Tag Archives: Rebecca O’Day

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.
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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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