Size Heat Pipes

 

  1. To ensure each heat pipe receives the same amount of heat, place them directly above the heat source, or very nearly so. This can be done with three round 6mm heat pipes or two flattened 8mm heat pipes (flattened to 2.5mm).
  2. Make sure each pipe can handle the heat load of 70 watts. Three 6mm heat pipes can carry 38 watts each = 114 watts, while the two 8mm flat can carry a total of 104 watts.
  3. Derate the heat pipe carrying capacity by 25% (good design practice). The derated 6mm option can carry 85.5 total watts what the 8mm option can carry 78 watts.
  4. Account for bending by derating 2.5% fo the 45 degree bend. Here we have a 90 degree bend so the two options can carry 81W and 74W respectively.

As can be seen from this analysis, both heat pipe configurations are adequate to transport heat from the evaporator to the condenser.  So why choose one over the other?  From a mechanical perspective it may simply come down to heat sink stack height at the evaporator, i.e. the 8 mm configuration has a lower profile than does the 6mm configuration.  Conversely, condenser efficiency may be improved by having heat input in three locations versus two locations, necessitating the use of the 6 mm configuration.

 

Heat Pipe Design Guidance for Heat Sink Integration

Once the correct heat pipe(s) have been identified the next step is integration into the heat sink. When heat pipes are used to move heat (vs spread heat) this is a two-step process: heat sink integration at the evaporator and heat sink integration at the condenser.

Interface Between Heat Pipe and Heat Source (Evaporator)

There are two often used methods for connecting heat pipes with the evaporator: indirect and direct.

 

Heat Pipe CPU Interface | Indirect vs Direct

 

The more cost effective method of mating heat pipes to a heat source is usually through a base plate. This can be done with either an aluminum or copper plate (shown on left). In addition to cost advantages, this method also allows heat to be distributed more evenly to each heat pipe in situations where the heat source is much smaller that the heat pipe contact area.

Direct interface from the evaporator to the heat pipes is usually reserved for situations where the base plate and associated additional TIM layer needs to be removed for performance reasons as shown in the image to the left. This comes with cost implications as the face of the heat pipes needs to be machined in order to make the needed thermal connection with the heat source.

 

Interface Between Heat Pipe and Fin Stack (Condenser)

The last step is properly integrating the heat pipe(s) into the condenser portion of the heat sink. It a situation where heat pipes are being used to spread heat to a local heat sink (image below left) flattened heat pipes are soldered to the base of the heat sink

 

Heat Pipe Soldered to Heat Sink Base | Attached Through FIns

 

 

When moving heat to a remote condenser, there are two common heat pipe mounting configurations. The first is identical to the method above. Namely, flattened heat pipes are soldered to a flat base or round heat pipes soldered to a grooved base.  If the fin stack is large, heat will need to be distributed more evenly by running the heat pipes through the center of the fin stack as seen in the above right image.

 

Heat Pipe Modeling Tips

When working in a CFD program like FloTherm or developing an Excel model, there comes a point where you need to input the heat pipe effective thermal conductivity. Here’s how to find these figures using our Heat Pipe Calculator. After entering the required inputs, the first table of the Calculator provided heat pipe effective thermal conductivity figures.

When early in the modeling cycle, there’s a fairly good way to cheat if you don’t have access to this calculator. Simply multiply the power input into each heat pipe by an estimate of its thermal resistance – this will give you the estimated heat pipe delta-T. For heat pipes from 3-8mm use 0.1 oC/W  or 0.075 oC/W for larger ones. Then enter in a thermal conductivity figure (start at 4,000 W/m-K and go up) until the modeled delta-T equals the roughly calculated detla-T.

 

Contact Us with Questions or To Receive a Quote

 

Related Links

Heat Pipes & Vapor Chambers – What’s the Difference?

Heat Pipes & Vapor Chambers – What’s the Difference?

by | Oct 27, 2019 | Heat Pipes & Vapor Chambers

 

Most thermal engineers are going to need a two-phase heat sink using either heat pipes or vapor chambers on numerous projects with which they are involved.  Reasons include: 

  • Keep out zones that prohibit a larger heat sink (thicker base, added fin area, etc.) 
  • Enclosure size and/or airflow can’t be increased. 
  • Transitioning to a solid copper heat sink, in whole or in part, adds too much weight and in some cases too much cost. 
  • Component power /density necessitates heat be moved to a remote location more than 40-50mm away from the heat source. 

In today’s blog we’d like to do a topline overview of structural differences and thermal design considerations between these very similar yet somewhat unique two-phase devices. If you need some background on the working principles of two-phase devices, please see our article “How do Heat Pipes Work”. 

Structural Design & Cost

Heat pipes and vapor chambers are composed of the same materials. The most combination is a copper enclosure with a sintered copper wick material. The devices have a small amount of water added and are evacuated to create a partial vacuum. Vapor chambers have an additional support structure (spacer) added for structural integrity and to allow vapor to flow freely. 

Difference between heat pipes and vapor chambers

Unlike heat pipes, which have a width to height aspect ratio on the order of 4:1, vapor chambers can have aspect ratios of 60:1 or more. Additionally, vapor chambers can be divided into two categories: a traditional vapor chamber with two stamped plates forming the upper and lower enclosure and a lower cost hybrid 1-piece vapor chamber that begins the manufacturing process as a very large heat pipe. 

Thermal Design Considerations

Heat Pipes

For decades, heat pipes have been the default two-phase device of choice for thermal engineers due largely to the cost delta relative to vapor chambers. They were used both for heat transport, for which they still have an advantage, and for heat spreading, typically using multiple pipes near one another. For lower power applications, perhaps requiring only a single, small heat pipe, or those where heat must be effectively transported, heat pipes keep dominance over vapor chambers due to their low cost and design flexibility. 

Heat pipes cooling notebook pc

Heat Pipes Cooling Notebook Computer (Wikipedia)

 

Greater total power and power densities eventually led to heat sinks using multiple heat pipes to solve the thermal challenge. Both images below are heat pipe heat sinks for a small form factor, high performance desktop PC. The one on the left uses a copper base plate in between the heat source and the heat pipes, as is common with heat pipe applications (indirect contact). As processor heat increased in the later generation of this product, the company encountered thermal issues, but did not want to radically redesign the thermal solution as can be seen on the image to the right. Here a vapor chamber replaced the copper base plate, spreading heat more evenly across the heat source and transferring it more effectively to the heat pipes. This is a notable example of how both types of two-phase devices can be used together. 

Heatsink using heat pipes and vapor chamber

Heat Pipe Heat Sink – (L) Using Solid Copper Baseplate, (R) Using Vapor Chamber (ixbtlabs.com & Celsia)

 

A potential alternative to this problem might have been to implement ‘direct contact’ heat pipes. But this solution has its drawbacks as well. As seen below, this design choice uses slightly flattened and machined heat pipes embedded in an aluminum mounting bracket to make direct contact with the heat source. While eliminating the base plate and added TIM layer – decreasing thermal resistance – it doesn’t spread the heat as effectively as a vapor chamber heat sink solution. 

Heat sink with flat heat pipes

Direct Contact Heat Pipes (Silverstonetek)

 

Traditional (2-Piece) Vapor Chamber

Most manufacturers of vapor chambers use a traditional two-piece design.  Studies and practical applications show that the performance of heat sinks using vapor chambers can be enhanced by 20-30% over their heat pipe counterparts. A two-piece vapor chamber design has cost implications of roughly the same magnitude versus a multiple heat pipe configuration. Nonetheless, vapor chamber usage has grown with increasing total powers, higher power densities, and space constraints of today’s devices. 

Because vapor chambers do an incredible job of spreading heat, allow for low-profile heat sinks, can be made into virtually any shape, embossed, and make direct contact with one or multiple heat sources, these devices are used in a wide variety of higher power applications. Below are two examples of heat sinks using two-piece vapor chambers. 

vapor chamber heat sinks

Traditional Two-Piece Vapor Chamber (anandtech.com & Celsia)

As mentioned earlier, the increased cost of this design sometimes limits its incorporation into thermal solutions. Another potential drawback is that there’s little design flexibility in the z-direction. Making a U-shape for instance, while conceivably possible, would be impractical from a manufacturability/cost perspective. 

Hybrid (1-Piece) Vapor Chamber

One-piece vapor chambers are a cost-reduced version of their two-piece counterparts yet keep the thermal performance characteristics while adding some unique capabilities (e.g., U-shape bending). Like heat pipes, a one-piece vapor chamber begins its life as a single copper tube, hence the 1-piece moniker. Like traditional two-piece vapor chambers, one-piece vapor chambers make direct contact with the heat source, have a multi-directional heat flow, and can support clamping forces of up to 90 PSI. But they’re less expensive to produce because they need less tooling, don’t use individual support posts, and don’t have to be welded on all four sides. Below are a few examples of one-piece vapor chambers. 

Inexpensive vapor chamber heat sinks

Hybrid One-Piece Vapor Chambers

 

The thing to remember about two-phase devices is that heat pipes favor moving heat over spreading it, while the reverse is true of vapor chambers. There are many thermal challenges where either could be used with satisfactory results so it’s important to do a thorough review process of both designs before settling on one. 

Please contact us, if you’d like to learn more about how Celsia can help with your next heat sink project. We’ve worked on everything from consumer devices to industrial test equipment that require heat sinks to cool anywhere from a few watts to a few kilowatts.

Vapor Chamber Cooling Design Guide

Vapor Chamber Cooling Design Guide

by | Oct 6, 2019 | Vapor Chamber

Vapor Chamber Cooling Design Guide

 

Electronics cooling using a vapor chamber is a fairly common design choice. This vapor chamber design guide is for the most prevalent types of applications: CPU/ASIC to amplifier applications with power ranging from around 20-250 watts, power density greater than 20 W/cm2, and heat source sizes of between 10-30mm square.  The focus is on vapor chamber cooling using a copper envelope with sintered copper wick and water as the working fluid. The following topics are covered in this guide.

  1. Vapor Chamber Cooling Design Parameters
  2. Vapor Chamber vs Heat Pipe
  3. Types of Vapor Chamber Design
  4. Vapor Chamber Usage Guide
  5. Vapor Chamber Thermal Conductivity & Performance
  6. Vapor Chamber Heat Sink Integration
  7. Dimensional Design Limits of Vapor Chambers

 

Vapor Chamber Cooling | Design Parameters

Cooling electronics using vapor chambers are subject to the following guidelines:

Power Handling Capacity

Vapor chambers can have the same power handling capacity as multiple heat pipes; from a few watts to over a kilowatt. However, if one heat pipe can meet thermal and physical requirements, it’s probably cheaper to use them – depending on post-production operations like machining. That’s why a move to vapor chambers from heat pipes usually involves applications with higher power and/or higher power densities. Anything less and heat pipes may suffice.

Power Density Capacity

Vapor chambers are particularly well suited for electronics cooling applications where power density is high – roughly above 20 W/cm2 yet below 500 W/cm2. In these situations, it’s usually critical that heat is spread quickly to a larger surface area.

Shapes & Dimensions

The traditional method for producing vapor chambers begins with two stamped plates, mirror images of each other, that eventually get diffusion bonded together. This gives the designer enormous leeway in the X and Y dimensions. Length and width max dimensions are governed by press and furnace size as well as application requirements. Consequently, you typically don’t find vapor chambers in excess of around 300 x 400mm.

Traditional Vapor Chamber | 1-Piece Bendable Vapor Chamber

 

A few manufacturers also have the capability to produce vapor chambers that start as a very large copper tube (25-70 mm diameter) which is sintered, flattened, and has an internal support structure added to it. We call these 1-piece vapor chambers. The main advantages are cost and the ability to be shaped into L and U configuration. The drawback is they can only be produced in rectangular shapes. Dimensional limits due to manufacturing capability for these typically range in the 100mm wide to 300mm length.

Both types of vapor chambers, particularly when designed with a sintered wick structure, are between 2.5-4mm thick depending on the power to be moved or spread.

 

Bending

Two-piece vapor chambers made of two stamped plates are generally not bent post stamping. Any small ‘steps’ or bends are done as part of the stamping process. However, one-piece vapor chambers that start as a tube are bend post-production in the factory.  While band radius changes somewhat depending on vapor chamber width, thickness and location of the bend, a typical bend radius is on the order of 7mm for smaller vapor chambers to 12mm for large ones. For more information, see the last section of this article: vapor chamber dimensional design limits.

 

Vapor Chamber Shapes

 

Surface Flatness

Vapor chamber surface flatness is particularly important because, unlike heat pipes, they are designed to make direct contact with the heat source. Flatness is controlled in the component contact areas to a nominal flatness is .002”/1” but post-machining, while adding cost, can bring this down to 0.001”/1”. This is typically only necessary when mating to higher power density components with similar flatness for very thin bond line thickness and low interface resistances.

 

Machined Vapor Chamber

 

Resistance to Heat Loads

Without modifications, vapor chambers can withstand deformation to around 110 oC. For a copper water vapor chamber to handle higher temperatures, the wall thickness needs to be increased, additional internal support structures are added, and/or an exoskeleton (metal plate) is used on one side of the vapor chamber (the other side is supported by the base of the heat sink). For comparison, heat pipes with their inherently stronger geometry can handle upwards to 200 oC.

 

Warped Vapor Chamber Caused by Excess Heat Load

 

Clamping Pressure

Vapor chambers are hollow and require internal support to withstand clamping pressures. Standard designs use supports for up to 60psi of pressure before becoming deformed. However, they can be altered to support up to 90psi.

Surface Treatment

All copper parts are passivated to protect against short term discoloration. Nickel plating is the most common coating used for both heat pipes and vapor chambers for corrosion protection or cosmetic reasons.

 

Vapor Chamber vs Heat Pipe

Vapor chambers differ from heat pipes in several ways. First, they are more isothermal than either solid metal or heat pipe based solutions. This allows a more uniform temperature across the die face (reduced hot spots) as well a more uniform temperature across the entire face of the vapor chamber (lower delta-T).

 

Benefits of using a vapor chamber vs heat pipes

Advantages of Vapor Chamber vs Heat Pipe

Second, heat sinks using a vapor chamber allow direct contact between the heat source and the device, reducing interface thermal resistance. Heat pipe solutions usually require an additional base plate and TIM layer.

Third, height constrained thermal solutions often benefit from vapor chambers because they a) make for a thinner base to which the fin stack is attached and/or b) allow for more fin area as heat pipes typically go through the center of the fin stack.

 

Types of Vapor Chambers

While everyone is familiar with a traditional vapor chamber that’s made from two stamped pieces of metal (2-piece design), there’s another method for producing these devices that offers some unique advantages.

For shapes other than a rectangle, a 2-piece vapor chamber is needed because the stamped plates can be created in virtually any shape along the XY planes. Additionally, they’re able to have a higher embossment should the heat source be recessed. Unfortunately, they come at a slight cost premium over a 1-piece and cannot be bent post-production

 

Pros and Cons of Traditional Vapor Chamber

 

A handful of manufacturers are now producing a 1-piece vapor chamber – so named because it begins life as a very large single copper tube which is flattened and has a corrugated spacer inserted for structural purposes. While its shape is limited to a rectangle, it can be bend in the Z-direction forming steps, L-shapes or U-shapes.

 

Pros & Cons of 1-Piece Vapor Chambers

 

Vapor Chamber Usage Guidelines

Use a vapor chamber when the heat sink design is conduction limited and here are a few simple rules, followed by some links to online calculators, that will help determine if a vapor chamber is a good solution. Here are some simple rules of thumb to remember:

Use Vapor Chambers When the Thermal Budget is Tight

The thermal budget is simply the maximum ambient temperature at which the end product will operate minus the maximum temperature of the component Tcase. For many outdoor or rugged applications, thermal budgets can be well below 40oC.

Vapor chambers should be used when the thermal budget is tight

Sum of the Delta-Ts Must be Below the Thermal Budget

That means that the sum of all individual delta-Ts (from TIM to Air) must be lower than the calculated thermal budget.  For typical applications in this category, we generally need the delta-T of the heat sink base to be 10oC or less. Visit our online calculator to see the difference in heat sink delta-Ts for your application.

Use Celsia’s online heat sink calculators to help determine if a vapor chamber should be used in place of an aluminum or copper base.

 

When the Ratio of Vapor Chamber to Evaporator Area is >10:1

Like heat pipes, vapor chamber thermal conductivity increases with length. This means that a vapor chamber the same size as the heat source will offer little advantage over a solid piece of copper. A good rule of thumb says that the area of the vapor chamber should be equal to or greater than 10X the area of the heat source.  In situations where the thermal budget is large or when a lot of airflow drives a small fin stack this may not be an issue. However, it’s often the case that the base of the sink needs to be considerably larger than the heat source.

As this Ratio is Reduced, Solid Copper Becomes an Option

 

Use a Vapor Chamber When the Primary Goal is to Spread Heat

While vapor chambers can sometimes be used to move heat to a remote heat sink, we most often see vapor chambers used to spread heat to a local heat sink. Heat pipes are ideal for connecting the heat source to a remote fin stack especially as this often involves a series of twists and turns.

 

Typical usage scenario for vapor chamber and heat pipes

Vapor Chambers Spread Heat | Heat Pipes Move Heat

 

Vapor Chamber Thermal Conductivity & Performance

When looking at the effective thermal conductivities of heat pipes and vapor chambers it appears that vapor chambers have lower thermal resistances than heat pipes do. This is due to the substantial cross-sectional area that vapor chambers have when compared to typical heat pipes. The average 6mm heat pipe has a cross-section of 28mm2 while even a small vapor chamber, 3mm x 40mm, has a cross-section of 120mm2 (dT = Q*L/(k*A).

If transporting the same power then the effective thermal conductivity goes down by the ratio of the cross-sections. A key point to remember is that although the VC has a lower effective conductivity, they offer performance advantages such as higher total capacities, better operation against gravity, direct contact to the heat source and somewhat lower delta-ts.

 

Vapor Chamber Heat Sink Integration

Vapor chambers can be attached to any kind of heat sink (extruded, skived, etc) but most often they are paired with zipper fins, also known as fin packs, or machined heat sinks.  There are two reasons for this. First, both of these heat sinks have very good thermal performance; zipper fins due to the ability to have very thin, closely spaced fins, and machined due to virtually infinite geometrical design options. Sometimes we see them successfully paired with die-cast housings with integrated fins used in extreme environments.

 

From Left: Zipper Fin Heat Sink, Machined Heat Sink, Die-Cast Heat Sink

 

Regardless of heat sink type, vapor chambers must be attached to the base/fins. They are soldered (most common) or epoxied to the base of the fin stack, the former having better thermal conductivity. Solders used for these assemblies have thermal conductivities on the order of 20 to 50 W/mK while epoxies are on the order of 1/10th of solder conductivities which makes them only useful for low power density applications <10 W/cm2.

 

Solder Thermal Conductivity & Melt Temperature

Solder Thermal Conductivity & Melt Temperature

 

Soldering takes place at temperatures generally above the max temp for vapor chambers so special care must be taken in designing solder fixtures. These fixtures must be able to withstand the internal pressures generated in the vapor chamber during the soldering process to prevent vapor chamber deformation. The pressure chart below indicates the internal vapor chamber pressures vs temperature.

 

Vapor Chamber Temperature Vs. Internal Presure

 

The solder fixture (shown below in purple) is designed to conform to that of the heat sink assembly, preventing it from deforming during the soldering process. The upper and lower portions are clamped or bolted together to prevent the vapor chamber from expanding.

 

Solder Fixture (Purple)

 

Celsia Vapor Chamber Dimensional Design Limits

The table below lists the specifications and tolerances for 1-piece vapor chambers. Because these vapor chambers begin as a very large tube, diameter is listed first followed by widths at various thicknesses as well as tolerances.  No table is provided for 2-piece vapor chambers as they can assume so many configurations although similar tolerances apply. With regard to Celsia’s 2-piece capabilities, 300 x 300mm is the largest possible form factor while sizes of roughly 75 x 150mm are the most common.

 

Vapor Chamber Specifications

Related Links

Methanol Heat Pipes, Acetone Heat Pipes, & Machined Vapor Chambers

Methanol Heat Pipes, Acetone Heat Pipes, & Machined Vapor Chambers

by | May 27, 2019 | Heat Pipes & Vapor Chambers

 

Heat pipe design is generally a straightforward process, but sometimes requirements can yield atypical heat pipe or vapor chamber designs, most notably methanol heat pipes, acetone heat pipes, and machined vapor chambers, in addition to a few interesting variations on standard designs.

Acetone & Methanol Heat Pipes

When operating temperature requirements are low and/or weight is a primary concern, acetone and methanol heat pipes can be alternatives to copper/water thermal solutions.

Above 0oC, standard heat pipes which use water as the working fluid operate normally: liquid turns to vapor which travels to the end of the device where the condenser cools it back to a liquid form that is transported via capillary action back to the evaporator. This process continues until the heat pipe suffers a structural failure or is heated to around 325 oC when vapor pressure is so high that there’s no liquid to be returned to the evaporator. Fortunately, heat pipes never reach those limits under normal operating conditions as most silicon chips start to fail at junction temperatures approaching 125 oC.

When heat pipes are required to operate below 0 oC, both acetone and methanol heat pipes eliminate the potential start-up issues with copper water heat pipes due to their freeze points being -48 oC and -75 oC respectively. As a general rule, engineers should consider methanol heat pipes before acetone ones as they will have a higher Qmax than their acetone counterparts. However, because methanol can only be used inside copper or stainless-steel enclosures / tubes while acetone can be used with aluminum, there is a weight penalty that might be important for applications such as space.

There are cases when it’s advantageous to use both methanol heat pipes and water heat pipes in the same heat sink design.  The chart below shows just such an application. The methanol heat pipes eliminate any start-up issues below freezing (water heat pipes), allowing proper functioning of the condenser area of a remote heat sink when the temperature is -15 oC.  Further, they allow the condenser area to heat above freezing, allowing the water heat pipes to fully function.

Machined Vapor Chamber Design

Not all applications needing two-phase devices can be adequately cooled with traditional heat pipe or vapor chamber solutions. This is where machined vapor chambers can be useful. Unlike traditional vapor chambers, these devices are machined to form complex shapes for all axes. The result is a device that can be attached to multiple heat sources on different planes, have other components affixed to them, and/or allow for unusually high clamping pressure without deforming while providing a tall stand-off to reach a recessed heat source.

In high-performance test equipment, there are many unique challenges. The vapor chamber on the left allows for multiple parts to be fastened to it which is then liquid cooled at its base. This part is machined from solid copper incorporating two internal chambers that use sintered copper as the wick structure.

Machined Vapor Chamber Designs

 

A machined vapor chamber incorporated into the heat sink on the right allows features such as a pedestal that allows the machined vapor chamber to reach a recessed heat source. The pedestal is hollow and is part of the vapor chamber that moves the heat to the copper fins.

Variations on Standard 2-Phase Designs

Sometimes the usual heat pipe customization toolkit just is not enough to meet heat sink performance requirements. Here are some design alternatives that engineers might consider.

Combo Vapor Chamber / Heat Pipe Design

With two-phase devices, engineers typically choose either a vapor chamber design or a heat pipe design. In this instance, again because of design constraints, we needed both. This example is from a gaming oriented small form factor desktop using a higher end Intel processor. The customer wanted to move to the next generation with lower power but much lower max operating temperature (that means smaller thermal budget), without any radical change to the current thermal solution. As I’m sure you’ve guessed, it didn’t take a great leap of engineering acumen to suggest we replace the solid copper base plate with a vapor chamber (see our heat sink performance calculator).

Combo Vapor Chamber & Heat Pipe Heat Sink

 

Given heat pipes were already being used to move and distribute the heat to the condenser, we achieved a 4-6 degree C performance improvement from this solution. With a bit of fan speed tweaking from the system manufacturer, we were able to achieve the required performance targets.

Hybrid Heat Pipe & Vapor Chamber Design

As often happens, we were brought into the heat pipe design process after PCB and enclosure layout was finalized. Keep out zones were fixed, condenser size was adequate for the airflow, but the tentatively planned 6mm flattened heat pipe didn’t adequately cover or cool the CPU and GPU that were upstream from the condenser. Note – this picture only shows the one connector but there was another toward the end for the CPU.

Hybrid Heat Pipe (condenser end) and Vapor Chamber (evaporator end)

 

Given design constraints and performance requirements, a two-heat-pipe alternative was not an option nor was a larger flattened heat pipe. Swaging accomplished the two diameter tube prior to completing the heat pipe manufacturing process including the addition of an internal support structure providing the strength for the required clamping pressures.

Square Heat Pipe Design

Heat pipes do not need to be round. In special applications square or rectangle cross sections provide the required interface to the electronics.

Square Heat Pipe Used for Theatrical LED Lighting Application

 

Due to optical issues, this heat pipe required 4 flat surfaces to which the LEDs are mounted. Additionally, it was required to work in any orientation. The design required a very specific wick structure in order to meet the targets. Initially, the design used a two-piece design, machined evaporator welded to a round tube. The second generation, cost reduced part is made from a single square tube.

U-Shape Vapor Chamber Design

Because vapor chambers are usually made from upper and lower stamped plates, they don’t lend themselves to shapes in the Z-direction. Over a decade ago, Celsia created a vapor chamber from a single very large tube, much like a heat pipe. The resulting width can be up to 110mm yet be as thin as 2.5mm. Of course, we add an internal support structure.

U-Shaped & Curved Vapor Chamber

 

The result is a vapor chamber that can be L or U-shaped yet still allowing for direct contact with the heat source. Although direct contact heat pipes are possible, they require machining which increases cost and they’re less effective at reducing hot spots on the die face. Additionally, the portion of the vapor chamber ‘legs’ that run along the inside of the condenser are slightly curved.