Tag Archives: ATS

Utilizing Fans in Thermal Management of Electronics Systems

Fans in Thermal Management

There are different types of fans that are used in thermal management of electronics with tube axial fans being the most common. (Wikimedia Commons)


The ongoing trend in the electronics industry is for increasingly high-powered components to meet the ever-growing demands of consumers. Coupled with greater component-density in smaller packages, thermal management is more and more of a priority to ensure performance and reliability over the life of an electronics system.

As thermal needs have grown, engineers have sought out different cooling methods to supplement convection cooling. While options such as liquid cooling have grown in popularity in recent years, still one of the most common techniques is to add fans to a system.

Through the years, fan designs have improved. Fan blades have been streamlined to produce great flow rate with less noise and fans have become more power-efficient to meet the desires of customers trying to use less resources and save costs.

While much has changed in the presentation of fans, there are many basic concepts that engineers must consider when deciding how to implement fans in a project.

This is part one of a two-part series on how to select the best fan for a project. Part one will cover the types of fans that can be used. Part two, which can be found at https://www.qats.com/cms/2017/03/10/analysis-of-fan-curves-and-fan-laws-in-thermal-management-electronics, will cover fan laws and analyzing fan curves.

COMMON TYPES OF FANS AND BLOWERS

As described by Mike Turner of Comair Rotron in an article for Electronics Cooling Magazine, “All You Need to Know About Fans,” fans are essentially low pressure air pumps that take power from a motor to “output a volumetric flow of air at a given pressure.” He continued, “A propeller converts torque from the motor to increase static pressure across the fan rotor and to increase the kinetic energy of the air particles.”

In a white paper from Advanced Thermal Solutions, Inc. (ATS) entitled, “Performance Difference Between Fans and Blowers and Their Implementation,” it was added that fans are at their core, dynamic pumps. The article added, that in dynamic pumps “the fluid increases momentum while moving through open passages and then converts its high velocity to a pressure increase by exiting into a diffuser section.”

The biggest difference between a fan and a blower is the direction in which the air is delivered. Fans push air in a direction that is parallel to the fan blade axis, while blowers move air perpendicular to the blower axis. Turner noted that fans “can be designed to deliver a high flow rate, but tend to work against low pressure” and blowers move air at a “relatively low flow rate, but against high pressure.”

The three types of fans are centrifugal, propeller, tube axial, and vane axial:

• In centrifugal fans, the air flows into the housing and turns 90 degrees while accelerating due to centrifugal forces before being flowing out of the fan blades and exiting the housing.
• Propeller fans are the simplest form of a fan with only a motor and propellers and no housing.
• Tube axial fans, according to Turner, are similar to a propeller fan but “also has a venture around the propeller to reduce the vortices.”
• Vane axial fans have vanes trailing behind the propeller to straighten the swirling air as it is accelerated.

The most common fans used in electronics cooling are tube axial fans and there are a number of manufacturers creating options for engineers. A quick search of Digi-Key Electronics, offered options such as Sunon, Orion Fans, Sanyo Denki, NMB Technologies, Delta Electronics, Jameco Electronics, and several more.

Fans in Thermal Management

A fan is added to a heat sink on a PCB in order to increase the air flow and heat dissipation from the board component. (Advanced Thermal Solutions, Inc.)

FACTORS TO CONSIDER WHEN PICKING A FAN

When selecting a fan, engineers must consider the specific requirements of the system in which they are working, including factors such as the necessary airflow and the size restrictions of the board or the chassis. These basic factors will allow engineers to search through the many available options to find a fan that fits his or her needs.

In addition, engineers may look towards combining multiple fans in parallel or in a series to increase the flow rate across the components without increasing the size of the package or the diameter of the fan.

Parallel operation means having two or more fans side-by-side. When two fans are working in parallel, then the volume flow rate will be increased, even doubled when the fans are operating at maximum. Turner added. “The best results for parallel fans are achieved in systems with low resistance.”

In a series, the fans are stacked on top of each other and results in increased static pressure. Unlike parallel operations, fans in a series work best in a system with high resistance.

The ATS white paper noted, “In real situations, the fans may interfere with each other. The end results is a lower than expected performance.” Turner warns that in either parallel or series configurations there is a point in the combined performance curve that should be avoided because it creates unstable and unpredictable performance, but analyzing fan performance and fan curves will be covered in more detail in part two of the blog.

Efficiency is a major factor when selecting a fan. As noted in an article from Qpedia Thermal eMagazine, “A large data center contains about 400,000 servers and consumes 250 MW of power. It has been estimated that about 20% of the total power supplied to a high end server is consumed by fans.”

Clearly, finding a fan that can work efficiently with lower power will save a considerable about of resources. The article details several methods for creating efficiency in designing a system that includes fans:

“Fan power consumption is traditionally reduced by controlling the motor speed to produce only the airflow required for adequate cooling, rather than operating continuously at full speed. Significant energy savings can be achieved beyond this technique through fan efficiency increase. Optimizing the motor and electronic driver, increasing fan aerodynamic efficiency through careful redesign, and optimizing fan-system integration are three ways of achieving this.”

Read more about the techniques for achieving efficiency at https://www.qats.com/cms/wp-content/uploads/2015/03/Designing_Efficient_Fans_for_Electronics_Cooling
_Applications.pdf
.

CLICK HERE FOR PART II.

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Cold Plates and Recirculating Chillers for Liquid Cooling Systems

Recirculating Chillers

ATS cold plates and recirculating chillers can be used in closed loop liquid cooling systems for high-powered electronics. (Advanced Thermal Solutions, Inc.)


The miniaturization of high-powered electronics and the requisite component density that entails have led engineers to explore new cooling methods of increasing complexity. As a result, there is a growing trend in thermal management of electronics to explore more liquid cooling systems and the reintroduction, and re-imagining, of cold plate technology, which has a long history that includes its use on the Apollo 11 space shuttle.i

Thermal management of high-powered electronics is a critical component of a design process. Ensuring the proper cooling of a device optimizes its performance and extends MTBF. In order for a system to work properly, engineers need to establish its thermal parameters from the system down to the junction temperature of the hottest devices. The use of cold plates in closed loop liquid cooling systems has become a common and successful means to insure those temperatures are managed.

Cold plate technology has come a long way since the 1960s. At their most basic level, they are metal blocks (generally aluminum or copper) that have inlets and outlets and internal tubing to allow liquid coolant to flow through. Cold plates are placed on top of a component that requires cooling, absorbing and dissipating the heat from the component to the liquid that is then cycled through the system.

In recent years, there have been many developments in cold plate technology, including the use of microchannels to lower thermal resistanceii or the inclusion of nanofluids in the liquid cooling loop to improve its heat transfer capabilities.iii

An article from the October 2007 issue of Qpedia Thermal eMagazine detailed the basic components of a closed loop liquid cooling system, including:

• A cold plate or liquid block to absorb and transfer the heat from the source
• A pump to circulate the fluid in the system
• A heat exchanger to transfer heat from the liquid to the air
• A radiator fan to remove the heat in then liquid-to-air heat exchanger

The article continued, “Because of the large surface involved, coldplate applications at the board level have been straight forward…Design efforts for external coldplates to be used at the component level have greatly exceeded those for PCB level coldplates.”

Exploring liquid cooling loops at the board or the component level, according to the author, requires an examination of the heat load and junction temperature requirements and ensuring that air cooling will not suffice to meet those thermal needs.iv

To read the full article on “Closed Loop Liquid Cooling for High-Powered Electronics,” click http://coolingzone.com/blog/wp-content/uploads/2017/01/Qpedia_Oct07_Closed_Loop_liquid_cooling_
for_high_power_electronics.pdf
.

Chillers provide additional support for liquid cooling loops

In order to increase the effectiveness of the cold plate and of the liquid cooling loop, recirculating chillers can be added to condition the coolant before it heads back into the cold plate. The standard refrigeration cycle of recirculating chillers is displayed below in Fig. 1.

Chiller,s Cold Plates

Fig. 1. The standard refrigeration cycle for recirculating chillers. (Adavanced Thermal Solutions, Inc.)

Several companies have introduced recirculating chillers to the market in recent years, including ThermoFisher, PolyScience, Laird, Lytron, and Advanced Thermal Solutions, Inc. (ATS). Each of the chiller lines has similarities but also unique features that fit different applications.

In order to select the right chiller, Process-Cooling.com warns that it is important to avoid “sticker shock” because of testing conditions that are ideal rather than based on real-world applications. The site suggests a safety factor of as much as 25 percent on temperature ranges to account for environmental losses and to ensure adequate cooling capacity.v

The site also noted the importance of speaking with manufacturers about the cooling capacity that is needed, the required temperature range, the heat load of the application, the length and size of the pipe/tubing, and any elevation changes.

“Look for a chiller with an internal pump-pressure adjustment,” the article stated. “This feature enables the operator to dial down the external supply pressure to a level that is acceptable for the application. Because the remaining flow diverts internally into the chiller bath tank, no damage will result to the chiller pump or the external application.”

When trying to decide on the right size chiller for your particular application, there are several formulas that can help make the process easier. Bob Casto of Cold Shot Chillers, writing for CoolingBestPractices.com, gave one calculation for industrial operations. First, determine the change in temperature (ΔT), then the BTU/hour (Gallons per hour X 8.33 X ΔT), then calculate the tons of cooling ([BTU/hr]/12,000), and finally oversize by 20 percent (Tons X 1.20).vi

Not every application will require industrial capacity, so for smaller, more portable chillers, Julabo.com had a secondary calculation for required capacity (Q).

Q=[(rV cp)material+(rV cp)bath fluid]ΔT/t

In the above equation, r equals density, V equals volume, cp equals constant-pressure specific heat, ΔT equals the change in temperature, and t equals time. “Typically, a safety factor of 20-30% extra cooling capacity is specified for the chilling system,” the article continued. “This extra cooling capacity should be calculated for the lowest temperature required in the process or application.”vii

Comparison of Industry Standard Recirculating Chillers

Recirculating Chillers

Applications for liquid cooling systems with chillers

Recirculating chillers offer liquid cooling loops precise temperature control and coupled with cold plates can dissipate a large amount of heat from a component or system. This makes chillers (and liquid cooling loops in general) useful to a wide range of applications, including applications with demanding requirements for temperature range, reliability, and consistency.

Chillers have been part of liquid cooling systems for high-powered lasers for a number of years to ensure proper output wavelength and optimal power.viiiix To ensure optimal performance, it is important to consider safety features, such as the automatic shut-off on the ATS-Chill 150V that protects against over-pressure and compressor overload. Other laser-related applications include but are not limited to Deep draw presses, EDM, Grinding, Induction heating and ovens, Metallurgy, Polishing, Spindles, Thermal spray, and Welding.x

Machine hydraulics cooling and semiconductors also benefit from the inclusion of chillers in liquid cooling loops. Applications include CVD/PVD, Etch/Ashing, Wet Etch, Implant, Inductively Coupled Plasma and Atomic Absorption Spectrometry (ICP/AA), Lithography, Mass Spectroscopy (MS), Crystal Growing, Cutting/Dicing, Die Packaging and Die Testing, and Polishing/Grinding.xi

One of the most prominent applications for liquid cooling, heat exchangers, cold plates, and chillers is in medical equipment. As outlined in an ATS case study,xii medical diagnostic and laboratory equipment requires cyclic temperature demands and precise repeatability, as well as providing comfort for patients. For Harvard Medical School, ATS engineers needed to design a system that could maintain a temperature of -70°C for more than six hours. Using a cold plate with a liquid cooling loop that included a heat exchanger, the engineers were able to successfully meet the system requirements.

Liquid cooling with chillers are also being used for medical imaging equipment and biotechnology testing in order to provide accurate results. ATS CEO and President Dr. Kaveh Azar will discuss the “Thermal Management of Medical Electronics” in a free webinar on Jan. 26 at 2 p.m. For more information or to register for the webinar, click https://www.qats.com/Training/Webinars.

Conclusion

Closed loop liquid cooling systems are not new but are gaining in popularity as heat dissipation demands continue to rise. Using cold plate technology with recirculating chillers, such as the ATS-Chill150V, ATS-Chill300V, and the ATS-Chill600V, to condition the coolant in the system can offer enhanced heat transfer capability.

Portable and easy to use, ATS vapor compression chillers are air-cooled to eliminate costly water-cooling circuits and feature a front LED display panel that allows users to keep track of pressure drop between inlet and outlet and the coolant level. They each use a PID controller.

Recirculating Chillers

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

References
i http://history.nasa.gov/SP-287/ch1.htm
ii https://heatsinks.files.wordpress.com/2010/03/qpedia_0309_web.pdf#page=12
iii http://www.sciencedirect.com/science/article/pii/S0142727X99000673
iv https://www.qats.com/cpanel/UploadedPdf/Qpedia_Thermal_eMagazine_0610_V2_lorez1.pdf#page=16
v http://www.process-cooling.com/articles/87261-chillers-evaluation-and-analysis-keys-to-selecting-a-winning-chiller?v=preview
vi http://www.coolingbestpractices.com/industries/plastics-and-rubber/5-sizing-steps-chillers-plastic-process-cooling
vii http://www.julabo.com/us/blog/2016/sizing-a-cooling-system-control-temperature-process-heating-operations
viii http://www.laserfocusworld.com/articles/print/volume-37/issue-6/features/instruments-accessories/keeping-your-laser-cool0151selecting-a-chiller.html
ix https://www.electrooptics.com/feature/keeping-it-cool
x http://www.lytron.com/Industries/Laser-Cooling
xi http://www.lytron.com/Industries/Semiconductor-Cooling
xii https://www.qats.com/cms/2016/10/04/case-study-thermal-management-harvard-medical-school-tissue-analysis-instrumentation/

ATS holding webinar on Thermal Management of Medical Electronics

Medical Webinar

DR. Kaveh Azar, founder, CEO and President of Advanced Thermal Solutions, Inc. (ATS), will present a free webinar on “Thermal Management in Medical Electronics” on Dec. 15, 2016.

On Thursday, Jan. 26, Advanced Thermal Solutions, Inc. (ATS) will host a free, online webinar on “Thermal Management of Medical Electronics”. The hour-long webinar will begin at 2:00 p.m. and there will be 30 minutes of question and answer time after its completion.

The webinar will be presented by thermal management expert Dr. Kaveh Azar, the CEO, President and founder of ATS. Dr. Azar will speak about the unique challenges that are present in finding a thermal solution for medical electronics and the importance of including thermal management in the design process.

The object of all thermal management is to ensure that the device junction temperature, the hottest point on a semiconductor, stays below a set limit. While this is true for all electronic systems, medical electronics pose unique thermal challenges that have to be overcome to meet the junction temperature requirements.

Medical electronics could have stringent material selection. For example, copper is a common metal chosen in thermal management, but can cause irritation or a neurodegenerative condition for patients and has to be used carefully. In addition, medical electronics may have spatial constraints, such as forceps that have only 2-4 millimeters of width, which is a constrained space with very little airflow.

Other challenges presented by medical electronics include the need for constant, reliable repeatability; temperature reliability within a range; and in some cases specific FDA requirements.

Dr. Azar will address each of these issues and more. To register for the free webinar on Thursday, Jan. 26, visit http://www.qats.com/Training/Webinars.

Discussion of Thermal Solution for Stratix 10 FPGA

An Advanced Thermal Solutions, Inc. (ATS) client was planning on upgrading an existing board by adding Altera’s high-powered Stratix 10 FPGAs, with estimates of as many as 90 watts of power being dissipated by two of the components and 40 watts from a third. The client was using ATS heat sinks on the original iteration of the board and wanted ATS to test whether or not the same heat sinks would work with higher power demands.

In the end, the original heat sinks proved to be effective and lowered the case temperature below the required maximum. Through a combination of analytical modeling and CFD simulations, ATS was able to demonstrate that the heat sinks would be able to cool the new, more powerful components.

ATS Field Application Engineer Vineet Barot recently spoke with Marketing Director John O’Day and Marketing Communications Specialist Josh Perry about the process he undertook to meet the requirements of the client and to test the heat sinks under these new conditions.

JP: Thanks again for sitting down with us to talk about the project Vineet. What was the challenge that this client presented to us?
VB: They had a previous-generation PCB on which they were using ATS heat sinks, ATS 1634-C2-R1, and they wanted to know if they switched to the next-gen design with three Altera Stratix 10 FPGAs, two of them being relatively high-powered, could they still use the same heat sinks?

Stratix 10 FPGA

The board that was given to ATS engineers to determine whether the original ATS heat sinks would be effective with new, high-powered Stratix 10 FPGA from Altera. (Advanced Thermal Solutions, Inc.)

They don’t even know what the power of the FPGAs is exactly, but they gave us these parameters: 40°C ambient with the junction temperatures to be no more than 100°C. Even though the initial package is capable of going higher, they wanted this limit. That translates to a 90°C case temperature. You have the silicon chip, the actual component with the gates and everything, and you have a package that puts all that together and there’s typically a thermal path that it follows to the lid that has either metal or plastic. So, there’s some amount of temperature lost from the junction to the case.

The resistance is constant so you know for any given power what the max will be. The power that they wanted for FPGAs 1 and 2, which are down at the bottom, was 90 watts, again this is an estimate, and the third one was 40 watts.

JP: How did you get started working towards a solution?
VB: Immediately we tried to identify the worst-case scenario. Overall the board lay-out is pretty well done because you have nice, linear flow. The fans are relatively powerful, lots of good flow going through there. It’s a well-designed board and they wanted to know what we could do with it.

I said, let’s start with the heat sinks that you’re already using, which are the 1634s, and then go from there. Here are the fan specs. They wanted to use the most powerful fan here in this top curve here. This is flow rate versus pressure. The more pressure you have in front of a fan, the slower it can pump out the air and this is the curve that determines that.

Stratix 10 FPGA

Fan operating points on the board, determined by CFD simulations. (Advanced Thermal Solutions, Inc.)

This little area here is sometime called the knee of the fan curve. Let’s say we’re in this area, the flow rate and pressure is relatively linear, so if I increase my pressure, if I put my hand in front of the fan, the flow rate goes down. If I have no pressure, I have my maximum flow rate. If I increase my pressure then the flow rate goes down. What happens in this part, the same thing. In the knee, a slight increase in pressure, so from .59 to .63, reduces the flow rate quite a bit.

Stratix 10 FPGA

CFD simulations showed that the fans were operating in the “knee” where it is hard to judge the impact of pressure changes on flow rate and vice versa. (Advanced Thermal Solutions, Inc.)

So, for a 0.1 difference in flow rate (in cubic meters per second) it took 0.4 inches of water pressure difference, whereas here for a 0.1 difference in flow rate it only took a .04 increase in pressure. That’s why there’s a circle there. It’s a danger area because if you’re in that range it gets harder to predict what the flow will be because any pressure-change, any dust build-up, any change in estimated open area might change your flow rate.

The 1634 is what they were using previously. It’s a copper heat pipe, straight-fin, mounted with a hardware kit and a backing plate that they have. It’s a custom heat sink that we made for them and actually the next –gen, C2-R1, we also made for them for the previous-gen of their board, they originally wanted us to add heat pipes to this copper heat sink, but I took the latest version and said, let’s see what this one will do. For the third heat sink, I went and did some analytical modeling to see what kind of requirement would be needed and I chose one of our off-the-shelf pushPIN™ heat sinks to work because it was 40 watts.

JO: Is the push pin heat sink down flow from the 1634, so it’s getting preheated air?
VB: Yes. This is a pull system, so the air is going out towards the fans.

Stratix 10 FPGA

CFD simulations done with FloTherm, which uses a recto-linear grid. (Advanced Thermal Solutions, Inc.)

This is the CFD modeling that ATS thermal engineer Sridevi Iyengar did in FloTherm. This is a big board. There are a lot of different nodes, a lot of different cells and FloTherm uses recto-linear grids to avoid waviness. You can change the shape of the lines depending on where you need to be. Sri’s also really good at modeling. She was able to turn it around in a day.

Stratix 10 FPGA

Flow vectors at the cut plane, as determined by CFD simulations. (Advanced Thermal Solutions, Inc.)

These are the different fans and she pointed out what the different fan operating curves. Within this curve, she’s able to point out where the different fans are and she’s pointing out that fan 5 is operating around the knee. If you look at all the different fans they all operate around this are, which is not the best area to operate around. You want to operate down here so that you have a lot of flow. If you look at the case temperatures, remember the max was 90°C, we’re at 75°C. We’re 15°C below, 15° margin of error. This was a push pin heat sink on this one up here and 1634s on the high-powered FPGAs down here.

Stratix 10 FPGA

JP: Was there more analysis that you did before deciding the original heat sinks were the solution?
VB: I calculated analytical models using the flow and the fan operating curves from CFD because it’s relatively hard to predict what the flow is going to be. Using that flow and doing a thermal analysis using HSM (heat sink modeling tool), we were within five percent. What Sri simulated with FloTherm was if a copper heat sink with the heat pipe was working super well, let’s try copper without the heat pipe and you can see the temperature increased from 74° to 76°C here, still way under the case temperature. Aluminum with the heat pipe was 77°; aluminum without the heat pipe was 81°, so you’re still under.

Basically there were enough margins for error, so you could go to smaller fans because there’s some concern about operating in the knee region, or you can downgrade the heat sink if the customer wanted. We presented this and they were very happy with the results. They weren’t super worried about operating in the knee region because there’s going to be some other things that might shift the curve a little bit and they didn’t want to downgrade the heat sink because of the power being dissipated.

Stratix 10 FPGA

Final case temperatures determined by CFD simulations and backed up by analytical modeling. (Advanced Thermal Solutions, Inc.)

JO: What were some of the challenges in this design work that surprised you?
VB: The biggest challenges were translating their board into a board that’s workable for CFD. It’s tricky to simplify it without really removing all of the details. We had to decide what are the details that are important that we need to simulate. The single board computer and power supply, this relatively complex looking piece here with the heat sink, and we simplified that into one dummy heat sink to sort of see if it’s going to get too hot. It all comes with it, so we didn’t have to work on it.

The power supply is even harder, so I didn’t put it in there because I didn’t know what power it would be, didn’t know how hot it would be. I put a dummy component in there to make sure it doesn’t affect the air flow too much but that it does have some effect so you can see the pressure drop from it but thermally it’s not going to affect anything.

JO: It really shows that we know how to cool Stratix FPGAs from Altera, we have clear solutions for that both custom and off-the-shelf and that we understand how to model them in two different ways. We can model them with CFD and analytical modeling. We have pretty much a full complement of capabilities when dealing with this technology.

JP: Are there times when we want to create a TLB (thermal load board) or prototype and test this in a wind tunnel or in our lab?
VB: For the most part, customers will do that part themselves. They have the capability, they have the rack and if it’s a thing where they have the fans built into the rack then they can just test it. On a single individual heat sink basis, it’s not necessary because CFD and analytical modeling are so established. You want two independent solutions to make sure you’re in the right ballpark but it’s not something you’re too concerned that the result will be too far off of the theoretical. For another client, for example, we had to make load boards, but even then they did all the testing.

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Technical Discussion of ATS Telecom PCB solution

Last year, Advanced Thermal Solutions, Inc. (ATS) was brought in to assist a customer with finding a thermal solution for a PCB that was included in a data center rack being used in the telecommunications industry. The engineers needed to keep in consideration that the board’s two power-dissipating components were on opposite ends and the airflow across the board could be from either side.

Telecom PCB

The PCB layout that ATS engineer Vineet Barot was asked to design a thermal solution for included two components on opposite ends and airflow that could be coming from either direction. (Advanced Thermal Solutions, Inc.)

The original solution had been to use heat sinks embedded with heat pipes, but the client was looking for a more cost-effective and a more reliable solution. The client approached ATS and Field Application Engineer Vineet Barot examined the problem to find the best answer. Using analytical and CFD modeling, he was able to determine that ATS’ patented maxiFLOW™ heat sinks would provide the solution.

Vineet sat down with Marketing Director John O’Day and Marketing Communications Specialist Josh Perry to discuss the challenges that he faced in this project and the importance of using analytical modeling to back up the results of the CFD (computational fluid dynamics).

JP: Thanks for sitting down with us Vineet. How was the project presented to you by the client?
VB: They had a board that was unique – where it would be inserted into a rack, but it could be inserted in either direction. So, we faced a unique challenge because airflow could be from either side of the board. There were two components on either side of the board, so if airflow was coming from one side then component ‘A’ would get hot and from the other side then component ‘B’ would get hot. The other thing was that the customer, who is a very smart thermal engineer, had already set up everything and he was planning on using these heat sinks that had heat pipes embedded in them. The goal was to try and come up with a heat sink that would do the same thing, hopefully without requiring the heat pipes.

JO: Can we talk for a second about the application? You mentioned that airflow was from either side, the board was going to be used in a data center or a telecom node?
VB: It was for a telecom company.

JP: Was there a reason he didn’t want to use a heat pipe?
VB: I think probably cost and reliability. We use heat pipes embedded in the heat sinks too, so it’s not a something we never want to use, but the client wanted to throw that at us and see if we had alternatives.

JP: Can you take us through the board and the challenges that you saw?
VB: As you can see from this slide, there are four main components and two of the hottest ones are on the edge. Airflow can be from right to left or left to right, so which one would be the worst-case scenario?

Telecom PCB

JO: From right to left, I think?
VB: Correct. This one is a straightforward one to figure out because not only is the component smaller but the power is also higher. Even though [air] can go both ways, there’s a worst-case scenario.

This was the customer’s idea – a straight-fin heat sink with a heat pipe and he put one block of heat pipe in there instead of two or three heat pipes that would normally be embedded in there. You can clearly see what the goal was. You have a small component in here, you want to put a large heat sink over the top and you want to spread the heat throughout the base of the heat sink. All the other components are also occupied by straight-fin heat sinks.

JO: Okay, at this point in the analysis, this is the rough estimate of the problem that you face?
VB: This is a straightforward project in terms of problem definition, which can be a big issue sometimes. This time problem definition was clear because the customer had defined the exact heat sink that they wanted to use. It’s not a bad heat sink they just wanted an improvement, cost-wise, reliability-wise.

This is the G600, which is the air going from left to right. The two main components are represented here and we want to make sure that the junction temperatures that the CFD calculated is lower than the maximum junction temperatures allowed, which they were. These heat sinks work. As we always like to do at ATS, we like to have two, independent solutions to verify any problem. That was the CFD result but we also did the analytical modeling to see what these heat sinks are capable of and the percent difference from CFD was less than 10 percent. Twenty percent is the goal typically. If it’s less than 20 percent then you know you’re in the ballpark.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JO: Do you use a spreadsheet to do these analytical modeling?
VB: HSM, which is our heat sink modeling tool, and then for determining what velocity you have through the fins, the correct way of doing this is to come up with the flow pattern on your own. You go through all the formulas in the book and determine what the flow will be everywhere or figure out what CFD is giving you for the fan curve and check it with analytical modeling. You can look at pressure drop in there, look at the fan curve and see if you’re in the ballpark. You can also check other things in CFD, for example flow balance. Input the flow data into HSM and it will spit out what the thermal performance is for any given heat sink. HSM calculations are based on its internal formulas.

JO: We effectively have a proprietary internal tool. We’ve made a conscious decision to use it.
VB: To actually use it is unique. Not everybody would use it. A lot of people would skip this step and go straight to CFD. We use CFD too but we want to make sure that it’s on the right path.

JP: What do you see as the benefit of doing both analytical and CFD modeling?
VB: CFD, because it’s so easy to use, can be a tool that will lead you astray if you don’t check it because it’s very easy to use and the software can’t tell you if your results are accurate. If you do any calculation, you use a calculator. The calculator is never going to give you a wrong answer but just because you’re using a calculator doesn’t mean that you’re doing the math right. You want to have a secondary answer to verify that what you did is correct.

JP: What was the solution that you came up with for this particular challenge?
VB: We replaced these heat sinks with the heat pipe with maxiFLOW™, no heat pipe needed. One of the little tricks that I used was to off-set the heat sinks a little bit so that these fins are out here and so the airflow here would be kind of unobstructed. And I set this one a little lower so it would have some fins over here, not much, that would be unobstructed. The G600 configurations worked out with the junction temperatures being below what the maximum requirement was without having to use any heat pipes for the main components. There is also a note showing that one of the ancillary components was also below the max. Analytical modeling of that was within 10-11 percent.

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

As you noted, this was the worst-case scenario, going from right to left and you can see because it’s the worst-case scenario this tiny little component here that’s 14 watts that’s having all this pre-heated air going into it, it’s junction temperature was exactly at the maximum allowed. That’s not entirely great. We want to build in a little bit of margin but it was below what was needed.

The conclusion here was that maxiFLOW™ was able to provide enough cooling without needing to use the heat pipes and analytical calculation agreed to less than 20 percent. We would need to explore some alternate designs and strategies if we want to reduce the junction temperature even further because that close to the maximum temperature is uncomfortable. The other idea that we had was to switch the remaining heat sinks, the ones in the middle, which are straight fin, also to maxiFLOW™ to reduce pressure drop and to get more flow through this final component.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JP: If you have an idea like that, is it something that you broach with the customer?
VB: They really liked the result. If this was a project where the customer said, ‘Yep, we need this,’ then we would have said here’s the initial result and we have an additional strategy. At that point the customer would have said, ‘Yeah this is making us uncomfortable and we need to explore further’ or they would have said, ‘You know what? Fourteen watts is a max and I don’t know if we’ll ever go to 14 watts or the ambient we’re saying is 50°C but we don’t know that it will ever get to 50°C so the fact that you’re at max junction temperature at the worst-case scenario is okay by us.’

JP: Do you always test for the worst-case scenario?
VB: It’s always at the worst-case scenario. It’s always at the max power and maximum ambient temperature.

JP: Was this the first option that we came up with, using maxiFLOW™? Were there other options that we explored?
VB: Pretty much. The way that I approached it was doing the analytical first. You can generate 50 results from analytical modeling in an hour whereas it takes a day and a half for every CFD model – or longer. These numbers here were arrived at with analytical modeling; the height, the width, the top width, were all from analytical modeling, base thickness to measure spreading resistance, all of that was done on HSM and spreadsheets to say this will work.

JP: Do you find that people outside ATS aren’t doing analytical?
VB: No one is doing it, which is really bad because it’s very useful. It gives you a quick idea if it’s acceptable, if this solution is feasible.

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.