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

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

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

 

At some point in a thermal system design project it may become apparent that the tried and true methods of increasing thermal efficiency – solid base, fin & fan – just aren’t sufficient. Reasons include:

  • Keep out zones 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.

Regardless of the reason, most thermal engineers are going to need a two-phase cooling solution using either heat pipes or vapor chambers on numerous projects with which they are involved. But, which one is likely the best choice? In today’s blog I’d like to do a topline overview of structural differences and thermal design considerations between these very similar yet somewhat unique two-phase devices.

How does a heat pipe work

 

It probably goes without saying, but the operating principles of all two-phase devices are identical. A wick structure (sintered powder, mesh screens, and/or grooves) are applied to the inside walls of an enclosure (tube or planar shape). Liquid (usually water) is added to the device and vacuum sealed at which point the wick distributes the liquid throughout the device. As heat is applied to one area, the liquid turns to vapor and moves to an area of lower pressure where it cools and returns to liquid form whereupon it moves back to the heat source by virtue of capillary action. In this sense, heat pipes and vapor chambers are the same thing.

For simplicity’s sake, I’ll be focusing on the most common type of two-phase device: an all copper vessel, using a sintered copper wick structure with water as the working fluid.

Structural Design & Cost

 

Difference between heat pipes and vapor chambers

 

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 difference 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 in close proximity to one another. For lower power applications, perhaps requiring only a single, small heat pipe, or those where heat must be effectively transported, heat pipes still maintain dominance 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 multiple heat pipes being used to solve the thermal challenge. Both images below are 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 subsequent 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 great 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 what some call ‘direct contact’ heat pipes. But, this solution has its drawbacks as well. As seen below, this design option uses slightly flattened and machined heat pipes cradled in an aluminum mounting bracket to make direct contact with the heat source. While eliminating the base plate and additional TIM layer – decreasing thermal resistance – it doesn’t spread the heat as effectively as a vapor chamber 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. While studies and practical application shows that the performance of heat sinks using vapor chambers can be enhanced by 20-30% over their heat pipe counterparts, a two piece design has cost implications of roughly the same magnitude versus a multiple heat pipe configuration. Nonetheless, vapor chamber usage has grown with the increasing power and power densities of today’s devices.

Because they 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 heat source, 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

Available from a growing number of manufacturers, one-piece vapor chambers are a cost reduced version of their two-piece counterparts, yet maintain the thermal performance characteristics while adding some unique capabilities (e.g. U-shape bending). Like heat pipes, a one-piece product begins its life as a single copper tube, hence the 1-piece moniker. Like traditional two-piece designs, 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 require 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. For sure, there are numerous thermal challenges where either could be used with good results so it’s important to do a thorough review process of both designs for settling on one.

Two-Phase Thermal Review Process

A typical, although not ideal, thermal design scenario is one in which several key variables are already defined. These include enclosure shape / size, component layout (with associated keep out zones) and airflow, as well as the total power, power density, and size of the heat source(s).

Step #1 – Start by looking at fin location

Given these constraints, the focus is on the heat sink itself and one should start by understanding if the condenser is remote or local to the heat source.

  • If remote by more than 40-50mm begin the investigation with heat pipes. Design flexibility is high allowing bending and flattening to conform to almost any shape in all three dimensions.
  • If local and copper alternatives have been ruled out then a vapor chamber solution that spreads the heat is the best starting point provided the perimeter ratio of the heat sink to the heat source is greater than 30:1.

Step #2: Run an Excel model to determine heat exchanger performance

Based on this input a simple excel model should be run to determine the performance of the heat exchanger. This tells us how much of the thermal budget – fin to air and air temperature rise – are being used. This provides information on how much is left for conduction and interfaces. Since these two components, fin to air and air temperature rise, are the largest resistances in the system, the tail that wags the dog, a design review at this point is normal to optimize the area for the fins and the air flow and pressure drops.

  • If the remaining thermal budget for conduction is less than 10oC, a look at copper or heat pipes. For small devices at low powers a single heat pipe is often sufficient. With the total power we can estimate the number of heat pipes required to carry the heat. For example, a single 6mm heat pipe can carry the power from a 45 watt device.
  • If it is less than 5oC then a vapor chamber may be required. For small devices at high powers typically a vapor chamber is the best solution. Vapor chambers, on the other hand, generally are not run to their performance limits so they are sized to cover as much of the base of the heat sink as possible. Due to their flat format there is a direct contact between the VC and the heat generating component.

Heat sink performance basic model

 

The above image is the summary page of such a model for an LED application. In this example two 8mm heat pipes were compared against a single 15mm wide vapor chamber of similar cost. Each ‘case scenario’ represents the use of different length fins. As you can see, the vapor chamber solution provides an additional 4-5 degrees Celsius of thermal headroom, but this delta is often higher.

Step #3 – Run a more sophisticated Excel model and CFD analysis to optimize the design

Available soon on Celsia’s website, but also downloadable from several other sources, a more comprehensive heat pipe and vapor chamber modeling solution will aid in refining the two-phase design as it accounts for changes to wick characteristics, vapor space, wall thickness, working fluid, case metal, and orientation. Subsequently, CDF modeling is often used to determine the performance of variations to the full heat sink assembly. However, sometimes the best use of time and money is simply to prototype and test a couple of thermal module iterations.

Heatsink performance advanced model

 

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.

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Vapor Chamber | Heatpipe Alternative

Vapor Chamber | Heatpipe Alternative

Vapor Chamber | Heatpipe Alternative

 

A vapor chamber is an effective heatpipe alternative that boosts heat sink performance by  5-10 oC for typical electronics cooling applications. Vapor chambers are a simple, relatively cost-effective, and very dependable device that can be used alone or in combination with heatpipes.

Vapor chambers operate under the same working principles as a heatpipe. They have a metal enclosure which is vacuum-sealed, an internal wick structure attached to the inside walls, and move liquid around the system using capillary action.  Unlike heatpipes, vapor chambers can achieve an impressive 60:1 width to height aspect ratio (flattened heat pipes are on the order of 4:1).

Let’s take a look at the conditions most likely to result in vapor chambers becoming the best solution versus heatpipes or solid metal solutions.

Vapor Chambers When the Thermal Budget is Very Tight

Heatpipes become a likely solution when the thermal budget is less than 40 oC, but as this budget shrinks vapor chambers become the likely hero. The main reason? Vapor chambers make direct contact with the heat source while heatpipes generally have a base plate between them and the heat source.

Note: thermal budget is the difference between the maximum semiconductor temperature (Max Tcase or Tjunction) and the maximum operating ambient temperature of the final system (Max Ambient).

And yes, you are correct that a heat source can make direct contact with a heat pipe solution. Two issues:

  1. The heatpipes have to be fly cut, adding an additional step and expense to the solution.
  2. The mounting block still has solid metal channels between the pipes, reducing thermal performance and possibly creating die face hot spots.

Vapor Chambers When You Need to  “Isothermalize”

Vapor chambers are ideal for applications where high power densities need to be dispersed quickly, hot spots across the die face need to be minimized, or 2+ heat sources are required to be close in temperature.  Below, 6 ASICS were required to remain within 2 oC of each other. The center cutout reduced weight.

When the goal is to achieve as uniform a temperature as possible, vapor chambers trump heatpipes by virtue of their large contiguous surface area that move heat in every direction. Heatpipes only move heat in a linear direction.

Vapor Chambers When Heat Sink Height is Constrained, Yet Fin Area Needs to Grow

Typically, heatpipes run through the center of a fin stack in order to maximize contact area and therefore transfer as much heat to the fins as possible. The downside – fin area is reduced. While this is not a problem if you’ve got the room to increase fin height, it poses a problem when that metric is constrained, as it is here in an add-in desktop graphics card application. A vapor chamber cooling design frees up needed fin area and provided direct contact to the heat source resulting in a 6 degree performance gain.

Vapor Chamber Conclusion

As mentioned in the opening to this article, heat sink performance improvement of 5-10 oC can be had by using a vapor chamber in place of heatpipes because they make direct contact with the heat source, can more evenly distribute heat across a large base, and allow for maximum fin area.

At Celsia, most of the applications for which we design benefit more from heatpipes than a vapor chamber, albeit those with application-driven changes to wick thickness and porosity to boost performance. But, that’s not to say we aren’t deeply invested in vapor chamber technology. The inherent performance benefits of these devices have been known since the 1960’s.

Vapor chamber cost relative to heat pipes, until a decade ago, was prohibitive for all but the most critical applications. Fortunately, the growing use of these devices coupled with innovative manufacturing techniques pioneered by us a decade ago (1-Piece Vapor Chamber) have driven the price down to near parity with 2-4 heat pipes. Below is a brief summary of the pros and cons of both types of vapor chambers, but more detail can be found here.

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Heat Pipe Design Oddities

Heat Pipe Design Oddities

Heat Pipe Design Oddities

 

Heat pipe design is generally a straightforward process, but sometimes custom requirements can yield atypical vapor chamber or heat pipe designs. I won’t be focusing on the specifics of the application or perhaps even the operating parameters. Rather, I’ll talk about only the requirements which drove each quirky solution.

Hybrid Vapor Chamber / Heat Pipe 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. We ended up swaging most of the length of a 6mm pipe before it was sintered and flattened. As you know, swaging a pipe reduces wall thickness and makes the resulting flattened device very week and potentially uneven over its length. By adding a support grid inside before flattening, we are able to keep the vapor within the device flowing freely while allowing a good degree of clamping pressure.

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.

Square Heat Pipe Design

This design requires a standard 12mm copper heat pipe to be TIG welded to a machined square end – both using a sintered wick. The device is then nickel plated.

 

Square Heat Pipe Design 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 Vapor Chamber Design

 

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.

Machined Vapor Chamber Design

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. The pedestal is hollow and is part of the vapor chamber that moves the heat to the copper fins.

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Bending Heat Pipes | How it Affects Vapor Chambers & Heat Pipes

Bending Heat Pipes | How it Affects Vapor Chambers & Heat Pipes

Bending Heat Pipes | How it Affects Vapor Chambers & Heat Pipes

 

The majority of applications for heat pipes, and to a lesser extent vapor chambers, require these products to be bent. Below are respective examples for high-performance graphics card, semiconductor equipment and networking applications.

 

Bent Heat Pipes and Vapor Chambers

Figure 1: Heat Sinks Using Bent Two-Phase Devices

 

 

But what effect does bending a heat pipe or vapor chamber have on their performance? To answer this, let’s first talk about bend parameters then we’ll move into the meat of it and discuss both evaporator and vapor resistances.

Although smaller bend radii are possible, heat pipe guidelines almost universally put their c/l (centerline) bend radius at 3X the diameter of the heat pipe being bent. In other words, for a 5mm round heat pipe bent into a U shape, the resulting OD would be 35mm.

Bend radius for a 1.5-3.5mm thick vapor chambers are about 10mm. For instance, for a 2mm thick vapor chamber the OD of a 180 degree bend would be 22mm. While these would have to stamped into place for a traditional two-piece vapor chamber, one-piece designs can be bent post production, even into a U-shape.

 

Figure 2: Bending Examples

Figure 2: Bending Examples

 

 

Two areas of thermal resistance in these devices must be closely examined. The first is evaporation resistance which is the deta-t due to conduction through the device wall and wick structure as well as phase change of the working fluid into vapor.

In the vast majority of applications, the evaporation resistance is the dominate resistance; therefore, making these devices somewhat length independent. This means that a two-phase device with a transport distance of 75 mm will have almost the same Tsource -Tsink as one with a 150 mm transport distance. This, in effect, doubles the effective thermal conductivity for the longer devices. For a typical water/copper heat pipe with 0.5mm wall and 0.4mm sintered wick, thermal resistance has a nominal value of about 0.1 oC/w/cm2

Figure 3: Heat Pipe

Figure 3: Heat Pipe

 

 

The second is vapor transport resistance which is the temperature loss in the transport region due to pressure drop and condensation delta-t’s. This resistance is typically a function of the power density in the vapor space – nominal power densities are 300-400 w/cm2 with a typical maximum of 800-1,000 w/cm2. The nominal thermal resistance of vapor transport is about 0.01o C/w/cm2. Because of the correlation between cross sectional area of the vapor space and vapor resistance, smaller diameter heat pipes or those that have been flattened increases vapor flow thermal resistance.

So how are these two key resistances affected by bending a two-phase device?

Because the evaporator is almost never placed at the arch of a bent two phase device, we would expect relatively little to no increase in its nominal value when comparing straight and bend two-phase devices. When testing a 3mm thick U-shaped vapor chamber with a 10mm bend radius we see this to be true. Test data showing the evaporation resistance before and after bending is shown in Figure 4. The results are identical within measurement error.

 

FIgure 4: Evaporator Resistance

Figure 4: Evaporator Resistance

 

 

Now let’s look at vapor transport resistance. Due to the pressure drop induced by bending the vapor chamber, we would expect vapor transport resistance to increase due to an increase in pressure drop, thus decreasing the thermal conductivity of the device. Again, testing supports this claim and is relatively consistent between heat pipes and vapor chambers. Bending a two-phase device 180 degrees increases vapor flow resistance by around 50%.

 

Figure 5: Vapor Resistance

Figure 5: Vapor Resistance

 

 

It’s important to note that while vapor transport resistance is significantly affected by bending, its relative contribution to overall thermal resistance is often small. Remember that for an un-bent part, evaporator resistance is a full order of magnitude greater than that of vapor flow resistance.

So, when we examine the overall thermal resistance, we see that the net effect is between 18-40%. You will also note a decrease in the total power. When relying on capillary pumping in the wick structure to return the fluid the bending affects the pore radius and porosity in the bend region. This has about a 10% effect on the Qmax post bending.

 

Figure 6: Total Thermal Resistance

Figure 6: Total Thermal Resistance

 

Given that one should generally design a two-phase device to operate at 70% of its Qmax (we’ll use total power of 70W), we see that the thermal resistance of an un-bent device from 0.047 degrees c/w (3.3 °C) to 0.063 (4.4 °C) for a device with a U-shaped bend. This translates to a delta-T of only 0.9 degrees Celsius.

Based on this we can extrapolate some rules of thumb.

  • Spec a straight heat pipe or vapor chamber with 30% thermal safety margin.
    • Example: A lead load of 70w should use a heat pipe designed with a Qmax of no less than 91w.
  • Add total bend radius of the heat pipe/VC. While not perfect this will get you very close to actual.
    • Example: one 90 degree bend and another 45 degree bend = 135 degrees of bend
  • For each 10 degrees of bend Qmax will decline by .56%.
    • In our example from above: 135 degree total bend divided by 10 multiplied by 0.56% = 7.6% decrease in Qmax.
  • So for our 70 w heat source with two bends totaling 135 degrees we’ll need a heat pipe with a Qmax of 70*(1+(.3+.076)) = 96.3w.

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