Engineering Design Guide for Heat Sinks and Heat Pipes

Engineering Design Guide for Heat Sinks and Heat Pipes

Billy: Welcome and thank you for joining us for today’s webcast, Engineer’s Design Guide for Heat Sinks and Heat Pipes. It’s 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 your moderator today.

Our webcast will 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 anytime during the presentation by entering it in 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 will be answered after the webcast. In order to view the presentation properly, please disable any pop-up blockers you may have on your browser.

This time I’d like to introduce our speakers, Bryan Muzyka, sales engineer of the Electronics Products Group at Advanced Cooling Technologies, graduated with a Bachelor of Science in Mechanical Engineering from Penn State University. He worked as research and development engineer for ACTs Aero Space group before taking on his current sales engineering. Bryan has firsthand experience designing, analyzing, integrating heat pipes into real world applications.

Also in the line for our Q&A is Scott Garner. Scott Garner is VP and manager of the EP Group. He has 20 years experience in the field of heat transfer and thermal management. He’s an inventor on 14 patents and has written a variety of technical publications and chaired and co-chaired numerous sessions on thermal management for a variety of professional conferences. His expertise is in working with customers to integrate heat pipes and two-phase heat transfer systems into electronics cooling systems. So this time I like to hand the webcast over to our speaker, Bryan Muzyka. Bryan?

Bryan: Thank you, Billy. As Billy said, this is a design guide for heat sinks and heat pipes. It is kind of a high level design guide, basically a heat pipe and heat sink 101 type course. So if you have any detailed question, I might go more in depth and we’re going to get into a presentation. Please feel free to ask those questions or contact us at a later time and we’ll be happy to get your responses to those.

So with that, I’ll just show a quick presentation overview. Basically, we will start with kind of an overview of the problems from electronics standpoint and talk about the thermal resistance network going from that source electronics to the ambient air. And then we’ll get into kind of our two different sections, which is the first one, heat sink design, and the second one, heat pipe design, and then we’ll kind of tie those together with an example of a case study analyzing a heat sink.

So the quick overview to heat sinks, the primary goal here to make sure that electronics do not exceed their maximum operating temperatures. Typical electronics will have specified maximum case temperatures and these can range anywhere from 70 to 100 degrees C. In air cool systems, which we’ll be talking in length today, the basic parameters require a sufficient amount of volume and delta T or temperature difference from your case temperature to ambient temperature to operate safety.

The typical areas of improvement which we’ll touch on today include the thermal interface material or TIM material, the heat spreading within the heat sink itself, and then the fin design. Here, the slide touches on the thermal resistance network of a heat sink. The blue block labeled 1 there shows a representation of your electronics device. The yellowish orange block, maybe your thermal interface material, and your heat sink is the fin piece above that. And then the area labeled 4 is your ambient air temperature.

So as you can see on the bottom, it gives you a representative thermal resistance network going all the way from the junction of the electronics to the ambient air temperature. The first thermal resistance in there is your thermal resistance from case to junction. This is typically specified by your electronics manufacturer and it’s typically very consistent. So you’ll have a very hard value to go on there. So usually you can design to the case temperature of the electronics.

The second resistance there is the thermal interface material resistance and we’ll get into a couple of rules of thumb there to help you select the proper thermal interface material. The third thermal resistance is the base temperature to the fin. So that’s basically conduction within your metal heat sink and that can vary based on your material properties and other things. We’ll talk about integrating heat pipes in the second session to increase that conductivity and reduce that thermal resistance.

The final thermal resistance is rejecting your heat to ambient air and that’s usually convection or radiation also sends back.

So in the next section we’ll talk about some designs pertinent to heat sinks. The first area we’ll talk about is basically going right down with thermal resistance network, touching on the TIM selection, TIM choices. We’ll then look at volumetric thermal resistance tools that will allow you to properly size your heat sink. And then we’ll look at some fin options, different selections you may have for fins. Finally, we’ll look at a case study that shows some thermal testing. Within that case study, we’ll look at optimizing your fin pitch to better reject the heat to the ambient air.

So the first area to look at is the thermal interface material. TIM materials are used for two primary reasons. The first one is shown in that figure to the right. It’s to reduce the air gaps caused by surface roughness or surface imperfections from your heat sink or your electronics packaging. The second reason is to bridge the CTE gap, though typically an electronics mounting device will be silicon-based and have a very low CTE, which is thermal expansion coefficient. Your heat sink will have a very high CTE. So there is a gap that could cause failure if you directly bond one to another. Therefore, thermal interface material can alleviate the CTE difference, as well as reduce the air gaps within that surface roughness.

Some of the common TIM materials include gap pads, which are basically small foam type pads that compress down between your heat sink and electronics. A thermal grease, which is somewhat of a paste that you can spread on and have the electronics touch the heat sink with. A foil, similar to like an aluminum foil or a graphite foil that can do the same type thing, and a phase change material that can alter phases between solid and liquid for various performance enhancements during liquefaction.

Some of the key areas of TIM performance include bond line thickness and thermal conductivity. Typically, what you’ll see is that you can select it off based on highest thermal conductivity. But oftentimes, if you have thicker bond line, that thermal conductivity enhancement does not pay off as well as if you had a lower thermal conductivity and a thinner bond line. So it’s kind of a tradeoff there to look at and decide which is your best foot forward is.

Now, the next area we’ll touch on is the volumetric thermal resistance calculation. This is a good first tool to sizing an overall heat sink. So airflow condition will basically determine what your overall volumetric thermal resistance is. From the required delta T, which you know because you know your maximum electronics temperature and you know your ambient temperature, you can come up with a given volume. So you can see the equation there. The volume is equal to the power times the thermal resistance divided by the temperature difference in your system.

The chart you see on the bottom is fairly reliable published data for volumetric thermal resistances and you can see there going from natural convection to any type of system that has airflow is a big drop in volumetric thermal resistance. So the addition of fans or blowers can greatly reduce your overall size of your heat sink required.

This next slide talks about some of the different fin options that engineers can consider when designing a heat sink. Each fin option has various different benefits and drawbacks. We’re just going to touch on a couple of the driving benefits to each design. Machined fins is a very good quick easy turnaround type of product. You can design them, you can send them to machine shops, have them manufactured very quickly with low, nonrecurring cost. The second item there is extruded fins. They will have volume cost benefits. They typically have an NRE charge with them, but as volume, they make sense to be looked out for a heat sink design.

Bonded fins, typically high performance and design flexibility. So some aspect ratios that might not be possible with extruded or machine fins can be possible with a bonded type fin. Folded fins, an example of folded fin, you can see in that middle picture to the right, the big enhancement there is additional surface area for improved performance. Another option is pin fins, which is the figure to the bottom. The big enhancement with pin fins is that you can have variable flow directions. So no matter if your airflow is coming in from the top to the bottom or from the left to the right in that figure, your fin will allow for good dissipation.

So now we’ll talk about a quick case study looking at a heat sink design. In this case, we had 150 watts that needed to be dissipated. It was 150 watts, which is a representative of one electronics chip and it was a natural convection setting. So the room temperature ambient was about 23 degrees and we selected an off the shelf heat sink sized about 7 inches by 2.8 inches by 12 inches. The fin pitch in this case was 8 millimeters, basically for availability and for a nice demonstration piece.

So looking at the next slide, what we did was took that heat sink and we tested it with a FLIR IR camera, which captured the temperature profile along the entire heat sink. Now what we show here is an 87-degree temperature at the electronics, which is a rise of about 64 degrees from ambient temperature. We estimated the certain amount of temperature rise in other areas including the thermal interface and conduction and what we did is we backed out a thermal resistance that you can see there, about 1,300 to 1,600 than a mere cube degree C per watt. What this shows is because it’s higher then the natural convection, volumetric resistance we showed a couple of slides ago, it’s showing that your fins are not very optimized for the natural convection setting.

So in this next slide, what we want to show is how fin pitch and thickness are important design characteristics to optimal heat transfer performance. So we took the original design point of 8-millimeter fin pitch and we optimized it. You can see kind of the heat sink design curve. Typically, in natural convection settings, fin spaced further apart, project heat better. Because if you have fins too tightly packed, they tend to choke flow, which causes large temperature rises. You can see, the optimal here was a little over 13 millimeter fin pitch. If you were to use that as opposed to the one that we showed with an 8-millimeter fin pitch, you’re getting about 10 degrees savings. So pretty significant savings just by sizing your fins correctly.

But now, we want to show what happens if your fins are sized correctly and you’re still not getting to the performance you needed. So here, what we’re showing is a pretty concentrated load in an extruded fin stack. This was a fin stack for a single IGBT mounted to it. With IGBTs, you have a very concentrated load, high powers going into that heat sink. What you had was a hot spot right in the center and it wasn’t able to spread the heat out to the outside fins to reduce the temperature enough to a safe operating temperature. So that transitions us into the second part of the presentation, which goes into heat pipe design and how to improve that conduction gradient.

Here we’ll give an overview of heat pipes and performance predictions there. We’ll talk about designing and modeling with heat pipes. We’ll look at the thermal conductivity enhancements you can have in relation to your heat sink design. Then we’ll revisit the case study with increased thermal conductivity and we’ll also look at ways to reduce weight and optimize performance with heat pipes.

So real quick, back to that extruded heat sink for the IGBT, the figure on the left uses a bare aluminum, while the figure on the right shows the enhancements that we could have with heat pipes. Basically improved conduction and spreading out of that concentrated load and it allows your fins to operate more efficiently and basically reduce your operating temperature of your electronics.

So how is this accomplished? The big advantage to a heat pipe is that it’s still a passive system. Going from a natural convection heat sink to a heat pipe heat sink, you’re not adding any additional power like you would with a fan. You’re just getting improved performance based on increased conduction rate. So the passiveness is because it’s a fields closed loop device, what happens internally is that you place your evaporator in by the electronics and it vaporizes an internal working fluid. For most systems, that working fluid is water because water has good thermal properties and good surface tension properties. The working fluid can vary based on your temperature requirements. You vaporize the fluid at the evaporator end. It forms an internal pressure gradient that spreads the vapor to the opposing end.

At the opposing end what you’re typically connected to like your fin side, you would condense that fluid back into a liquid form and it would be absorbed into the wick structure. The wick structure would them pump that fluid using a capillary force back to the evaporator. The wick structure is typically a fine, poor radius material. It could be a thinner powder, it could be a simple groove, or it could be some sort of a screen mesh and basically it creates that capillary force that drives that liquid back, similar to dipping a paper towel into a glass of water and watching the fluid weak up with the paper towel. So very similar principles, but it allows for you to have passive liquid return and operate very efficiently.

So when in your system, a heat pipe will typically have somewhere between a two to five degree temperature difference across the length of the pipe, which at pretty much any length, will give you increased performance to a bulk metal. So the big advantages of a heat pipe are, as I mentioned, the effective thermal conductivity. This can range anywhere from 10,000 for a short 6-inch pipe to 200,000 for some of the longer pipes that are used in things like satellite systems on orbiting space. The second big benefit is that they’re passive so they’re not adding any additional power or costlier system and it provides good isothermality characteristics so they keep things at very constant temperatures.

Heat pipes are governed by several limitations. The biggest limit we want to point out from a design perspective is the capillary limit. For terrestrial systems, the capillary limit is typically the first limit you’ll hit and this is basically the wick’s ability to overcome the various pressure drops in the system. So your wick structure will form a capillary pressure and that pressure must be higher than your pressure drops, which include the vapor pressure drop, the liquid return pressure drop, and then the pressure drop from gravity. So when we design heat pipes, we’ll look at the worst case orientation and we’ll design accordingly. Some of the tools you can use is the heat pipes calculator, which we’ll show in the next slide. Then we’ll get into kind of design guide on how you can size and fit heat pipes into your system.

So getting into the design guide, there’s basically three main steps to designing heat pipes within your system. The first step is to assure that heat pipes can move the total power for your design requirements. This will input size, orientation and various things and it will output kind of the capillary curve. One of the tools that ACT has created to help with that is an online heat pipe calculator where design engineers can actually go in and calculate these capillary limits and then take the appropriate amount of heat pipes and size of heat pipes that they want for their system.

The second step is fitting heat sink with your system. We have a design guide online that talks about bending, flattening, and various integration techniques. Then the third technique is performance prediction. If you want to predict the performance of your system, we have some modeling guidelines in here, as well as online, that can help engineers model heat pipes for their various system parameters.

This is a screen shot of what our calculator will output. The items you see in red, which are evaporated length, condenser length, and then orientation, which is the amount of the length against gravity the heat pipe will operate at. Basically, it will allow for the curve of various diameter heat pipes and the capillary limits associated with that curve.

So if you know your operating temperature limit, in this case we are looking at something in between 60 and 80 degrees, you can see what size heat pipes you would need for various powers. In this case, we’re looking at about a 50-watt transport requirement and we would size somewhere above that, maybe a 6-millimeter or a quarter inch heat pipe.

The next slide is the heat pipe design guide. This talks about some standard heat pipe sizes and some bending and flattening rules of thumb. The bending rule of thumb that we typically use is to bend no tighter than three times the outside diameter of the heat pipe. You can go slightly tighter than that. However, we’ll eventually see some crinkling in the copper.

Flattening profile, typically you don’t want to go any flatter than two-thirds times the outside diameter. There you are kind of changing the shape of the heat pipe which will affect the vapor space internally. So a good rule of thumb is two-thirds times the outside diameter. Some of the integration techniques that are used with heat pipes are primarily solder and epoxy. Epoxy is a nice, quick, and easy way to integrate. Solder is often a better joint and thermally a better joint as well. Some of the items for reliability, you can see there, we have some reliability guides online that talk about various shock and vibration and different performance parameters that you can look at on your own time.

So here we have some basic modeling techniques. One of the easiest modeling techniques is to assume heat pipes are a solid element and then increase or decrease the conductivity until you get a two to five-degree temperature difference across the length. Typically a good starting point is about 10,000 and then adjust accordingly. This is a nice, easy way for engineers to model heat pipes in their system.

The next slide here just talks about high cape plates. This is an ACT term for high conductivity, which include embedded heat pipes into the plate. This is where we’ll look at a lot of different scenarios to increase base pipe conductivity. One of the areas we’ve improved on here is the plate thickness. As we’ll show later, this is a good way to reduce overall weight of your system. But a high cape plate, which includes heat pipes and solder, is basically the same weight as a bulk aluminum plate of the same size and it typically has a very similar structural strength so you’re not adding any weight or decreasing any strength. You’re just increasing the overall conductivity of your plate.

That plate to the bottom right there, you can see in the next slide, a thermal profile of that plate. Basically by embedding heat pipes in there, you are able to reduce the temperature by about 20 degrees in that plate, so very significant reduction in temperature and has a fairly high power application.

One of the other modeling techniques we had in this slide, that last bullet point there is if you’re modeling high cape plates, instead of modeling each individual heat pipe, one good way to look at it is just using a bulk conductivity of 600 watts per meter-K. That’s a good rough estimate, but it also can give you a very good result as a first iteration approach.

Now we’re going to look at that case study one more time. After optimizing the fin pitch, you might not still been there as far as your performance. So we looked at here was just doing exactly what I said last slide and increasing that base plate conductivity to 600 watts per meter-K.

By doing that, we showed a 14% improvement over the baseline aluminum. We want to show here is that oftentimes, thermal performances, the driver and the design, but we want to get into next is what happens if weight is the overall driving force and your aluminum heat sink was operating okay, it was just too heavy.

So here what we did is we used some of our in-house tools to design a heat pipe heat sink that would operate similar to the aluminum heat sink we talked about earlier, but reduce the weight. So we sized that heat sink. We’re able to cut about three-tenths of an inch off the fin height and off the base height and we’re able to reduce the overall length by about 1.7 inches. Overall, that was nearly a two-pound reduction in weight.

This slide has the testing results, basically going back to that test we ran with the 12-inch length aluminum heat sink and then we built that heat sink I showed last slide with the embedded heat pipes and then reduced dimension and we tested it and you can see that. It has identical thermal performance as far as the electronics temperature goes. You could see that there’s not as much of a thermal gradient because of the improved spreading, but that just increased the fin efficiency and was able to hold your electronics temperature at the same temperature.

So the conclusions in wrapping up, proper heat sink design is necessary for efficient air cool systems. One of the takeaways is a quick tool using volumetric thermal resistance is readily available and good published data and that could be used very easily. One of the biggest steps is fin pitch optimization. That can provide significant improvements if you’re seeing some sort of choked airflow or not getting to those published results. The next area is that heat pipes can offer extreme heat transfer improvements. If you’re having some conduction-related issues, they can often offer three to four times the thermal conductivity of their aluminum.

The takeaways now after watching this presentation, you should have the tool to size the proper heat sink and you should also have enough design guides to size and rally your heat pipes within your system and also do some basic modeling and performance predictions. So last slide, I’ll just have some references. We referenced a couple Electronics Cooling articles. At this time, we want to thank you for your time and we would be happy to answer any questions.

Billy: Thanks, Bryan. This time, we begin our Q&A, I’d like to welcome to the line Scott Garner, VP and manager of the Electronics Product Group at Advanced Cooling Technologies. We have a few questions already in. The first question, “Can you define bond line thickness?”

Scott: Bond line thickness is the especially the effective thickness of that interface material, whether it’s a gap pad or thermal grease. Bryan talked about the tradeoff of conductivity and bond line thickness. Obviously you want to tie a conductivity and as thin a bond line or as thin a gap of interface material as you can achieve, but they typically are fairly viscous materials. So that bond line thickness is the thickness of that interface material and it can be controlled by the amount of TIM material that you put down, the pressure and the temperature of which are components attached to your heat sink.

Billy: This question came in about midway through the presentation, “What was being held constant as the fin spacing was varied?”

Scott: This was a natural convection analysis so there wasn’t any airflow velocity or pressure drop requirement to hold constant. It was just a natural convection heat sink. So we varied the fin pitch and kept the fin thickness constant in that analysis. As you can see from that curve, in this case, many times you want to add more surface area and more fins to dissipate heat load. But in natural convection, if you get those fins too close, you choke the air flow and it doesn’t allow air to flow over you heat sink. So in this case, going to a wider pitch helped increase the airflow. We can do the same analysis with force convection. In that case, we would typically hold the fin curve constant.

So we don’t hold velocity or pressure drop but we actually input the fin curve into the model and then we can take a look at similar plots for fin pitch and fin thickness to select the optimum fin for that given geometry and selected fin.

Billy: This will have to be our last question. It’s from an attendee here. “Have you used heat pipes along with thermal electric generators? If so, what specific advantage has it offered?”

Scott: Yeah. We have done that quite a bit on both sides of the thermal electrics, both were generation and for cooling. So a thermal electronic generator, you input electronic power and force temperature across it or if you maintain it to delta T across it, it will output some electrical power. We use them in some government-funded R&D programs for waste heat recovery and integrated them with heat pipes. Again, we can put heat pipes heat sinks on either side either picking up heat, in that case, an exhaust stream and focusing on the small surface area thermal electric.

Also then on the back side, you need to dissipate that heat from the thermal electric and again, utilizing heat pipes to get to a much larger surface area than is available just off the contact area the thermal electronic itself. Many times, you can put a thermal electric directly on a device to maintain temperature control. Again, there that heat that needs to dissipated from the back side of the thermal electric, and standard heat sinks, as we talked about, or heat pipe enhanced heat sinks can help dissipate that heat because you do need a much larger surface area to dissipate it to the air typically.

Billy: All right. We’ll have to end it there. That concludes today’s webcast. Again, if we didn’t have the chance to answer your questions, our sponsors will do their best to address them after today’s presentation. Thanks to Bryan Muzyka, Scott Garner, and everyone for joining us. Just a reminder, that this webcast will be available on demand at www.techbriefs.com for the next 12 months. Have a great day.