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

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

 

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.

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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