Tag Archives: Josh Perry

Nanofluids in Electronics Cooling Applications

Posted on October 4, 2017 by | 7 comments

By Dr. Reza Azizian
Research Scientist
Advanced Thermal Solutions, Inc.

1. Introduction

Power requirements for electronic devices have risen steadily in recent years, with the rate of increase sloping upwards, and that has necessitated enhanced thermal management solutions to preserve performance and maintain the mean time between failures (MTBF) of these devices. There are a variety of solutions that can be implemented for cooling high power electronic devices from air to liquid cooling. While air cooling is the default, liquid cooling is necessary when high-power electronic devices dissipate more than of 300-520 W/cm2. [1,2]

The addition of nanoparticles to a coolant (i.e. Nanofluid) is another alternative that can provide further improvement within a liquid cooled system.

Nanofluids

Engineered suspensions of nanoparticles in liquids have become known recently as nanofluids. The nanoparticles dispersed in a base fluid are typically metal or metal oxide particles with a size range of 1-100 nanometers. When suspended in the base fluid, nanoparticles create a colloidal solution that has been shown to eliminate the issues of erosion, sedimentation and clogging that plagued earlier solid-liquid mixtures that used larger particles.

Dispersion of nanoparticles in a base fluid alters the fluid’s overall thermo-physical properties (such as enhancing the thermal conductivity). Researchers were able to demonstrate as much as 20% enhancement in heat transfer performance of the single phase, liquid cooled system when nanoparticles were introduced. [3]

This white paper tries to offer a basic overview of nanoparticles and nanofluids for engineers unfamiliar with this topic, including a basic introduction of what nanofluids are, terminology and selecting criteria for engineers, price points, market trends, basic applications, challenges and conclusions. The end of this paper contains both a definition of terms for this topic and a list of further references.

2. Nanofluid Composition

Nanofluids are made up of two basic components: nanoparticles and a base fluid. Surfactant is also used in some cases to increase colloidal stability.

The nanoparticles used in nanofluids are typically metals, metal oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil. [4] The mixture of nanoparticles into a base fluid creates a colloid, as shown in Fig. 1.

Nanofluids in Electronics Cooling

Figure 1. Colloidal dispersion of nanoparticles in a base fluid (Nanofluids). [5]

The addition of nanoparticles to a base fluid enhances thermal conductivity and viscosity of the resultant nanofluids. The degree of enhancement depends on various factors such as volume fraction of the particles being added, particle shape, etc. The reasons for the enhancement are attributed to several microscopic phenomena including particle dynamic effect, liquid layering on the surface of the nanoparticles, and particle clustering. [6]

3. Markets and Applications

Several key markets and applications for nanofluids include:

  • Heat Transfer Enhancements
    • Power Systems (nuclear and conventional); heat exchangers (chemical industry); heat pipes; thermal management of power electronics and other semiconductors; air conditioning; and refrigeration systems
  • Mass Transfer Enhancements
    • Fermentations; protein/cell separation; drug delivery; and catalysts
  • Rheology
    • Lubricants (deep hole drilling)
  • Magnetic Nanofluids (Ferrofluids)
    • Rotary seals; cancer therapy (hyperthermia); NMR imaging; and magnetic cell sorting

4. Selecting the right nanofluid from the thermal standpoint

In the case of single-phase heat transfer, it has been shown that the heat transfer coefficient and the amount of pressure loss can be predicted by means of traditional correlations and models (Eq. 1 and 2 for heat transfer coefficient of laminar and turbulent flow respectively), as long as the measured temperature and loading dependent nanofluid properties are used. [7, 8]

Where hnf is the convective heat transfer coefficient of nanofluids; Knf, µnf, ρnf, cp,nf, are the thermal conductivity, viscosity, density and specific heat of nanofluid respectively; V is the fluid velocity, x is the distance from the entrance of the tube and D is the tube diameter.

It is clear that if accurate values of the thermo-physical properties for a specific nanofluid are available, the thermal behavior can be predicted. Apart from the specific heat and density, which can be identified by mixture rules, thermal conductivity and viscosity are the two most important parameters for overall thermal evaluation of a specific nanofluid. It is expected that by dispersing nanoparticles with higher thermal conductivity in comparison to the base fluid overall thermal conductivity of the resultant nanofluid will be higher than that of the original base fluid.

This is beneficial because having a working fluid with higher thermal conductivity would result in a reduction of pumping power while achieving the same amount of heat removal. It can also be shown that if one can increase the thermal conductivity of the working fluid by a factor of three (enhancing overall thermal conductivity is basically what nanofluid does), the heat transfer in the same apparatus doubles. [3] To give readers an idea, to increase the heat transfer of conventional fluid by a factor of two, pumping power must usually be increased by a factor of eight. Therefore, using nanofluid with high value of thermal conductivity can provide large pumping power reduction in the system.

Effective medium theory (i.e. Eq. 3) with Ck = 3 (for a dilute system of spherical particles) can be used as a rule of thumb criteria of the thermal conductivity estimation for the specific nanofluid. Obviously, measuring thermal conductivity and identifying Ck provides much more accurate value.

where knf and kbf are the thermal conductivity of nanofluid and base fluid respectively; Ck is the constant that should be found by matching Eq. 3 to the experimental data; and ∅ is the volume fraction of the particles.

However, this is just half of the story, in addition to thermal conductivity, it was also evident that viscosity has a significant impact on the overall performance of a heat transfer fluid. Adding nanoparticles to the base fluid results in a higher viscosity of the resultant nanofluid. Clearly, pumping a fluid with increased viscosity through a heat exchanger requires an increase in pumping energy, thereby reducing the overall benefit of a fluid with higher thermal conductivity. [9] Unfortunately, most of the equations for predicting viscosity of nanofluids under-predicts the experimental data by a huge margin. It was shown that for the first order estimation value of Cµ = 10 can be used in Eq. 4.

where µnf and µbf are the viscosity of nanofluid and base fluid respectively and Cµ is the constant that should be found by matching Eq. 4 with the experimental data.

This shows that generally the enhancement in viscosity is higher than enhancement in thermal conductivity (Cµ = 10 compared to Ck = 3). Then the key question to be answered is: “Is there any benefit of using nanofluids as a heat transfer fluid?” [10]

Criteria for the overall effectiveness of nanofluids as heat transfer fluids have been proposed, [9,11] which suggest that the increase in the viscosity has to be more than four times larger than the comparable increase in thermal conductivity for a nanofluid to be not beneficial at all.

Therefore, the overall quick guide line for selecting a nanofluid for a specific application would be as follows:

A. Use Eq. 3 and 4 to estimate the thermal conductivity and viscosity of the nanofluid (measured values of the thermal conductivity and viscosity are obviously preferred).

B. If the measured thermal conductivity and viscosity of the nanofluid of interest meets the requirements of Cµ <4 Ck move to the next step. If not, the choice of nanofluid should be changed.

C. Calculated/measured thermal conductivity and viscosity values should be implemented in Eq. 1 or Eq. 2 (depending on the flow) for the first order estimation of heat transfer coefficient enhancement.

In applications that involve boiling, it was shown that the boiling of nanofluids result in a deposition of nanoparticles on the heated surface (Fig. 2). [12,13] Deposition of nanoparticles on the surface provides a porous, hydrophilic layer that alters the wettability of the surface. Due to the nano-scale porous nature of this layer, capillary wicking also plays an important role. Lateral flow of the liquid due to the capillary wicking supplies a fresh liquid to the dry region beneath the bubble and delays the irreversible growth of the hot spot, which results in critical heat flux (CHF). [13] The exact mechanism behind the observed enhancement in the CHF value is still subject of debate in heat transfer community. [14]

Figure 2. Scanning electron microscopy of the heated surface after boiling with (a) DI-water and (b) nanofluid. [12]

5. Cost estimation of Nanofluids

The overall cost of a nanofluid is determined by the price of the chosen base fluid and the price of the nanoparticles themselves. Prices are higher when the nanoparticles are more difficult to synthesize or the process for creating the stable colloidal suspension is more challenging. Table 1 presents an estimation of the price for some common nanoparticles with water as the base fluid. The thermal conductivity of the resultant nanofluid is a function of the nanoparticle’s thermal conductivity. Clearly, nanoparticles with higher value of thermal conductivity provide higher enhancement.

6. Geographic Considerations for Nanofluid R & D

Nanotechnology is a field that has worldwide investment in R&D both from governments and private enterprises. The Industrial Research Institute’s “2016 Global R&D Funding Forecast” [15] notes that the most important technologies from a world-wide Industrial R&D investment standpoint include nanotechnology for both biological and industrial applications. Indeed, nanotechnology research for industrial applications (including thermal management of electronics) is forecast to have a 30% increase in R&D investment over the next two years. Only Information Technologies is forecast to have a larger increase at 31%.

The United States nanotechnology industry is led by the National Nanotechnology Initiative, which (in 2017 alone) has budgeted $1.4 billion for R&D. The budget supports nanoscale science, engineering, and technology R&D at 11 agencies, including the National Science Foundation, National Institute of Health, Department of Energy and the Department of Defense. [16] In addition, the United States has published 6,926 scientific papers on nanotechnology as of 2015. [17]

Other countries that are also heavily investing in nanotechnology include:

– China is second to the United States in investment for nanotechnology research having invested $1.3 billion in R&D and having published 2,898 scientific papers.
– Japan has $850 million dollars invested in nanotechnology research and has published 2,626 papers.
– Germany also has $850 million dollars invested with 1,767 published scientific papers.
– South Korea has $400 million dollars invested and 1,461 published scientific papers.

There has been a major effort in the European Union (EU) to make nanotechnology available for commercial purposes. In the European Commission’s “Horizon 2020” framework for research and innovation, nanofluids were labeled a “Key Enabling Technology,” although it also noted that many nanofluids are at Technical Readiness Level (TLR) 1-3, making them several years away from full commercial viability. [18] The European Commission is devoting €80 billion to spearhead research into various technologies, in addition to private investments.

While the amount of investment shows the overall commitment of a country to this technology, another metric is the concentration of nanotechnology startup companies by geography. For example, in the United States, the highest concentrations of nanotechnology startup companies are in California, Massachusetts, New York and Texas. The densest locations (with 30 or more companies) are Boston, Mass.; San Francisco, Calif.; San Jose, Calif.; Raleigh, N.C.; Middlesex-Essex, Mass.; and Oakland, Calif. [19]

7. Regulatory Considerations

Many nanoscale materials are regarded as “chemical substances” under the U.S. Environmental Protection Agency (EPA) Toxic Substance Control Act (TSCA). [20] TSCA requires manufacturers of new chemical substances to provide specific information to the Agency for review prior to manufacturing chemicals or introducing them into commerce. The EPA can take action to ensure that chemicals that may pose an unreasonable risk to human health or the environment are effectively controlled. The EPA’s bulletin, “Control of Nanoscale Materials under the Toxic Substances Control Act” outlines the regulatory framework for nanoscale materials. Most engineers implementing nanofluids for thermal management would be acquiring these materials from another manufacturer.

8. Key Market Drivers

Market drivers are forces that compel products and services to change in order to meet consumer or business demand. A useful construct for those drivers is a STEEPLE analysis, which considers seven broad factors of market drivers: Socio-Cultural, Technological, Economic, Environmental, Political, Legal and Ethical. This section deals with two to three key technological drivers for the most relevant heat transfer enhancement applications such as: power plants, power electronics and other semiconductors, and heat pipes.

8.1. Power Systems (nuclear and conventional)

Virtually all developed countries rely on a national power grid for their nations to grow and prosper. Nanofluids can play an important role in this space by improving the outcome and cost-efficiency of existing power plants. The use of nanofluids to cool a power plant can significantly reduce the resources consumed and the cost in electricity to run it. These cost savings and the greater efficiency that nanofluids provide can also eliminate the need for building additional power plants, which is a massive financial boost for a country, state, or municipality.

In power systems, the key technological drivers to consider for the implementation of nanofluids include:

o Nuclear [21]
– Improving the energy efficiency of a nuclear power plant by improving the functionality of existing power systems.
– Cost and regulatory avoidance by not having to build additional infrastructure since the efficiency of existing infrastructure can be improved to the degree that more building is unnecessary.
– Safety of nuclear power plants by improving the cooling of nuclear rods (in a pressurized water reactor, Fig. 3 [22]) by pushing the point at which a rod might overheat (CHF) to a higher value.

Figure 3. Pressurized water reactor (PWR). [22]

o Solar [23]
– Improvement to heat transfer by increasing the heat transfer performance of the fluid in the solar collectors for hot water supply (Fig. 4).

Figure 4. Solar hot water system. [24]

o Conventional [24]
– Improvement in power plant efficiency through various methods including higher heat capacity of the heat transfer fluid.
– Reduction in the size of heat exchangers.
– Reducing vapor pressure to reduce plant risk.
– Improvement in the stability of heat transfer fluids with a goal of keeping the consistency of energy conversion stable.

8.2. Power electronics and semiconductors [25, 26, 27]

With the advent of cloud computing, electric and hybrid vehicles, and the Internet of Things (IoT), electronics have become increasingly integrated into our lives. This integration of and the growth in applications drives an increasing need for more performance but at lower power and in smaller packages.

In power electronics and semiconductors the key technology drivers are:

• Increasing speeds of semiconductors with simultaneous increase in component densities.
• Increasing the performance of fluids for thermal management of electronics and migration from air to liquid cooling systems.
• Improvement in energy efficiency of electronic systems.
• Improving rack density for computing systems through reducing systems to sub-1U platforms.
• Improvement in power module life (MTBF).
• Thermal management of ever-increasing power densities.

8.3. Heat Pipes

Heat pipes are increasingly used as a key component in thermal management solutions. Their ability to transfer large amounts of heat from one section of a system to another makes them invaluable for keeping semiconductor junction temperatures within their proper range.

Key technology drivers for heat pipes and nanofluids are:

• Improving the heat transfer performance and temperature uniformity in heat pipes.
• Reduction of the thermal resistance (because of higher thermal conductivity of nanofluids).
• Increasing maximum allowable heat load of heat pipes (Qmax) by improving the wettability and providing larger surface area for thin-film evaporation in an evaporator (because of particle deposition as a result of evaporation).

9. Applications and Specifications

Nanofluids are a broad category of engineered fluids. The composition and selection of a particular colloid is generally specific to a given application category. The following are some examples of applications that could use nanofluids and some general specifications as to why an engineer may want to choose them.

9.1. Computer servers for cloud computing

Cloud computing, with its large scale of servers, continues to grow and with this trend so too does the amount of energy data centers consume. Moving from air cooling to liquid cooling can create a more energy- and computationally-efficient data center. When using water as the base fluid with nanoscale particles of zinc oxide, copper oxide or carbon nanotubes, for example, the colloid’s capability to conduct heat is improved 10-40% over water alone. The lower range of improvement is with a conventional heat exchanger, while the higher range requires a specially designed heat exchanger. [29]

9.2. Power Electronics

Insulated-gate bipolar transistor (IGBT) modules, power brick and power semiconductors have ever-increasing heat dissipation requirements reaching 300 W/cm2 and higher. At those heat dissipation levels the constraints of cabinet size, system density, noise levels and fan size make using liquid cooling with nanofluids a good thermal management strategy. As in other applications, an improvement of 10-40% may be possible by using nanofluids rather than water alone.

The implementations of liquid cooling can vary, but commonly involve heat pipes, vapor chambers or cold plates (either single or two-phase) with a pump. Because of the unique thermal capabilities of nanofluids, all three implementations can be smaller, lighter, quieter and thinner than when deployed without nanofluids. [30]

10. Trends

The idea of using liquid cooling for power electronics applications is no longer confined to theoretical observations or laboratory experiments. There is widespread use of heat pipes, for example, and personal computers frequently incorporate elaborate liquid cooling systems. Nanofluids represent an enhancement to the technologies that are increasingly being used for cooling electronics.

Much of the ongoing research is focused on immersion cooling and boiling. In tests, boiling with nanofluids has been shown to improve the value of critical heat flux (CHF) by as much as 200% as a result of nanoparticle deposition on the surface of component. This has led to other research on, potentially, engineering the surface of high-powered electronic devices with nanoparticles to improve heat dissipation without the need for an engineered liquid.

Researchers are also continually testing new materials to disperse in different fluids. For instance, aluminum oxide, iron oxide, zinc oxide, cerium oxide, and bismuth oxide are just some of the options that Nanophase Technologies Corporation offers to customers. In addition, there is work being done with carbon nanotubes that have shown promising results in heat transfer. [31] New materials, such as graphene, are also being developed and tested to determine their potential for application in thermal management.

Other trends in the technology include the use of different materials such as copper, aluminum, and newly developed polymers for the loop of a liquid cooling system or different wicking materials in the production of heat pipes and vapor chambers. In addition, improvements are being made in heat exchangers, pumps, and other components of liquid cooling systems.

11. Challenges

While nanofluids have gained momentum and notoriety in the past two decades, there are still several challenges to their widespread commercialization. [32] One of those challenges is that the concept of nanofluids is still relatively young. The term was only coined in the early 1990s and, while the number of studies have increased in recent years, there are no long-term assessments of how the addition of nanoparticles could affect a cooling system over time. Nanoparticles could collect and cause degradation in a pump or heat exchanger, for example. There is a widespread belief in the stability and reliability of nanofluid solutions, but it has not been possible to document that over the course of a long period of time.

Also, there are questions that have been raised about the environmental impacts of disposing nanofluids from a system, whether as vapor in the case of a nuclear power reactor or as liquid when replacing the fluid in a data center system. While it is possible that nanoparticles in high quantities could be harmful, there are few studies that have focused on the environmental impacts [33] or the impact on humans. [34]

12. Conclusions

This white paper summarized the potential applications of nanofluid in electronic cooling. It was shown that the thermal performance of the liquid cooled system can be further improved by dispersing nanoparticles in a base fluid (i.e. nanofluid). The guide for selection of the right nanofluid for specific application is provided. It was shown that the heat transfer coefficient enhancement in nanofluid can be predicted simply by implementing thermo-physical properties of the nanofluid of interest in the traditional models/correlation for single phase convective heat transfer. It was also shown that the large enhancement in CHF value can be achieved by boiling nanofluid on a heated surface. Based on these basic principles employing nanofluid in different applications, their opportunities and challenges were discussed.

Acknowledgment

The author would like to thank Ms. Rebecca O’Day and Mr. Joshua Perry for their help in preparing this manuscript.

Definition of Terms
———————————————-
There are several terms that are used in discussion nanoparticles and nanofluids in electronics cooling applications. These terms are listed below and defined for reference.

Base Fluid – Fluids generally used to contain nano particles to create a colloidal suspension. Base fluids typically but not exclusively used include water, ethylene glycol, engine oil, Tolouene, refrigerants.

Colloid – Nanoscale or microscale particles suspended in another medium.

Nanoparticle – Particles with size on the order of 1-100 nm. [35]

Nanoparticle Types: Nanoparticles can be made of materials with good thermal conductivity such as metallic oxides (Al2O3, CuO, SiO2, Fe3O4), nitride ceramics (AlN, SiN), Carbide ceramics (SiC, TiC), metals (Cu, Ag, Au), semiconductors (TiO2, SiC), carbon nanotubes (SWCNT, DWCNT, MWCNT).

Nanofluid – Fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloids of nanoparticles in a base fluid.

References
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4. S.K. Das, S.U.S. Choi, H. E. Patel, “Heat Transfer in Nanofluids—A Review,” Heat Transfer Engineering 27 (2006) 3-19.
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6. S. Akilu, K.V. Sharma, A.T. Baheta, R. Mamat, “A Review of Thermophysical Properties of Water Based Composite Nanofluids” 66 (2016) 654-678.
7. U. Rea, T. McKrell, L-W. Hu, J. Buongiorno, “Laminar convective heat transfer and viscous pressure loss of alumina–water and zirconia–water nanofluids”, International Journal of Heat and Mass Transfer 52 (2009) 2042–2048.
8. W. Williams, J. Buongiorno, L-W. Hu, “Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes”, ASME Journal of Heat Transfer 130 (2008) 042412-7.
9. D.C. Venerus, J. Buongiorno, R. Christianson, et al., “Viscosity Measurements on Colloidal Dispersions (Nanofluids) for Heat Transfer Applications”, Applied Rheology 20 (2010) 44582-7.
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For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

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Technical Discussion: Designing Heat Sinks for Cooling QSFP Optical Transceivers

Posted on August 14, 2017 by | 3 comments

During a recent project designing a thermal solution for a customer’s PCB (printed circuit board) layout, Advanced Thermal Solutions, Inc. (ATS) Field Application Engineer Peter Konstatilakis also analyzed the thermal properties of a series of SFP (small form-factor pluggable) optical transceivers on the edge of the board.

QSFP Optical Transceivers

ATS engineer Peter Konstatilakis holds the heat sinks that he designed for cooling QSFP optical transceivers. (Advanced Thermal Solutions, Inc.)

From that project came the idea of examining the thermal challenges presented by SFP and QSFP (quad SFP) and designing a heat sink solution that future customers could use to solve potential issues that stem from the increased power requirements of the compact transceivers that are frequently used in the transmission of data.

After conducting an analytical analysis, running computer simulations, and testing the heat sinks in the state-of-the-art ATS labs, Peter demonstrated a new heat sink design and optimized layout sequence that showed 30 percent improvement on QSFP heat sinks currently on the market.

In addition, he showed that having heat sinks with fewer fins upstream and heat sinks with more fins downstream provided a near isothermal relationship between the first and last QSFP, an important consideration for QSFP arrangements.

Peter recently sat down with ATS Vice-President of Marketing and eCommerce Rebecca O’Day and Marketing Communications Specialist Josh Perry to discuss the project, his research, and the successful design of the new QSFP cooling solution.

JP: What prompted the work on QSFP heat sinks? Why did we start looking into this technology?
PK: Optics are pretty big now with all the higher information rates, 400 gigabyte cards, which is 400 gigabytes of throughput and that’s a lot. They need these multiple high-powered SFP or QSFP to do that. So, higher power demands call for ATS expertise in thermal management.

RO: Optics are really expanding. It’s not just routers and things like that, but they’re also used in storage, array networks, video…so this kind of thing could really be able to expand.
PK: Anywhere that you are transferring data, which is basically everywhere – the Cloud, big servers, the internet itself. They’re being used a ton.

JP: Was the impetus for designing QSFP heat sinks something that was prompted by a customer or did we think about the technology and recognize that it needed to be cooled?
PK: We had worked on SFP cooling for a customer first, so that helped us understand the area a bit more. Also, from what we were hearing from customers, QSFP that were being designed had higher throughput, which means higher power. And it is also good to have products that we can market, even if it isn’t for every customer, and show that we can handle the optical transceiver arena.

JP: What was the first step in designing the heat sinks? Did you know a lot about QSFP or did you have to do a lot of research?
PK: There is definitely a lot to think about. You can’t use a TIM (thermal interface material) because the QSFP isn’t fixed in the cage; it can be hot swapped. After a few insertions and removals, it will gunk up the TIM.

JP: Was that something you knew before?
PK: It was something I knew before, but there is also a specification document for this technology written by the SFF (Small Form Factor) Committee, which is a standard controlled document that engineers design to for this form factor and it stated in there not to use a TIM. When we looked at it with the customer, it made sense and when we asked the customer they agreed.

RO: If there is no TIM, how does the interface work? Is it a direct interface? Is it flat enough?
PK: You have to specify a good enough flatness and surface roughness, within cost means, that will still have a low contact resistance. That was one of the challenges as well as understanding the airflow of typical QSFP arrangements because you have four in a row, so you’re going to have preheated air going into the fourth QSFP.

JP: When designing the heat sinks, what were the issues that you needed to consider?
PK: One consideration was getting as much surface area as we can, so that required extending the heat sink off the edge of the cage and we also had fins on the bottom of the heat sink. Usually, you only have fins above the cage but there was some room underneath, about 10 mm depending on what components are around, which provides additional surface area.

We also found that when you extend the surface the spreading resistance becomes an issue as well, so you need to increase the thickness of the base to help spread the heat to the outer extremities of the heat sink. You want the first QSFP and the last QSFP case temperatures’ to be isothermal due to laser performance (an electrical parameter), whereas each individual heat sink should be isothermal to get the most out of all the heat sink surface area (a heat transfer parameter).

‘Cold’ spots insinuate a lack of heat transfer to that location and thus poor use of that surface area. Then it was about the airflow and having the front heat sinks be shorter with fewer fins and the back two to be taller with denser fin arrays.

ATS heat sinks designed specifically for cooling QSFP optical transceivers. (Advanced Thermal Solutions, Inc.)

JP: Was the difference in fin arrays between upstream and downstream heat sinks how you optimized the design to account for the preheated air?
PK: What is really important is to keep each QSFP at the same temperature, within reason, because they all work together. So, if one is a higher temperature than another, the laser performance is going to be affected and it will affect the stack. You want to have them as isothermal as you can; the case temperature from the first QSFP to the last.

We figured when we were going through the design, you could have a shorter heat sink up front with fewer fins to help the airflow pass to the downstream QSFP. The upstream QSFP wouldn’t need as much cooling because they’re getting the fresher air and faster airflow. So, if you relax the front heat sinks and make the ones in the back more aggressive, then you’re going to get better cooling downstream.

What happens is the front heat sinks aren’t as effective. This is fine as long as the upstream QSFP case temperatures are lower than the downstream QSFP. The overall effect is that the upstream QSFP temperatures will be closer to the temperature of the downstream QSFP, keeping the stack as isothermal as possible.

This is where the limit lies. Minimizing the upstream QSFP heat sinks, which in turn minimizes the amount of preheat to the downstream QSFP and allows as much airflow to enter downstream QSFP. At the same time ensuring the upstream QSFP temperatures are equal to or just lower than the downstream QSFP. This keeps the downstream QSFP temperatures at a minimum, while also keeping the transceiver stack close to isothermal.

JP: Were there any unexpected challenges that you had to account for?
PK: There was a challenge in testing and making sure that the thermocouples (which you can see in the picture below) contact the heat sink surface correctly and all of them at the same point. I had to glue it, so it may touch the case of the heat sink or it may not, depending on how the glue set, so I had to put a little thermal grease inside the pocket just to have the thermocouple make good contact with the heater block itself.

The test setup to measure cooling performance of individual heat sinks on a QSFP connector cage when airflow is from one side only. (Advanced Thermal Solutions, Inc.)

The metal piece (heater block) mimics the QSFP and we put a cartridge heater in the middle to heat it up and then we put a groove where the thermocouple is attached as I just explained.

Other than that…it was really just the flatness. It was hard to test and get reliable data between several heat sinks because there is going to be some flatness variation between them. Sometimes there isn’t enough to show a variation, but if I’m seeing different data with a different heat sink on the same heater block then the flatness and surface roughness is affecting it.

RO: On the flatness issue, in theory someone could spend a lot of money and make sure that it was completely flat but there’s a certain point where it has to be flat enough.
PK: Obviously there are diminishing returns after a certain point, so you have to find that line. There are no calculations that explain flatness and surface roughness, so at the end of the day it comes down to testing.

RO: I find it interesting that the testing was a challenge because it appears to us on the outside that this is a standard approach but then you get into it and have to ask how are we going to measure the temperature accurately:
PK: There is always something that comes up which you didn’t think about until you start doing the testing and you have to make a change and modify it to make it work. That is where experience comes in handy. The more testing you do, the more you’ve seen and you can take care of the problem before it arises.

RO: It’s a good example of what we can do at ATS. We don’t have to test with a full, expensive board or the full optical arrangement, instead we can come up with inexpensive (low startup cost) ways to test that will provide quick, accurate data to help the customer get to market.

JP: So, we tested three different arrangements for the heat sinks?
PK: Yeah. There were two different designs with changes in the density of the fins. Based on the CFD (computational fluid dynamics) and in the lab, the best outcome was having the less dense fins in front for the first two heat sinks and having the denser fin arrays downstream. As we expected, more airflow was able to make it to the back heat sinks and were able to cool them more effectively.

QSFP Heat Sinks

This graph shows the difference in temperature between the ATS heat sinks at various air flows. (Advanced Thermal Solutions, Inc.)

We were seeing less than a degree difference, especially at higher airflows, between the first heat sink and the last and that was pretty impressive. That configuration also provided the lowest temperature for the final two QSFP. Those are going to be the limiting factors; they’re going to be the highest temperature components no matter what since they’re receiving preheated air. That’s why it’s important to minimize the preheated air and maximize the airflow downstream by designing shorter, low fin-density heat sinks upstream.

If you put a dense heat sink up front, you’re going to restrict airflow downstream and you’re going to pull more heat out of the component because it is a better heat sink. With this you’re going to dump more heat into the air and send it to the downstream QSFP. So, it is worth keeping some heat in the upstream components, which has a double effect of keeping all of the QSFP temperatures as isothermal as possible. As long as the upstream components aren’t going over the case temperature of the last component, then you’re fine.

RO: It’s almost counter-intuitive. The general thermal design says to pull as much heat away from the component as quickly as possible and dissipate it, but you’re saying it was better to leave some of the heat in place.
PK: For the upstream QSFP, absolutely. There is margin because it is receiving so much fresh air.

That is really because we’re working in a system environment where choices upstream affect the airflow downstream. If it wasn’t a system and you’re looking at a single component, then sure you want to get rid of all the heat. And again, leaving heat in also allows the QSFP components to be as isothermal as possible.

JP: It sounds like it worked the way that you expected going in?
PK: Yeah it did. I’m not going to sit here and pretend it always happens that way but what we thought would happen did happen and we were able to design it analytically before we went into CFD and testing.

JP: Were there certain calculations that you use when working with a system?
PK: We can look at the fan curve. Each heat sink has its own pressure drop and the way you use a fan curve is to analyze the four heat sinks, add the pressure drops together, and then examine the fan curve (the amount of airflow varies with the pressure that the fan sees) with the higher the pressure, the less airflow. So, we’re able to estimate the amount of airflow across the system based on the total pressure drop.

We also use Q=mCpΔT and that way we can determine, based on the amount of power coming from the component, what is the air temperature that is leaving the heat sink. It is a little conservative because we’re saying that all of the heat is going into the next heat sink, which isn’t true because a little is escaping to other locations, but being conservative doesn’t make a difference when comparing designs.

Analyzing the airflow into each heat sink and the temperature into each heat sink lets us know what we have to design for; just because you’re putting more surface area doesn’t mean you have a good solution.

RO: This is a good example of how thermal management is more than just removing the heat, but also analyzing how the heat travels and thinking about it as a system. It’s much more complicated.

JP: How important is for ATS to be able to see potential thermal challenges in new technology, like this, and work through the problem even if it isn’t for a specific design or customer?
PK: It always helps to have more experience. It’s knowledge for the future. We’ve already seen it, we’ve already dealt with it, and we can save time and cost for the customer.

Whenever we run into this issue, we can say we tested that in the lab and explain the solution that we found. We don’t need to do more analysis, but provide the customer with a solution.

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

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Posted in Heat Sinks, SFP/QSFP, Uncategorized

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Heat Sink Design: ATS Engineers Bring Ideas to Life

Posted on June 29, 2017 by | 1 comment

Marketing Communications Specialist Josh Perry sat down with Product Engineering Manager Greg Wong to discuss the process that Advanced Thermal Solutions, Inc. (ATS) engineers go through to create a heat sink and find a thermal solution for customers.

Watch the full conversation in the video below and scroll down to read the transcript of the interview.

JP: Greg, thanks again for joining us here in marketing to explain what it is that goes into designing a heat sink for a customer. So, how does that process begin?
GW: We usually start with a few basic parameters; we call them boundary conditions. So, we start with a few boundary conditions, basics like how much airflow we have, how much space constraint we have around a heat sink, and how much power we’re dissipating, as well as the ambient temperature of the air coming into the heat sink.

So, those are the real basic parts that we need to start out with and sometimes the customer has that information and they give it to us, and usually we double-check too, and then other times the customer has parts of the information, like they know what fan they want to use and they know what kind of chassis they’re putting it in and we take that information and we come up with some rough calculations so we can arrive at those things like air flow and stuff like that.

JP: When you get the data from the customer, how do you determine what the problem is, so that way you can move forward?
GW: We usually start out with an analytical analysis. So, we put pen to paper and we start out with basic principles of heat transfer and thermal resistance and stuff like that so we can understand if what we’re trying to achieve is even feasible and we can come up with some basic parameters just using that analytical analysis.

Like we can calculate what kind of heat sink thermal resistance we need or we can calculate how much air flow we need or, if we have several components in a row, we can calculate what the rough air temperature rise is going to be along that chain of parts. So, there’s a lot we can do when we get the basic information from the customer just on pen and paper.

JP: What’s the next step beyond analytical?
GW: Well, we can do some lab testing or a lot of times we also use CFD simulations and, if our customer has a model they can supply us, we can plug that into the CFD simulations and we can come up with an initial heat sink design and we can put that into the simulations as well and then we set those up and run them.

The great thing, having done these analytical analyses beforehand, we know what to expect from CFD simulations. So that way, if the simulations don’t run quite right, we already have an understanding of the problem, we know what to expect, because CFD is not 100 percent reliable.

I mean, you can go and plug all this stuff in there but you really have to understand the problem to know if the CFD is giving you a good result. So, oftentimes that’s the next stage of the process and from there we can actually produce low-volume prototypes right here in Norwood (Mass.), in our factory. We have CNC machines and manual milling machines, lathes, all that kind of stuff, and we can produce the prototypes and test them out here in our labs.

JP: How much of a benefit is it to be able to create a prototype and to be able to turn one around quickly like that?
GW: Oh, it’s great. I mean, if we had to wait to get parts from China it will take weeks to get. We can turn them around here in a few days and the great thing about that is we can test them in our labs and, you know, when it comes to getting results nothing beats the testing.

I mean, you can do analytical analysis, you can do CFD simulations, but when you actually test the part in a situation that is similar to what the actual thing is going to be that’s where the real meat comes down.

Heat Sink Design

ATS engineers take customer data and using analytical modeling and CFD simulations can design the right cooling solution to meet the customer’s specific thermal needs. (Advanced Thermal Solutions, Inc.)

JP: So, we test the prototypes before sending them out to the customer? We do the testing here or do we send it to them first?
GW: It all depends on what the customer requires. Sometimes the customer has a chassis that we really can’t simulate in our labs, so we might send the prototype heat sinks to the customer and the customer will actually put them into their system to test them out.

Other times, a customer might have a concept and they don’t actually have a product yet, so we’ll mock something up in our labs and we’ll test it and it all just depends what the customer needs and also how complex the problem is.

If it’s a simple heat sink and pretty simple airflow, we might not need to test that because we understand that pretty well, but the more complex the chassis is and how the airflow bends and stuff like that, the greater benefits we get out of lab testing.

JP: Well, I appreciate it Greg. Thank you for taking us through the process of making a heat sink and solving thermal problems for our customers.
GW: Sure Josh. We love seeing new thermal challenges and coming up with ways of keeping stuff cool.

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.

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Posted in CFD, Chassis, Consulting, Engineering, heat sink, heat sink design, Heat Sinks, PCB, Thermal Design

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How Did Thermal Performance of Aluminum Heat Sink Compare to Copper?

Posted on February 2, 2017 by | 4 comments

Advanced Thermal Solutions, Inc. (ATS) was recently tasked with creating a more cost-effective and lighter solution for a customer that was looking to replace a relatively large heat sink, which was dissipating the heat from four components on a printed circuit board (PCB). The customer did not want a skived heat sink, so ATS engineers created a custom aluminum heat sink embedded with copper heat pipes to draw the heat away from the components.

ATS engineers worked on a comparison of a copper heat sink with an aluminum heat sink that had embedded heat pipes running above the components. Analysis showed that the aluminum heat sink nearly matched the thermal performance of the copper and was within the margin required by the client. (Advanced Thermal Solutions, Inc.)

ATS engineers used analytical modeling and CFD simulations to examine the thermal performance of two aluminum heat sink designs: one with heat pipes that stopped at the edge of the components and the other with heat pipes that ran above the components. Analysis demonstrated that the design with heat pipes running above the components kept junction temperatures within 2°C of the original copper heat sink and an average difference of less than 1°C.

Peter Konstatilakis, a Field Application Engineer at ATS who worked with the client on this analysis, sat down with Marketing Communications Specialist Josh Perry to discuss the technical details behind the thermal analysis and the results that were presented to the customer.

JP: Thanks for taking the time to talk about this project Peter. What was it that they approached you with? What was the problem or the challenge?
PK: There was a long lead time with sourcing this copper; it’s a relatively large and heavy part.  This size bar of copper isn’t typically stocked. So, we were having sourcing issues with this non-standard copper stock and they were having weight and cost issues. They had to cut this heat sink in half for testing because they were overweight on the board. Through shock and vibe testing, if the heat sink is too heavy then it can actually rip out of the board.

An alternative was to make the heat sink through a manufacturing process called skiving. Skived heat sinks have a fin count tolerance, so you may have more fins than are specified or you might have less fins, and some of the fins may be curved, which poses cosmetic issues with skived heat sinks; the fins aren’t perfectly straight. It’s not really an issue thermally, especially if companies don’t see the heat sinks too often, but this client’s customers see the boards, see the heat sinks, and they wanted them to look perfect.

Instead of having to get this copper, we thought, why don’t we make an aluminum heat sink with heat pipes? That’s sort of where this came from.

JP: So the problem with skiving a heat sink was mostly an issue with aesthetics?
PK: Yeah, exactly. The tolerance on the fin spacing was +/- three fins, due to the high number of fins. I did a quick analytical analysis with our heat sink calculation tool and the difference in thermal resistance was maybe 1%. That was because the heat sink has such a large surface area and losing a fin or two only changes the performance by a percent or less. On a smaller heat sink, you will see a greater difference. I told the customer but they said that they still didn’t want to go with skived for aesthetic reasons. Instead, we extruded aluminum and then we put heat pipes in the base.

JP: Why was it necessary to add heat pipes to the heat sink?
PK: The big thing, in this case, is the spreading. You can see the locations of the components and then how large the heat sink is. There’s definitely a lot of spreading resistance in the base because there’s so much distance between the heat sink and all the components, so that’s the main issue that we were trying to take care of with the heat pipes. An aluminum heat sink with heat pipes is definitely a lot lighter than a copper heat sink, about three times lighter. Overall it’s much easier to source and also much cheaper. I think it’s again about three times as much for copper.

JP: When this challenge came across your desk, what was the first thing that you looked at? How did you approach the challenge?
PK: What I did was look at our analytical tool again and I modeled this heat sink in all copper. Since there are four components it’s a little complicated, but I modeled them as one component in the middle of the heat sink with gap pad and everything and got the performance of that heat sink. Once I did that, I ran CFD simulations on the copper heat sink with the components placed as they are in the application and the performance values were within 15%. So, doing that, we knew that we had a good CFD model.

After running the baseline simulations on the copper, I moved onto the aluminum heat sink knowing that we had a good CFD model and that we could trust the results. I used the aluminum heat sink and put heat pipes in the base. I started with heat pipes out in front of the components and then the next simulation was with heat pipes above the components. Obviously, if the heat pipes are above the component then you’ll get a little better spreading resistance and the heat will flow better.

Aluminum Heat Sinks

The first of two aluminum heat sink designs had heat pipes that stopped at the components. This design was not as effective as when the heat pipes ran above the components. (Advanced Thermal Solutions, Inc.)

JP: How significant of a difference was it?
PK: From the base line of the copper heat sink, it was around a 1-2°C difference, on average.

After looking at these two simulations, I met with Dr. Kaveh Azar (founder, CEO and President of ATS) to discuss the results. With the heat pipes above the components, we were seeing an average difference of less than 1%. It performs worse by less than 1% and I’m currently doing a couple of other simulations to see if we can improve that by adding more heat pipes, making the heat pipes wider, or even running less conservative heat pipes since the conductivity I’m running with is 2000 W/m-K axially and 400 W/m-K through the cross section. Really, the axial conductivity should be around 20,000-50,000 W/m-K, and the copper wall and wick effective conductivity is around 100-200 W/m-K due to the low conductivity of the porous copper sintered wick. The conservative values I used were to get the simulation up and running, while I’ll end up analytically determining the respective heat pipe conductivity.

I’m also doing an all-aluminum simulation just so we can see what that looks like and so we can see how much better the copper heat sink is in general.

This turned into just looking at the heat sink and trying to put heat pipes in them to seeing if we could also vary the length and see if we could get better performance. Your pressure drop increases as the length increases, so the higher the pressure drop then the lower the air flow is going to be in the system, the lower the airflow then the lower the performance. There is sweet spot for the length. I’m looking at that with our analytical calculator. And then the base thickness as well, we’re looking at that too.

Aluminum Heat Sinks

The results of the CFD analysis showed that the average temperature difference between the copper and the second aluminum heat sink design was less than one degree. (Advanced Thermal Solutions, Inc.)

JP: With the aluminum heat sink within 1% of the copper, did that make switching from copper worth it for the customer?
PK: It definitely did. If you’re within 1% and the customer has a little margin already, then it’s worth it because it’s three times lower cost, lower weight, and it will look better because it’s extruded rather than skived.

JP: Just to clarify, what is the difference between skiving and extruding?
PK: Extruding, basically, is pushing a hot piece of metal through a die that is in the shape of a heat sink, so it’s like putting play-doh through a die. You get a heat sink with the fin pitch and everything, where skiving uses a copper block and they come in with a blade and peel the fin out. The blade comes in and pushes a layer up. You can skive aluminum as well and they’re about the same cost, but you can’t extrude copper for a heat sink.

This showed our thermal capability to the customer. It showed that we can design custom heat sinks. We can make them more cost-effective, better performing, whatever they need.

JP: When you’re working through these types of challenges, how much of it becomes a foundation of knowledge that you can then take to another customer’s project?
PK: The more experience that you have, the better. Like any field, the more experience you have then you can look at something and know right off the bat if it’s going to work or not. It also helps in terms of understanding how to model certain applications and where to start with the design.

JP: Did we run these simulations here or did we have (ATS engineer) Sridevi Iyengar run the simulations in India?
PK: We did it here. Sri does a lot, but she uses FloTHERM and I’m quicker with Autodesk CFDesign. FloTHERM can be used for bigger systems because it takes less of a mesh. Generally, FloTHERM only works in rectangular coordinates, where CFDesign works with tetrahedrons, allowing the simulation of angled objects. Since it works with tetrahedrons though, it takes longer to mesh and run than FloTHERM. You can’t really have anything angled in FloTHERM and obtain accurate results. We ended up having to angle the heat pipes in order to contact the components, which are in a different plane than the rest of the heat sink.

JP: I know it is a priority at ATS, but why was it important to have an analytical component, not just CFD, in finding a solution?
PK: Analytical modeling is used to ensure that the CFD results make sense. When you see the graphs from CFD, it looks appealing to the eye and you get drawn to it. It’s science and engineering that is made visible, whereas heat transfer and fluid dynamics (for air) are invisible to the naked eye. Another method of ‘seeing’ heat transfer is using an infrared thermal camera or liquid crystal thermography, while a water tunnel or inducing smoke into the flow can be used to see fluid flow. The analytical also gives us a good first judgement and solid design direction.

Optimization for the length of the heat sink and the base thickness, I did with our analytical tool. CFD simulations take a lot of time, so I can narrow down the number of designs and determine what we want to simulate. Rather than doing 10 different simulations, when each takes on average three or four hours, I can get instant results and say, okay, a 5 mm base is the sweet spot, so let me try in CFD 4 mm thickness, 5 mm, and 6 mm; narrowing it down to three simulations.

Analytical modeling gives us quick what-if scenarios, which we say a lot, and it definitely helps give you an understanding of what to expect. If the numbers are way off then I know something is wrong in the CFD model and I check to see if my mesh and other parameters are correct. It humbles you almost and it helps you understand the application and what you’re simulating. The last thing you want to do is give a customer incorrect data.

It gives you two independent solutions. We say analytically this solution is validated, so we have faith in the model. Now, here is the model and it shows better what we want to do.

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

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Posted in Analysis, CFD, Consulting, heat sink design, Thermal Analysis

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Discussion of Thermal Solution for Stratix 10 FPGA

Posted on October 28, 2016 by | Leave a comment

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 https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

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