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## Design Considerations When Using Heat Pipes (Pt. 2)

Now that we’ve determined which heat pipes can effectively carry the required heat load (Pt 1), we need to ensure we select and size the heat exchanger to dissipate the heat into the surrounding air -based on available air flow. As mentioned in the prior post, this article is not intended to provide detailed training/equations on the proper design of heat pipes and heat sinks, but rather to offer guidance on the number and size of heat pipes used as well as to provide some tips for estimating heat sink size and determining attachment methods of the sink to the PCB.

## Types of Heat Sinks

There are numerous choices from zipper pack fins to extruded fin stacks, each with their own cost and performance characteristics.  While the heat sink choice can markedly affect heat dissipation performance, the biggest performance boost for any type of heat exchanger comes with forced convection.

Table 1: Types of Heat Sinks

## Estimating Required Heat Sink Volume

By using the simple equation below, we can estimate the overall volume of the heat sink required to cool a heat source.

V= (Q*Rv)/Delta T

Volume is the outer dimensions of the heat sink. Q is power in watts and Rv is the volumetric thermal resistance (cm3-C/W). Table X provided some well established guidelines for the latter under different air flow rates.  The first thing to notice is the enormous positive affect forced air has on thermal performance. Even with a gentle air flow, volumetric thermal resistance of the heat sink is reduced by two-thirds.

Table 2: Air Flow vs Volumetric Thermal Resistance

Let’s say we have a 50W heat source (Q) who’s specified maximum operating junction temperature (Tjunction) is 100o C. Further, let’s assume we’re designing this device to operate in a max ambient temperature of 70o C – a rugged environment. By subtracting the ambient temperature from the maximum allowable electronics temperature, you arrive at the delta t: in this example it’s 100 – 70 = 30o C delta t.

If we are designing a rugged computer, for instance, we’ll probably have a semi or fully sealed enclosure meaning that our heat sink needs to be designed for natural convection. The resulting heat sink volume would be:

V = (50*500)/30= 833 cm3 = a really large heat sink of 9.4 x 9.4 x 9.4 cm. Obviously, we wouldn’t have a heat sink with 9.4 cm high fins in a natural convection environment so the length and width dimensions would have to grow substantially.  But, we might surmise that this application would probably need to use the chassis lid as the heat sink.

There’s an example of just such an application that I talked about in a prior post on ruggedized electronics and whose heat load, ambient operating temperature, and fanless design match the above example.  Let’s look at just the lower half of the design. The bottom side of the PCB contains the processor and chipset at roughly 50W. A copper plate in conjunction with two heat pipes are used to spread and move heat to the chassis case, whose published dimensions are 25cm (L) X 15cm (W).  We know the overall thickness of the assembled case is 7.5 cm and it looks to me the top and bottom lids are each roughly 2.5 cm thick. This would result in an overall volume of 938 cm3 which is within 15% of the required 833cm3 we calculated earlier.

Figure 1: PerfecTron MIL-810 Computer

Had this same heat source been used in conjunction with forced air, the required heat sink volume would have been reduced by up to ten fold.

Although beyond the scope of this article, the next steps in heat sink design would be some detailed excel modeling followed by CFD analysis. This will enable engineers to fully understand optimal base thickness, fin pitch, fin height, and base/fin material among other things.

## Mating Heat Pipes to the Condenser

Whether we’re dealing with a heat exchanger that is local or remote to the heat source, the options for mating a heat pipe(s) to them are identical.

### Grooved Base

It should go without saying that simply soldering a round pipe to a flat surface is far from optimal.  Circular or semi-circular grooves should be extruded or machined into the heat sink. It’s advisable to size the grooves about 0.1mm larger than the diameter of the heat pipe in order to allow enough room for the solder.

The heat sink in Figure 2 uses both a local and remote heat sink.  The extruded heat exchanger is design to accommodate slightly flattened heat pipes, helping to maximize the contact between the copper mounting plate and the heat source. A remote stamped fin pack is used to further increase thermal performance. These types of heat exchanger are particularly useful because the pipes can run directly through the center of the stack, decreasing conduction loss across the fin length.  Because no base plate is required with this fin type, weight and cost can be reduced. Again the holes through which the heat pipes are mounted should be 0.1 mm larger than the pipe diameter.

Had the pipe been completely round at the heat source, a thicker grooved mounting plate would have been required as seen in Figure 3.

Figure 2 (L): Heat Pipes in Grooved Base     Figure 3 (C): Heat Pipes in Grooved Mounting Block     Figure 4 (R): Direct Contact Heat Pipes

### Direct Contact

If conduction losses due to the base plate and extra TIM layer are still unacceptable, further flatting and machining of the heat pipes allows direct contact with the heat source as seen in Figure 4 above. Performance gains from this configuration usually lead to between a 2-8o C reduction in temperature rise.  In cases where direct contact of the heat source to the heat pipes is required a vapor chamber, which can also be mounted directly, should be considered due to its improved heat spreading capacity.

## Attaching the Heat Sink to the Heat Source

The primary reason for considering a heat pipe solution is improved performance. As such, I’m not going to talk about the use of thermal tape or epoxy as the primary means of attaching the heat sink to the die. We generally stick with one of several mechanical attachment methods where we can meet Mil or NEBS shock and vibe requirements. Additionally, the thermal resistance of thermal interface material (TIM) improves as pressure between the die and the heat sink is increased.

An inexpensive attachment method for small (low mass) heat sinks are stamped mounting plate. Although it requires two PCB holes, this method offers better shock and vibe protection thermal tape or epoxy and some TIM compression – although still only 5 PSI.

Figure 5 (L): Stamped Mounting Plate           Figure 6 (C): Spring Loaded Push Pins                Figure 7 (R): Spring loaded Metal Screws

Figure 6 shows spring loaded plastic or steel push pins further increase TIM compression up to around 10 PSI. Installation is fast and simple but removal requires access to the back of the PCB. Push pins should not be considered for anything more than light duty shock and vibe requirements.

Spring loaded screws offer the highest degree of shock and vibe protection (Figure 7) as they are the most secure method of attaching a heat sink to the die and PCB. They offer the highest TIM preload at roughly 75 PSI and work very well with heavy heat sinks.

Celsia is a custom heat sink supplier specializing in heat pipe and vapor chamber thermal assemblies.  We are a U.S. based company, with wholly-owned Taiwan manufacturing facilities, that prides itself on solving difficult cooling challenges under tight deadlines.  Please contact us to schedule a thermal review of your next application.

## Should I Use Heat Pipes or Vapor Chambers to Cool My Electronics Application

I’m often asked which two-phase cooling technology, heat pipes or vapor chambers, is best suited for a particular electronics application. While there are some good rules of thumb the answer isn’t always simple. Nonetheless, this blog post will cover the basics with additional information covered in ann online webinar found in the Resources section of this site.

Let’s start by saying that a transition from traditional heat sinks to either type of two-phase technologies should only be considered when the design is conduction limited and/or when non-thermal goals such as weight or size can’t be achieved with other materials such as solid aluminum or copper.

Once it’s determined that traditional heat sinks just won’t meet thermal requirements, how do mechanical engineers begin to determine the next best option?

## Are You Moving or Spreading the Heat?

While there’s no hard line of distinction, think of the difference like this.

Space constrained applications and/or those with two or more heat sources sharing a common condenser (fin stack) are good candidates for moving the heat to a remote location of more than around 40mm distance. In these cases, the design flexibility of heat pipes, by virtue of their bendability in all directions, are the default choice. And this is especially true if only a single heat pipe is needed. For reference, a 3mm sintered metal heat pipe using water as the working fluid is adequate for cooling 5-20 watts of power while an 8mm pipe can carry 20-90 watts. The range is affected by the porosity and thickness of the wick structure, the amount of working fluid, the degree of bending required and the amount of flattening done to the round pipe.

In my experience heat pipes are used in 95%+ of applications where heat needs to be moved to a remote condenser. However, when conduction loss needs to be reduced further, such as in the laser diode example below, vapor chambers become a viable option. Here three heat sources share a common condenser, reducing temperature rise by roughly 15% over a similar heat pipe alternative.

450W RGB Laser Diode Solution for a 3D Projector

When it comes to spreading heat to a local condenser, the choice between heat pipes and vapor chambers becomes a lot more complicated….

Heat pipes are probably a best choice if:

• There’s plenty of air flow
• You’ve got lots of room for fins
• Nominal power densities are <25 w/cm2
• Ambient temperatures are normal – let’s call this below 45oC
• Cost is a key consideration – every penny counts!

A vapor chamber should be considered if:

• Power densities are high – certainly by the time they hit 50 w/cm2
• Reducing hot spot across the die is a key concern
• Z-direction (height) is constrained, yet fin area needs to be increased
• Atypical ambient temperatures and/or low air flow
• Performance is a key consideration – every degree counts!

Before I review a couple of examples, it’s important to note some of the reasons vapor chambers can perform better than heat pipes for spreading heat. First, vapor chambers make direct contact with the heat source, whereas heat pipes usually require a solid base plate increasing conduction loss –from the base plate itself and from an additional TIM layer between the base plate and the heat pipes. Second, heat spreading via vapor chambers is multi-directional while with heat pipes it’s linear. Third, a vapor chamber solution often allows for additional fin area. These advantages typically allow vapor chamber solutions to have between 10-30% better performance than their heat pipe counterparts. This translates to around 3-9oC for most applications.

Despite the performance difference, vapor chambers were regarded as a very niche solution due in large part to their cost premium over heat pipes, especially for non-consumer (low volume) applications. But manufacturing innovations from an increasing number of suppliers helped narrow the gap.

Let’s take a look at a couple of examples. The first is for a telecom infrastructure manufacturer that wanted to understand both the cost and performance difference between competing heat pipe and vapor chamber designs. The heat source was in the 85 watt range with ambient temperature of 55 degrees Celsius. We compared a four 8mm heat pipe solution to a single vapor chamber design. As the image below shows, the former required a base plate as secondary machining of the heat pipes was not possible due to the wall thickness of the pipes. Testing showed the vapor chamber design to outperform the alternative by a full 4oC, allowing the assembly to meet specifications.

The second example is from one of the first two graphic card solutions to use vapor chambers. Increasing GPU power required that the current heat pipe based solution be redesigned while maintaining the heat sink stack height. Here two 8mm pipes were replaced with a vapor chamber. Elimination of the base plate allowed us to increase fin height and overall fin area while the better heat spreading of the vapor chamber all contributed to a 6oC better solution.

## How Does Bending Affect Heat Pipe & Vapor Chamber Performance?

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.

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

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

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

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

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

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.

## Heat Pipes and 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.

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.

## 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 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.

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.

Direct Contact Heat Pipes (Silverstonetek)

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.

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.

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

(I’ll be presenting a detailed version at Semi-Therm next month)

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.

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.

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.