Aviation Thermal Management

Billy: Welcome and thank you for joining us for today’s webcast “Aviation Thermal Management – When to Use Heat Pipes, HiK Plates, Vapor Chambers, and Conduction Cooling” sponsored by Advanced Cooling Technologies and Tech Briefs Media Group. I’m Billy Hurley, Associate Editor with Tech Briefs Media Group, and I’ll be you moderator today. Our webcast would last approximately 30 minutes and there will be a question and answer period at the end of the presentation.

If you have a question you may submit it at any time during the presentation by entering it at the box at the bottom of your screen. Our presenters will answer as many questions as possible at the conclusion of the presentation. Those questions not addressed during the live event would be answered after the webcast. In order to view the presentation properly, please disable any popup blockers you may have on your browser.

At this time, I would like to introduce our speakers, John Hartenstine, Manager of Aerospace Products, has been with ACT since 2005, He has over 27 years’ experience in research and product development engineering focusing on advanced thermal management including heat pipes. He is a co-inventor on three US patents and has co-authored over 30 papers.

Also in line is Doctor Bill Anderson. Bill Anderson is the chief engineer at Advance Cooling technologies. He has over 30 years’ experience in two-phase heat transfer. He has designed and developed a number of unique heat transfer devices. For the last few years, Dr. Anderson has been developing high temperature heat pipes and radiators for nuclear fission and electric propulsion, as well as working on Thermal Management Systems for full authority digital engine control and other Avionics boxes.

Now before we begin, I also want to mention that if you would like a PDF of today’s presentation, please request a copy in the question box at the bottom of your screen. Our presenters will then contact you and send the PDF after the presentation. So at this time I’d like to hand over the webcast to our first speaker John Hartenstine. John?

Dr. Hartenstine: Great. Thank you Billy. The webinar this afternoon is entitled “Aviation Thermal Management – When to Use Heat Pipes, HiK Plates, Vapor Chambers, and Conduction Cooling.” We will get started with a quick overview of what we are going to cover today. We’ll start with the motivation followed by some discussion on the baseline aluminum plates and getting into benefits of use and selection criteria for heat pipes, HiK plates, vapor chambers and encapsulated conduction cooling. And then taking a look at some trait studies. Finally we will wrap up the presentation and we will take some questions.

The motivation for this webinar is to address on the unique cooling challenges facing avionic design engineers where key components must be maintained below the specific temperatures. There are several types of thermal technology with the design engineer toolbox including conduction cooling, heat pipes, HiK or high-connectivity plates and vapor chambers. One of the questions that is often asked is what are the designed criteria used when conducting trait studies involving these technologies? So this webcast will address that.

All these are standard method for removing heat from electronics over short distances to a location where the heat can be removed by liquid or forced air cooling.

So we’ll start off with just some basics with conduction cooling, simple baseline conduction, there is no two-phase component here. Conduction cooling from least expensive to most expensive is standards aluminum followed by heat spreaders. [Inaudible 00:03:25] like diamond and encapsulated conduction plates. Today we will focus on aluminum and encapsulated conduction plates.

So the simplest method to cool electronics is by conduction through aluminum plate, typically 6063 and 6061 are constantly used to cool the electronics, and they also provide structural support. At times there is a need to enhance the baseline of aluminum, you cannot meet the power and mass requirements, or you will need to consider other conduction technologies or heat pipes. Copper is not commonly used mainly due to its density.

We are starting with some baseline conduction cooling and then we are going to get into HiK plates and vapor chambers. But before we do that we need to just address some basics with heat pipes.

Heat pipes are passive two phase heat-transfer devices. They utilized the latent heat of fluid to very effectively transfer heat across their length. Looking at the figure at the top left portion of the slide, the evaporator area would be placed beneath your heat generating components. Heat is gathered and input into the heat pipe which causes the fluid to vaporize, the vapor then moves along the center of the heat pipe to a [quarter] region passively due to the inherent pressure gradient within the pipe.

At the quarter region it condenses back into a liquid. And then the liquid is pumped back to the evaporator using capillary reaction provided by wick structure. An analogy were to be, if you take a napkin and dip it into your coffee how your pulls it up. That is the same popping mechanism how the fluid is returned from the condenser to the evaporator.

Overall the heat pipes have a temperature differential of two to five degrees across the length that can be used by themselves or in conjunction with other metal components within the system. They have thermal connectivities anywhere from 10,000 to 200,000 W/m K and heat fluxes in the 50-70 W/m per square centimeter range.

Temperature, height, against gravity and acceleration are typical limitations for these passive two-phase cooling devices. Heat pipe performance curves that you can see on the right are plotted from ACT’s online calculator where we plotted power as a function of temperature for a specific heat pipe design. And this case it’s an eight inch long pipe with a two inch long evaporator, a two inch condenser. The plot on the top is for operation horizontal. The plot below is for [four inches] against gravity. The curves are for a number of different pipe diameters.

So if you know from these curves that for a standard wick, these curve are for a standard wick improved performance can be achieved with enhancements to the wick design. One of the things you can see from these curves is that the water pipes, you can effectively transfer heat above 30 degrees C taking a look at the slope of the curve.

One of the questions that has often been asked is how far can a heat pipe operate against gravity. Water heat pipes can operate roughly nine to ten inches above the conductor. Also, heat pipes can work at lower temperatures, but for a water heat pipe below zero obviously, the pipe will freeze so it’s not a typically an issue for electronics cooling because the environment itself is providing the cooling. Once the power is turned on, the heat pipe will thaw out and start to transport power. It’s also important to note that properly manufactured heat pipes can operate over many free fall cycles.

So let take a look at spot cooling. Heat pipes are used for three typical purposes: one if you’re looking to move power from point A to point B. Two, if you are looking to spread the thermal load, and three if you are looking to isothermalize the surface.

So in this case, spot cooling refers to cooling discrete components, moving heat off a chip to a remote heat sink. So if you take a look at the picture in the top right, what’s shown there, you have two copper water heat pipes that are soldered into an aluminum mounting plate. You can see [bosses] within those plates. Under those bosses were processors, so what were doing is pulling heat off those processors, effectively using heat pipes, and then transferring the thermal load up to a liquid cold rail at the top.

So what are some of the selection parameters for spot cooling heat pipes? These have the same benefits as regular heat pipes, they are relatively low cost, t hey have a high connectivity, passive operation, nine to ten inch maximum height, and heat fluxes up to around 75 watts a square centimeter. Lightweight and flexible, they can be made into countless geometries. They also have the ability to meet demanding environmental conditions such as stringent operations and survival temperatures, shock and vibrations etcetera.

They are typically not structural elements, and they transfer heat in one direction.

Continuing on, HiK plates are embedded, heat pipe plates where you take the isothermal product [inaudible 00:08:19] of heat pipes, embed them into a standard aluminum plate with either epoxy or solder to increase the overall connectivity.

The heat pipes are strategic placed to get good thermal results without affecting current geometry or mounting features.

The heat pipes plus soda are assembled in a way to that of aluminum with connectivity nearly three to five times greater than that of raw aluminum itself. These plates can also be used as structural components within the systems.

Shown here is a thermal analysis of an aluminum plate containing many high powered electrical components with and without embedded pipes. The picture in far left are model results of aluminum plates without any heat pipes. You can see they are three hot spot locations on that plate.

The picture in the middle is the same aluminum plate but now with heat pipes embedded in the plate strategically placed. You can see the max temperature has dropped about 20 degrees and is fairy uniform in temperature. The picture on the right is the actual hardware itself and silver lines you see they are actually the location where heat pipe was embedded in to the plate.

So what are the selection parameters for HiK plates? Again same as heat pipe benefit, the high connectivity passive type of heat fluxes. We have been able to achieve HiK plate as thin as 1.83 millimeters without reducing the overall power capacity.

In addition, maximum height for HiK plates is around 18 to 20 inches with heat pipes positioned across the plate from each other. There are some options with the plate materials. With aluminum magnesium and aluminum silicon carbide.

With aluminum ,we have achieved thermal connectivity between 600-1200W/m K, with magnesium between 450 to 800 W/m K.

This type of technology strongly use for conduction cooled cards, for example the top two pictures you see on the right show a conduction cool card with heat pipes that have been incorporated to the plates itself. Pulling, power off the sensitive component deep within the card, and transferring the power with very little thermal resistance over to the edge so they can transfer the thermal load into the card guides within a chassis.

So next we are going to look at a case study evaluating HiK plates. The objectives here is to see what kind of reduction in weight and size can be realized using a HiK plate compared to aluminum extrusion to maintain the same firm performance.

So for a standard heat sink, it was aluminum heat sink, 12 inch long evaporator, .6 inch thick base thickness, it had a weight of 9.6 pounds.

By introducing heat pipes into it, we introduced five heat pipes into the design, three over the heat input location and two outside of that area to improve with spreading.

We were able to reduce the overall length from 12 to 10 inches, the thickness from .6 to .28 which resulted in a weight reduction from 9.6 to 6.3 pounds, a 35% reduction.

If you take a look at the same conditions, here are some actual thermal images to demonstrate the improvement. The HiK heat sink shown on the right more effectively spreads spreads the heat as can be seen the yellow area surrounding the source even though the heat sink is shorter and is lower and thinner.

The improvement is directly attributable to the addition heat pipes which can be seen as a red light in the picture in the right.

Let’s talk about vapor chambers. Vapor chambers, like conventional cylindrical heat pipes, they transport heat from a heat source to a heat sink with a very small temperature gradient. Vapor chamber heat pipes are often used to accept heat from a small high heat flux source, and transfer heat to a much large lower heat flux sink, where the heat can be effectively dissipated.

The main benefits of the vapor chambers is the day they are nearly isothermal with one or two degrees, can be used to cool multiple component and be made as thin as three millimeters.

They also have a very low thermal resistance. Heat fluxes are so much of that of heat pipes, but you can increase the heat flux with wick enhancements.

Some of the main limitations are that they are higher in cost compared to HiK plates and cannot be used as an overall structure. Another limitation for standard water vapor chambers is the max temperature is around 105 degrees.

From the pictures that you see there you can see that vapor chamber internal on the lower left, the assembled vapor chamber in the center, and then the typical vapor chamber component on the right.

So what are some of the selection parameters for vapor chambers? Again the main benefits, same with heat pipes is they have high connectivity passive, maximum dimensions of around 10 inches by 20 inches. Heat fluxes around 75 watts per square centimeter for typical wick designs.

But there can be enhancements with envelope material to promote direct [inaudible 00:13:29] such as instead of using all copper design, using aluminum nitride direct on copper.

And you can see some of the pictures for the aluminum nitride direct on copper shown in the smaller picture in the center and up picture above the three there.

Next we’ll briefly address encapsulated conduction cooling. First materials like diamond or diamond composite exhibit high conductivity but are expensive over large areas. Metal composites and Pyrolytic graphites are brittle, hygroscopic and can have a relatively low strength. But this has been improved by encapsulating the graphite with a metallic shell for protection and strength with a high conductivity core. Also in addition some of the reports have documented thermal connectivities for this type of structure around 550 W/m K.

So in summary with encapsulated pyrolytic graphite, the in-plane connectivity of the graphite itself is between a 1000-1500 W/m K.

The out-of-plane connectivity around 10, and the thermal [vias], which we will discuss next, lowers the thermal connectivity, which is the overall function material structure itself.

So expanding a little more on the encapsulated Pyrolytic Graphite. With the Encapsulate graphite, thermal load is transferred into the structure through thermal vias. So you can see the triangular shaped vias as well as APG and encapsulate in the figure on the right. Also you can see some of the steps there for manufacturing. The manufacturing is a fairly rigorous and involved process and can be relatively expensive compared to the other two-phase heat transfer devices.

And again thermal connectivity is supported in the 550 W/m K range.

So what is the selection criteria for the encapsulated conduction cooling? The main benefits are that they can be made to be thin, they have a thermal conductivity that’s higher compared to aluminum, they are not affected by acceleration or gravity, so they can be used in application were sustained higher accelerations are required or where the heat pipe cannot be oriented favorably.

We also have a wide temperature range, especially at low temperature where water heat pipes are not effective. They have lower density compared to two-phase system and has a long thermal transport length.

Some of the limits are they have a higher cost compared to passive two-phase heat transfer devices, a lower thermal conductivity compared to two-phase, and also high flux chip locations are fixed at the design compared to common vapor chambers.

In addition, conductions are shown to drop off as a function of thermal cycling.

So we run some trait studies, taking a look at a general plate, common plate 9.0 inches long, 4.0 inches wide, with a thickness of 0.12 inches thick. We put a 50 watt heat load over 5 square centimeter at one end of the plate, and we evaluated two sink conditions.

The first one was 0.5 inch long cold rail set at 25 degrees at the far end of the plate, and the other one was simulated forced air heat sink on the entire back side of the plate.

So we took a look at five case studies. The first was with a standard of 6063 aluminum plate. The second was with encapsulated conduction plate with a uniform connectivity of 550 W/m K. A HiK plate with a uniform conduction of 1000 W/m K, and then the same HiK plate with heat pipes positioned into an optimal particular configuration. We also took a look at copper water vapor chambers.

So the figures that you see below at the bottom, you see a cold rail HiK plate, and a Convection HiK plate. For the cold raile HiK plate you can see the location of where the heat pipes have been placed. So in this case we are trying, the heat pipes have been placed under the heat source, and we are trying to transport a thermal load out to the 25 degree cold rail at the other end.

For the convection, the portion for a HiK plate to take heat off the back side of the plate, we have heat pipes mounted directly under the source, but then the heat pipes are spread uniformly throughout the plate to get better spreading and improve the back side heat transfer performance.

So in the cold rail cooling case, the aluminum plate exhibits the maximum temperature of 210 degrees C. The maximum temperature drops, it’ll be regressed from from 550 W/m K for the encapsulated conduction plate to a generalized high K plates to an optimized high K plate and finally a vapor chamber. So as you can see there is a significant reduction in temperature using passive two phase cooling approach. You get improved results with higher effective thermal conductivity.

So for the convectional cooling case we see a similar progression in thermal performance across the different cooling technologies. On the optimum’s HiK plate design, you can see the thermal patterns from the heat pipes spreading the thermal load across the plate’s surface, which improves the convection-side heat transfer, that’s the view third from the left.

In addition you can see that the vapor chamber shows further improvements over a HiK plate as a result of [2D] spreading.

An additional case study was generated to take a look at cooling discrete heat sources over a common plate. Evaluating HiK plates and vapor chambers. In this case convection cooling off the back side of the plate provided the heat sink. There were multiple 1 square cm location and heat load ranging from 10 to 200 W/m K placed over the plate. As you can see, as you look at the results here, that the low heat flux components almost disappear and the higher heat fluxes tend to spread the thermal load. One take away from here is if you want to keep something Isothermal as possible you would want use a vapor chamber.

So as a result, we generate a table to summarize each technology discussed, evaluating density, spreading, thermal connectivity, maximum heat flux, minimum thickness, maximum height and relative cost. You can see from top to bottom progressing from least expensive to most expensive. And so this table was[inaudible 00:19:56] conducting thermal traits to their applications.

A couple of things I want point out here, under the spreading column you see 2-D and 1-D cooling. We also see, as I said earlier, for spot-cooling, for regular heat pipe it’s a 1-D cooling, but for a HiK plate it show a 1.5-D cooling. What’s meant by that? So we have the heat pipe that is transferring thermal load from one end to the other, but in a HiK plate its embedded within the aluminium structure, you’re also seeing some spreading within that plate so you get some additional benefits there.

In addition under the maximum heat flux column, you see you see a designation that says ‘depends on geometry’. What this means is it’s not a heat flux limit but a limit on chip temperature. One thing I want to point out is two-phase heat transfer devices weigh more compared to aluminum or encapsulated graphite for given thickness, but they have a higher thermal connectivity, so as a result specific thermal conductivity wins out by comparison.

So a selection criteria summary, I will just go through this quickly. Aluminum plates 200W/m K 2-D, cheapest solution. Heat pipes 10,000 to 100,000 W/m K 1-D, use for discrete point cooling. HiK plates 600 to 1200 W/m 1.5-D, for strategic thermal spreading. Vapor chamber 5000 to 100,000 W/m K 2-D, expensive compared to HiK plates but they can be isothermal to within 1 to 2 degrees.

Encapsulated Conduction Systems, a reported 550 W/m K 2-D heat transfer, expensive but they have a sustained but they can have high accelerations, operation at lower temperature and protocol heighths greater than 20 inches.

So wrapping upm, we hope that this presentation this afternoon provided assistance to design engineers provide some direction as to the use and benefits and selection criteria for these technologies.

I want thank you for joining us and thank you to those who’ve submitted questions. Bill Anderson has been reviewing the questions as they’ve been coming in and I want to answer as many questions as possible within the allotted time remaining. If you don’t get an answer to your question, we will respond with an email shortly.

Billy: Thanks John. At this time we would like to begin our Q/A so if you have a question out there you might submit it by entering it in the box at the bottom of your screen. And now I would like to welcome to the line Dr Bill Anderson. Bill we already have some question in the queue. The first one here we have, what are the approximate delivery times for heat pipes, HiK plates, vapor chambers, and enhanced conduction cards?

Dr. Anderson: Okay for heat pipes and small volume, you are typically talking about three to four weeks after we receipt of order. For HiK plates and vapor chambers, it’s typically eight to ten weeks. And for encapsulated conduction is greater than ten weeks depending on the complexity of the design.

Billy: Can heat pipes be used at temperatures lower than 25 degrees Celsius?

Dr. Anderson: Definitely, the 25 degree C limit is really referring to only the heat pipes with water as a working fluid. Ammonia heat pipes for space craft thermal control operating down to around minus 65, ethanol pipes operate down to about minus 50. And other fluids can operate at lower temperatures well below minus 200 degrees C.

Billy: Welcome and thank you for joining us for today’s webcast “Aviation Thermal Management – When to Use Heat Pipes, HiK Plates, Vapor Chambers, and Conduction Cooling” sponsored by Advanced Cooling Technologies and Tech Briefs Media Group. I’m Billy Hurley, Associate Editor with Tech Briefs Media Group, and I’ll be you moderator today. Our webcast would last approximately 30 minutes and there will be a question and answer period at the end of the presentation.

If you have a question you may submit it at any time during the presentation by entering it at the box at the bottom of your screen. Our presenters will answer as many questions as possible at the conclusion of the presentation. Those questions not addressed during the live event would be answered after the webcast. In order to view the presentation properly, please disable any popup blockers you may have on your browser.

At this time, I would like to introduce our speakers, John Hartenstein, Manager of Aerospace Products, has been with ACT since 2005, He has over 27 years’ experience in research and product development engineering focusing on advanced thermal management including heat pipes. He is a co-inventor on three US patents and has co-authored over 30 papers.

Also in line is Doctor Bill Anderson. Bill Anderson is the chief engineer at Advance Cooling technologies. He has over 30 years’ experience in two-phase heat transfer. He has designed and developed a number of unique heat transfer devices. For the last few years, Dr. Anderson has been developing high temperature heat pipes and radiators for nuclear fission and electric propulsion, as well as working on Thermal Management Systems for full authority digital engine control and other Avionics boxes.

Now before we begin, I also want to mention that if you would like a PDF of today’s presentation, please request a copy in the question box at the bottom of your screen. Our presenters will then contact you and send the PDF after the presentation. So at this time I’d like to hand over the webcast to our first speaker John Hartenstein. John?

Dr. Hartenstine: Great. Thank you Billy. The webinar this afternoon is entitled “Aviation Thermal Management – When to Use Heat Pipes, HiK Plates, Vapor Chambers, and Conduction Cooling.” We will get started with a quick overview of what we are going to cover today. We’ll start with the motivation followed by some discussion on the baseline aluminum plates and getting into benefits of use and selection criteria for heat pipes, HiK plates, vapor chambers and encapsulated conduction cooling. And then taking a look at some trait studies. Finally we will wrap up the presentation and we will take some questions.

The motivation for this webinar is to address on the unique cooling challenges facing avionic design engineers where key components must be maintained below the specific temperatures. There are several types of thermal technology with the design engineer toolbox including conduction cooling, heat pipes, HiK or high-connectivity plates and vapor chambers. One of the questions that is often asked is what are the designed criteria used when conducting trait studies involving these technologies? So this webcast will address that.

All these are standard method for removing heat from electronics over short distances to a location where the heat can be removed by liquid or forced air cooling.

So we’ll start off with just some basics with conduction cooling, simple baseline conduction, there is no two-phase component here. Conduction cooling from least expensive to most expensive is standards aluminum followed by heat spreaders. [Inaudible 00:03:25] like diamond and encapsulated conduction plates. Today we will focus on aluminum and encapsulated conduction plates.

So the simplest method to cool electronics is by conduction through aluminum plate, typically 6063 and 6061 are constantly used to cool the electronics, and they also provide structural support. At times there is a need to enhance the baseline of aluminum, you cannot meet the power and mass requirements, or you will need to consider other conduction technologies or heat pipes. Copper is not commonly used mainly due to its density.

We are starting with some baseline conduction cooling and then we are going to get into HiK plates and vapor chambers. But before we do that we need to just address some basics with heat pipes.

Heat pipes are passive two phase heat-transfer devices. They utilized the latent heat of fluid to very effectively transfer heat across their length. Looking at the figure at the top left portion of the slide, the evaporator area would be placed beneath your heat generating components. Heat is gathered and input into the heat pipe which causes the fluid to vaporize, the vapor then moves along the center of the heat pipe to a [quarter] region passively due to the inherent pressure gradient within the pipe.

At the quarter region it condenses back into a liquid. And then the liquid is pumped back to the evaporator using capillary reaction provided by wick structure. An analogy were to be, if you take a napkin and dip it into your coffee how your pulls it up. That is the same popping mechanism how the fluid is returned from the condenser to the evaporator.

Overall the heat pipes have a temperature differential of two to five degrees across the length that can be used by themselves or in conjunction with other metal components within the system. They have thermal connectivities anywhere from 10,000 to 200,000 W/m K and heat fluxes in the 50-70 W/m per square centimeter range.

Temperature, height, against gravity and acceleration are typical limitations for these passive two-phase cooling devices. Heat pipe performance curves that you can see on the right are plotted from ACT’s online calculator where we plotted power as a function of temperature for a specific heat pipe design. And this case it’s an eight inch long pipe with a two inch long evaporator, a two inch condenser. The plot on the top is for operation horizontal. The plot below is for [four inches] against gravity. The curves are for a number of different pipe diameters.

So if you know from these curves that for a standard wick, these curve are for a standard wick improved performance can be achieved with enhancements to the wick design. One of the things you can see from these curves is that the water pipes, you can effectively transfer heat above 30 degrees C taking a look at the slope of the curve.

One of the questions that has often been asked is how far can a heat pipe operate against gravity. Water heat pipes can operate roughly nine to ten inches above the conductor. Also, heat pipes can work at lower temperatures, but for a water heat pipe below zero obviously, the pipe will freeze so it’s not a typically an issue for electronics cooling because the environment itself is providing the cooling. Once the power is turned on, the heat pipe will thaw out and start to transport power. It’s also important to note that properly manufactured heat pipes can operate over many free fall cycles.

So let take a look at spot cooling. Heat pipes are used for three typical purposes: one if you’re looking to move power from point A to point B. Two, if you are looking to spread the thermal load, and three if you are looking to isothermalize the surface.

So in this case, spot cooling refers to cooling discrete components, moving heat off a chip to a remote heat sink. So if you take a look at the picture in the top right, what’s shown there, you have two copper water heat pipes that are soldered into an aluminum mounting plate. You can see [bosses] within those plates. Under those bosses were processors, so what were doing is pulling heat off those processors, effectively using heat pipes, and then transferring the thermal load up to a liquid cold rail at the top.

So what are some of the selection parameters for spot cooling heat pipes? These have the same benefits as regular heat pipes, they are relatively low cost, t hey have a high connectivity, passive operation, nine to ten inch maximum height, and heat fluxes up to around 75 watts a square centimeter. Lightweight and flexible, they can be made into countless geometries. They also have the ability to meet demanding environmental conditions such as stringent operations and survival temperatures, shock and vibrations etcetera.

They are typically not structural elements, and they transfer heat in one direction.

Continuing on, HiK plates are embedded, heat pipe plates where you take the isothermal product [inaudible 00:08:19] of heat pipes, embed them into a standard aluminum plate with either epoxy or solder to increase the overall connectivity.

The heat pipes are strategic placed to get good thermal results without affecting current geometry or mounting features.

The heat pipes plus soda are assembled in a way to that of aluminum with connectivity nearly three to five times greater than that of raw aluminum itself. These plates can also be used as structural components within the systems.

Shown here is a thermal analysis of an aluminum plate containing many high powered electrical components with and without embedded pipes. The picture in far left are model results of aluminum plates without any heat pipes. You can see they are three hot spot locations on that plate.

The picture in the middle is the same aluminum plate but now with heat pipes embedded in the plate strategically placed. You can see the max temperature has dropped about 20 degrees and is fairy uniform in temperature. The picture on the right is the actual hardware itself and silver lines you see they are actually the location where heat pipe was embedded in to the plate.

So what are the selection parameters for HiK plates? Again same as heat pipe benefit, the high connectivity passive type of heat fluxes. We have been able to achieve HiK plate as thin as 1.83 millimeters without reducing the overall power capacity.

In addition, maximum height for HiK plates is around 18 to 20 inches with heat pipes positioned across the plate from each other. There are some options with the plate materials. With aluminum magnesium and aluminum silicon carbide.

With aluminum ,we have achieved thermal connectivity between 600-1200W/m K, with magnesium between 450 to 800 W/m K.

This type of technology strongly use for conduction cooled cards, for example the top two pictures you see on the right show a conduction cool card with heat pipes that have been incorporated to the plates itself. Pulling, power off the sensitive component deep within the card, and transferring the power with very little thermal resistance over to the edge so they can transfer the thermal load into the card guides within a chassis.

So next we are going to look at a case study evaluating HiK plates. The objectives here is to see what kind of reduction in weight and size can be realized using a HiK plate compared to aluminum extrusion to maintain the same firm performance.

So for a standard heat sink, it was aluminum heat sink, 12 inch long evaporator, .6 inch thick base thickness, it had a weight of 9.6 pounds.

By introducing heat pipes into it, we introduced five heat pipes into the design, three over the heat input location and two outside of that area to improve with spreading.

We were able to reduce the overall length from 12 to 10 inches, the thickness from .6 to .28 which resulted in a weight reduction from 9.6 to 6.3 pounds, a 35% reduction.

If you take a look at the same conditions, here are some actual thermal images to demonstrate the improvement. The HiK heat sink shown on the right more effectively spreads spreads the heat as can be seen the yellow area surrounding the source even though the heat sink is shorter and is lower and thinner.

The improvement is directly attributable to the addition heat pipes which can be seen as a red light in the picture in the right.

Let’s talk about vapor chambers. Vapor chambers, like conventional cylindrical heat pipes, they transport heat from a heat source to a heat sink with a very small temperature gradient. Vapor chamber heat pipes are often used to accept heat from a small high heat flux source, and transfer heat to a much large lower heat flux sink, where the heat can be effectively dissipated.

The main benefits of the vapor chambers is the day they are nearly isothermal with one or two degrees, can be used to cool multiple component and be made as thin as three millimeters.

They also have a very low thermal resistance. Heat fluxes are so much of that of heat pipes, but you can increase the heat flux with wick enhancements.

Some of the main limitations are that they are higher in cost compared to HiK plates and cannot be used as an overall structure. Another limitation for standard water vapor chambers is the max temperature is around 105 degrees.

From the pictures that you see there you can see that vapor chamber internal on the lower left, the assembled vapor chamber in the center, and then the typical vapor chamber component on the right.

So what are some of the selection parameters for vapor chambers? Again the main benefits, same with heat pipes is they have high connectivity passive, maximum dimensions of around 10 inches by 20 inches. Heat fluxes around 75 watts per square centimeter for typical wick designs.

But there can be enhancements with envelope material to promote direct [inaudible 00:13:29] such as instead of using all copper design, using aluminum nitride direct on copper.

And you can see some of the pictures for the aluminum nitride direct on copper shown in the smaller picture in the center and up picture above the three there.

Next we’ll briefly address encapsulated conduction cooling. First materials like diamond or diamond composite exhibit high conductivity but are expensive over large areas. Metal composites and Pyrolytic graphites are brittle, hygroscopic and can have a relatively low strength. But this has been improved by encapsulating the graphite with a metallic shell for protection and strength with a high conductivity core. Also in addition some of the reports have documented thermal connectivities for this type of structure around 550 W/m K.

So in summary with encapsulated pyrolytic graphite, the in-plane connectivity of the graphite itself is between a 1000-1500 W/m K.

The out-of-plane connectivity around 10, and the thermal [vias], which we will discuss next, lowers the thermal connectivity, which is the overall function material structure itself.

So expanding a little more on the encapsulated Pyrolytic Graphite. With the Encapsulate graphite, thermal load is transferred into the structure through thermal vias. So you can see the triangular shaped vias as well as APG and encapsulate in the figure on the right. Also you can see some of the steps there for manufacturing. The manufacturing is a fairly rigorous and involved process and can be relatively expensive compared to the other two-phase heat transfer devices.

And again thermal connectivity is supported in the 550 W/m K range.

So what is the selection criteria for the encapsulated conduction cooling? The main benefits are that they can be made to be thin, they have a thermal conductivity that’s higher compared to aluminum, they are not affected by acceleration or gravity, so they can be used in application were sustained higher accelerations are required or where the heat pipe cannot be oriented favorably.

We also have a wide temperature range, especially at low temperature where water heat pipes are not effective. They have lower density compared to two-phase system and has a long thermal transport length.

Some of the limits are they have a higher cost compared to passive two-phase heat transfer devices, a lower thermal conductivity compared to two-phase, and also high flux chip locations are fixed at the design compared to common vapor chambers.

In addition, conductions are shown to drop off as a function of thermal cycling.

So we run some trait studies, taking a look at a general plate, common plate 9.0 inches long, 4.0 inches wide, with a thickness of 0.12 inches thick. We put a 50 watt heat load over 5 square centimeter at one end of the plate, and we evaluated two sink conditions.

The first one was 0.5 inch long cold rail set at 25 degrees at the far end of the plate, and the other one was simulated forced air heat sink on the entire back side of the plate.

So we took a look at five case studies. The first was with a standard of 6063 aluminum plate. The second was with encapsulated conduction plate with a uniform connectivity of 550 W/m K. A HiK plate with a uniform conduction of 1000 W/m K, and then the same HiK plate with heat pipes positioned into an optimal particular configuration. We also took a look at copper water vapor chambers.

So the figures that you see below at the bottom, you see a cold rail HiK plate, and a Convection HiK plate. For the cold raile HiK plate you can see the location of where the heat pipes have been placed. So in this case we are trying, the heat pipes have been placed under the heat source, and we are trying to transport a thermal load out to the 25 degree cold rail at the other end.

For the convection, the portion for a HiK plate to take heat off the back side of the plate, we have heat pipes mounted directly under the source, but then the heat pipes are spread uniformly throughout the plate to get better spreading and improve the back side heat transfer performance.

So in the cold rail cooling case, the aluminum plate exhibits the maximum temperature of 210 degrees C. The maximum temperature drops, it’ll be regressed from from 550 W/m K for the encapsulated conduction plate to a generalized high K plates to an optimized high K plate and finally a vapor chamber. So as you can see there is a significant reduction in temperature using passive two phase cooling approach. You get improved results with higher effective thermal conductivity.

So for the convectional cooling case we see a similar progression in thermal performance across the different cooling technologies. On the optimum’s HiK plate design, you can see the thermal patterns from the heat pipes spreading the thermal load across the plate’s surface, which improves the convection-side heat transfer, that’s the view third from the left.

In addition you can see that the vapor chamber shows further improvements over a HiK plate as a result of [2D] spreading.

An additional case study was generated to take a look at cooling discrete heat sources over a common plate. Evaluating HiK plates and vapor chambers. In this case convection cooling off the back side of the plate provided the heat sink. There were multiple 1 square cm location and heat load ranging from 10 to 200 W/m K placed over the plate. As you can see, as you look at the results here, that the low heat flux components almost disappear and the higher heat fluxes tend to spread the thermal load. One take away from here is if you want to keep something Isothermal as possible you would want use a vapor chamber.

So as a result, we generate a table to summarize each technology discussed, evaluating density, spreading, thermal connectivity, maximum heat flux, minimum thickness, maximum height and relative cost. You can see from top to bottom progressing from least expensive to most expensive. And so this table was[inaudible 00:19:56] conducting thermal traits to their applications.

A couple of things I want point out here, under the spreading column you see 2-D and 1-D cooling. We also see, as I said earlier, for spot-cooling, for regular heat pipe it’s a 1-D cooling, but for a HiK plate it show a 1.5-D cooling. What’s meant by that? So we have the heat pipe that is transferring thermal load from one end to the other, but in a HiK plate its embedded within the aluminium structure, you’re also seeing some spreading within that plate so you get some additional benefits there.

In addition under the maximum heat flux column, you see you see a designation that says ‘depends on geometry’. What this means is it’s not a heat flux limit but a limit on chip temperature. One thing I want to point out is two-phase heat transfer devices weigh more compared to aluminum or encapsulated graphite for given thickness, but they have a higher thermal connectivity, so as a result specific thermal conductivity wins out by comparison.

So a selection criteria summary, I will just go through this quickly. Aluminum plates 200W/m K 2-D, cheapest solution. Heat pipes 10,000 to 100,000 W/m K 1-D, use for discrete point cooling. HiK plates 600 to 1200 W/m 1.5-D, for strategic thermal spreading. Vapor chamber 5000 to 100,000 W/m K 2-D, expensive compared to HiK plates but they can be isothermal to within 1 to 2 degrees.

Encapsulated Conduction Systems, a reported 550 W/m K 2-D heat transfer, expensive but they have a sustained but they can have high accelerations, operation at lower temperature and protocol heighths greater than 20 inches.

So wrapping upm, we hope that this presentation this afternoon provided assistance to design engineers provide some direction as to the use and benefits and selection criteria for these technologies.

I want thank you for joining us and thank you to those who’ve submitted questions. Bill Anderson has been reviewing the questions as they’ve been coming in and I want to answer as many questions as possible within the allotted time remaining. If you don’t get an answer to your question, we will respond with an email shortly.

Billy: Thanks John. At this time we would like to begin our Q/A so if you have a question out there you might submit it by entering it in the box at the bottom of your screen. And now I would like to welcome to the line Dr Bill Anderson. Bill we already have some question in the queue. The first one here we have, what are the approximate delivery times for heat pipes, HiK plates, vapor chambers, and enhanced conduction cards?

Dr. Anderson: Okay for heat pipes and small volume, you are typically talking about three to four weeks after we receipt of order. For HiK plates and vapor chambers, it’s typically eight to ten weeks. And for encapsulated conduction is greater than ten weeks depending on the complexity of the design.

Billy: Can heat pipes be used at temperatures lower than 25 degrees Celsius?

Dr. Anderson: Definitely, the 25 degree C limit is really referring to only the heat pipes with water as a working fluid. Ammonia heat pipes for space craft thermal control operating down to around minus 65, ethanol pipes operate down to about minus 50. And other fluids can operate at lower temperatures well below minus 200 degrees C.

Billy: Here’s another question, what set the minimum survival temperature limit for two-phase systems?

Dr. Anderson: What we have normally seen is people suggest a minus… the specifications we have seen normally say minus 55 degrees C, and so that’s what we specify in this webinar. That’s actually not a real limit, we have had some titanium water heat pipes, [cold-soaked] by NASA down to minus 115 degrees C, and successfully restarted.

Billy: What’s the largest HiK plate that can be fabricated?

Dr. Anderson: As John mentioned we can only go about 20 inches from top to bottom when they are both, you cool the top and bottom… if you are going to have near horizontal system, then you can have a much larger HiK plate. We have fabricated a HiK plate for both water and methanol heat pipes that will strip the inches by 24 inches. The effective thermal conductivity from one end to the other that was 2500 W/m K, so we were well… kind of giving it the range you only see normally with [that end].

Billy: Here is a question from an attendee, what is the heat reduction after application of heat pipe, for example after a 100 W chip with a 180 degrees Fahrenheit?

Dr. Anderson: Really we would have to know what kind of heat fluxes, size of the chips, how far you are transferring stuff, the heat sink and all, because all of those factor in. But typically what you have is, you get heat out spot-cooling with a heat pipe, you will have the temperature drops from whatever the thing is mounted on into the copper… heat pipe envelope and wick, about a 2 degrees C drop along the heat pipe length, and then you have the temperature drop going back to the sink.

I mean if you say those details we can provide more exact information, but generally with the HiK plates and all we’re typically seeing a 22 degrees C drop with just a spot-cooling heat pipe we can get much larger drops.

Billy: Are there other working fluids other than water? And is internal corrosion an issue?

Dr. Anderson: When you are talking about electronics cooling, water is by far the best fluid in terms of it’s physical properties. There are a lot other fluids, like I said you can have ammonia and methanol at lower temperatures. At higher temperatures you can have alkaline metals things going up, to super alloys going up to 1100 degrees C and higher [refractory] metals. So there’s a whole range of working fluids that you select based on your temperature range.

But one heat pipe fluid typically covers up to a 150 degrees C, maybe a little more. In terms of corrosion, we don’t normally see corrosion problems with heat pipe wicks and envelopes that are known to be compatible with a working fluid. People have tested copper water heat pipes, or… [I’ve] tested them for 30 years without problems. I think aluminum ammonia heat pipes have had similar life tests. You can have very,very long life without any maintenance or corrosion issues.

Billy: What impact would phase change materials have on performance of heat pipes or HiK plates?

Dr. Anderson: Okay, when you’re talking about high acceleration type system, where your heat pipe might stop operating momentarily, the phase change materials actually help you, because they give you a bumper when the heat pipe is not operating but keeping the temperature low. In other cases when you are talking about storing something, it really has no effect on heat pipe performance, so.

Billy: What is the minimum diameter of a water heat pipe?

Dr. Anderson: 0.062 inches. 16th of an inch. We can actually in some cases, we can flatten things down to get a little flatter, but that kind of a minimum diameter to be carrying significant amounts of power.

Billy: Here is another question from an attendee, what kind of plating do you put on a plate and how does that affect performance? Is there a large difference in conduction between the use of solder, air, or a thermal grease when using heat pipes in a plate?

Dr. Anderson: When heat pipes are put in a plate, they’re typically soldered in place if there is a real thermal performance limit. If it’s a slightly less one, we will epoxy them. We normally don’t put thermal grease unless it’s something want be making and breaking. But if you want to [inaudible 00:28:40] what the effect is, basically that’s just an additional thermal resistance you have to take into account when you are modeling the system.

Billy: Can you use heat pipes or vapor chambers to transfer heat over a range of temperatures?

Dr. Anderson: Definitely, the heat pipe, for example, water heat pipe, you can have them operating from a roughly 25 degrees C, up to 270 degrees C. The actual operating point that the heat pipe operates at is determined by resist-, the power in, geometry and all going into the heat pipe and then the heat sink conditions. When you have a water heat pipe, it doesn’t automatically operate at 100 degrees C, because that is the atmospheric pressure at which water boils, it can operate over that very wide range.

Billy: We have time for one or two more questions. Here is one. What is the maximum distance that the heat pipe can have between the evaporator and the condenser such as using a fluid of NH3?

Dr. Anderson: Okay. It really depends on your set up, with a water heat pipe, you can go fairly long distances, maybe 20 or 30 inches as long as it’s kind of vertical, much shorter if it’s horizontal. Ammonia heat pipes are typically used in spacecraft with very large grooves, they have very high permeability, you can get them operating over like three meters or so carrying significant power, but they only basically work in space.

Billy: This will be our last question, can you speak at all to any technology focused towards automotive power electronics cooling, for let’s say hybrid cars?

Dr. Anderson: I guess I could just say we are looking at that, but we don’t have any products right now specifically for cars.

Billy: All right, we’ll end it there. That concludes today’s webcast. Again if we did not a get a chance to answer your question today, our sponsors have your questions, and will do their best to address them after today’s presentation.

So thanks to everyone for joining us and thanks again to John Hartenstein and Dr. Bill Anderson. Just a reminder, this webcast will be available on demand at www.techbriefs.com for the next 12 months. Have a great day. Here’s another question, what set the minimum survival temperature limit for two-phase systems?

Dr. Anderson: What we have normally seen is people suggest a minus… the specifications we have seen normally say minus 55 degrees C, and so that’s what we specify in this webinar. That’s actually not a real limit, we have had some titanium water heat pipes, [cold-soaked] by NASA down to minus 115 degrees C, and successfully restarted.

Billy: What’s the largest HiK plate that can be fabricated?

Dr. Anderson: As John mentioned we can only go about 20 inches from top to bottom when they are both, you cool the top and bottom… if you are going to have near horizontal system, then you can have a much larger HiK plate. We have fabricated a HiK plate for both water and methanol heat pipes that will strip the inches by 24 inches. The effective thermal conductivity from one end to the other that was 2500 W/m K, so we were well… kind of giving it the range you only see normally with [that end].

Billy: Here is a question from an attendee, what is the heat reduction after application of heat pipe, for example after a 100 W chip with a 180 degrees Fahrenheit?

Dr. Anderson: Really we would have to know what kind of heat fluxes, size of the chips, how far you are transferring stuff, the heat sink and all, because all of those factor in. But typically what you have is, you get heat out spot-cooling with a heat pipe, you will have the temperature drops from whatever the thing is mounted on into the copper… heat pipe envelope and wick, about a 2 degrees C drop along the heat pipe length, and then you have the temperature drop going back to the sink.

I mean if you say those details we can provide more exact information, but generally with the HiK plates and all we’re typically seeing a 22 degrees C drop with just a spot-cooling heat pipe we can get much larger drops.

Billy: Are there other working fluids other than water? And is internal corrosion an issue?

Dr. Anderson: When you are talking about electronics cooling, water is by far the best fluid in terms of it’s physical properties. There are a lot other fluids, like I said you can have ammonia and methanol at lower temperatures. At higher temperatures you can have alkaline metals things going up, to super alloys going up to 1100 degrees C and higher [refractory] metals. So there’s a whole range of working fluids that you select based on your temperature range.

But one heat pipe fluid typically covers up to a 150 degrees C, maybe a little more. In terms of corrosion, we don’t normally see corrosion problems with heat pipe wicks and envelopes that are known to be compatible with a working fluid. People have tested copper water heat pipes, or… [I’ve] tested them for 30 years without problems. I think aluminum ammonia heat pipes have had similar life tests. You can have very,very long life without any maintenance or corrosion issues.

Billy: What impact would phase change materials have on performance of heat pipes or HiK plates?

Dr. Anderson: Okay, when you’re talking about high acceleration type system, where your heat pipe might stop operating momentarily, the phase change materials actually help you, because they give you a bumper when the heat pipe is not operating but keeping the temperature low. In other cases when you are talking about storing something, it really has no effect on heat pipe performance, so.

Billy: What is the minimum diameter of a water heat pipe?

Dr. Anderson: 0.062 inches. 16th of an inch. We can actually in some cases, we can flatten things down to get a little flatter, but that kind of a minimum diameter to be carrying significant amounts of power.

Billy: Here is another question from an attendee, what kind of plating do you put on a plate and how does that affect performance? Is there a large difference in conduction between the use of solder, air, or a thermal grease when using heat pipes in a plate?

Dr. Anderson: When heat pipes are put in a plate, they’re typically soldered in place if there is a real thermal performance limit. If it’s a slightly less one, we will epoxy them. We normally don’t put thermal grease unless it’s something want be making and breaking. But if you want to [inaudible 00:28:40] what the effect is, basically that’s just an additional thermal resistance you have to take into account when you are modeling the system.

Billy: Can you use heat pipes or vapor chambers to transfer heat over a range of temperatures?

Dr. Anderson: Definitely, the heat pipe, for example, water heat pipe, you can have them operating from a roughly 25 degrees C, up to 270 degrees C. The actual operating point that the heat pipe operates at is determined by resist-, the power in, geometry and all going into the heat pipe and then the heat sink conditions. When you have a water heat pipe, it doesn’t automatically operate at 100 degrees C, because that is the atmospheric pressure at which water boils, it can operate over that very wide range.

Billy: We have time for one or two more questions. Here is one. What is the maximum distance that the heat pipe can have between the evaporator and the condenser such as using a fluid of NH3?

Dr. Anderson: Okay. It really depends on your set up, with a water heat pipe, you can go fairly long distances, maybe 20 or 30 inches as long as it’s kind of vertical, much shorter if it’s horizontal. Ammonia heat pipes are typically used in spacecraft with very large grooves, they have very high permeability, you can get them operating over like three meters or so carrying significant power, but they only basically work in space.

Billy: This will be our last question, can you speak at all to any technology focused towards automotive power electronics cooling, for let’s say hybrid cars?

Dr. Anderson: I guess I could just say we are looking at that, but we don’t have any products right now specifically for cars.

Billy: All right, we’ll end it there. That concludes today’s webcast. Again if we did not a get a chance to answer your question today, our sponsors have your questions, and will do their best to address them after today’s presentation.

So thanks to everyone for joining us and thanks again to John Hartenstine and Dr. Bill Anderson. Just a reminder, this webcast will be available on demand at www.techbriefs.com for the next 12 months. Have a great day.