Author Archives: Josh Perry

What are the benefits of using Pin Fin Heat Sinks in thermal management of electronics

Posted on August 22, 2017 | 1 comment

Engineers tasked with designing modern electronics face a number of issues. Expectations are for more functionality, more power, and more components in ever-smaller packages but also with quick turnaround for production and staying within tight budget parameters.

Thermal management is a critical aspect of the design process and, as demand for component-density and miniaturization continues to increase, engineers need cooling solutions that fit into small spaces, will not cause project cost overruns, and will provide the best heat transfer possible for today’s modern,  processors.

Heat sinks and convection cooling remain the go-to solutions for most systems and high-efficiency Pin Fin heat sinks are designed to meet the requirements of modern electronics cooling with little extra cost added. In particular, the pin fin heat sink geometry is designed to provide increased surface area for heat transfer, low thermal resistance from base to fins at high airflow (200-plus LFM), and work in environments where the direction of airflow is ambiguous.

Pin Fin Heat Sinks

Pin fin heat sinks from Advanced Thermal Solutions, Inc. (ATS). Pin fin heat sinks provide low thermal resistance at high LFM. (Advanced Thermal Solutions, Inc.)

How does the Pin Fin geometry work?

Barry Dagan, an engineer at Cool Innovations, Inc., wrote a piece for New Electronics in 2009 that explained how the pin fin structure uses the ambient airflow to enhance its thermal performance. [1]

“Any heat sink removes heat by ‘breaking’ the boundary layers of still air that are wrapped around its surface because still air is a very good thermal insulator,” Dagan explained. “The boundary layers are broken by accelerating the flow of air into the heat sink – either using fans and forced airflow or via the chimney effect. In either case, the faster the airstream, the more likely the boundary layers are to break and the more effective the heat sink will be.”

He added, “The round, aerodynamic pin design reduces resistance to surrounding airstreams that enter the pin array, while simultaneously increasing air turbulence. The omnidirectional pin configuration, which allows air to enter and exit the heat sink in any direction, exposes the heat sink to the fastest possible air speed.”

In an earlier article for EE Times, Dagan also noted that the pin fin geometry “allows for a high degree of customization.” Engineers can make adjustments to the overall height, pin height, base thickness, footprint, pin diameter, and pin density to find an optimal cooling solution for their particular project. [2]

“Pin fins can also be catered for situations where both footprint and height are restricted,” Dagan wrote. “For example, the pin fin technology enables the design of heat sinks with a footprint of half an inch squared and a total height as low as 0.15 in.”

A study conducted by Younghwan Joo and Sung Jin Kim that was published in the International Journal of Heat and Mass Transfer indicated that the heat dissipation per mass of optimized pin fin heat sinks was greater than optimized plate-fin heat sinks in most applications. [3]

Pin Fin Heat Sinks

Pin Fin heat sinks on a PCB. (Advanced Thermal Solutions, Inc.)

In a comparison of heat sinks conducted at Advanced Thermal Solutions, Inc. (ATS) and published in Qpedia Thermal eMagazine, a 33-mm tall elliptical pin fin heat sink under forced convection had the lowest thermal resistance of the 10 heat sinks that were tested. [4]

The ATS family of pin fin heat sinks, made from extruded aluminum, range in sizes from 10 mm by 10 mm to 60 mm by 60 mm. Heights range from 2-25 mm. Through testing in ATS wind tunnels, the pin fin heat sinks demonstrated thermal resistance as low as 2.5°C/W and added little weight to the board. [5]

How are pin fin heat sinks attached to a board?

Pin fin heat sinks are versatile and can be attached to a variety of component packages, including BGA, QFP, LCC, LGA, CLCC, TSOP, DIP, LQFP, and many others. Because pin fin heat sinks are lightweight, standard thermal tape or epoxy can be used to securely attach them to components.

In addition, pin fin heat sinks work with mechanical attachments such as z-clips and ATS maxiGRIPTM or superGRIPTM, which are two-component attachment systems that provide secure hold without damaging the PCB and only minimal addition to the component footprint.

Pin Fin Heat Sinks

Pin fin heat sinks attached to a PCB with ATS maxiGRIP heat sink attachment system. (Advanced Thermal Solutions, Inc.)

How do pin fin heat sinks provide cost savings?

In his article for EE Times, Dagan explained, “Pin fin technology provides cost-effective heat sink solutions for medium and high-volume applications due to low associated tooling charges and minimal waste of raw materials.” [2]

For example, the ATS family of standard and custom pin fin heat sinks are all available for less than $2.00, with the vast majority of heat sinks available for less than a dollar. This means that engineers can find high-efficiency heat sinks and save money in the budget, which can be put to other design considerations, such as higher-powered fans to increase airflow, better heat sinks attachments, or additional chips and other board components. [5]

This is particularly beneficial for the growing maker market, which is working on new technology or enhancing current technology but generally with far smaller budgets than traditional OEM.

A 2012 article from The Economist, entitled “A Third Industrial Revolution,” discussed the impact of additive manufacturing techniques and how it was now possible to make parts through processes like 3-D printing that are cheaper and faster than traditional methods. According to the article, this will not just affect large manufacturers but also trickles down to a community of makers and smaller companies, what the article labeled “social manufacturing.” [6]

The article added, “As manufacturing goes digital, a third great change is now gathering pace. It will allow things to be made economically in much smaller numbers, more flexibly and with a much lower input of labour, thanks to new materials, completely new processes such as 3D printing, easy-to-use robots and new collaborative manufacturing services available online. The wheel is almost coming full circle, turning away from mass manufacturing and towards much more individualised production.”

Pin Fin Heat Sinks

Pin fin heat sinks provide cost-effective cooling solutions for small manufacturers and the maker market. (Advanced Thermal Solutions, Inc.)

A study, also from 2012, from MAKE magazine and Intel surveyed the maker community to get data about the (self-proclaimed) hobbyists, builders, tinkerers, and engineers. Out of the total respondents, 79 percent said that they worked in hardware and software, with electronics in second place at more than 60 percent. Thirty-four percent of respondents said that they were involved in making products for income and 19 percent of the total said that they paid for projects with outside funding. [7]

Crowdfunding can only take a project so far and for makers trying to earn money from designs, it is crucial to find cost-effective solutions both to ensure a project comes in under budget and to maximize profits from the sale of the design.

Pin fin heat sinks can be added at low-cost and provide the necessary thermal performance to push a design process along. For the maker market and its (at times) limited resources, high-efficiency pin fin heat sinks provide thermal performance on a budget with the versatility to fit into a variety of systems and designs.

References
[1] http://www.newelectronics.co.uk/electronics-technology/pin-fin-heat-sinks-point-the-way-to-more-efficient-cooling/18641/
[2] http://www.eetimes.com/document.asp?doc_id=1204099
[3] Younghwan Joo and Sung Jin Kim, “Comparison of thermal performance between plate-fin and pin-fin heat sinks in natural convection,” International Journal of Heat and Mass Transfer, April 2015, 345-356.
[4] https://www.qats.com/cms/wp-content/uploads/2014/03/HowAirVelocityAffects_Qpedia08.pdf
[5] https://www.qats.com/News-Room/Press-Releases-Content/1184.aspx
[6] http://www.economist.com/node/21552901
[7] http://www.nyu.edu/social-entrepreneurship/speaker_series/pdf/Maker%20Market%20Study%20FINAL.pdf

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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Posted in Heat Sinks

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Technical Discussion: Designing Heat Sinks for Cooling QSFP Optical Transceivers

Posted on August 14, 2017 | 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 [email protected].

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Posted in Heat Sinks, SFP/QSFP, Uncategorized

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Industry Developments: Cooling QSFP Optical Transceivers

Posted on August 7, 2017 | 5 comments

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

Rapid advancements in fiber optic technology have increased transfer rates from 10GbE to 40/100GbE within data centers. With the emergence of 100GbE technologies, the creation of data center network architectures free from bandwidth constraints has been made possible. The major enabler of this performance increase is the QSFP optical transceiver.

QSFP is the Quad (4-channel) Small Form-Factor Pluggable optical transceiver standard. A QSFP transceiver interfaces a network device, e.g. switch, router, media converter, to a fiber optic or copper cable connection as part of a Fast Ethernet LAN.

The QSFP design became an industry standard via the Small Form Factor Committee in 2009. Since then, the format has steadily evolved to enable higher data rates. Today, the QSFP MSA (multi-source agreement) specification supports Ethernet, Fibre Channel (FC), InfiniBand and SONET/SDH standards with different data rate options.

QSFP

Fig. 1. The Small QSFP Form Factor Allows More Connectors and Bandwidth than Other Fiber Optic Transceiver Formats. Note the Cooling Fins on Each Receiver Device. [1]

Thermal Issues

The small QSFP form factor has significantly increased the number of ports per package. The increased density of transceivers can lead to heat issues. The optical modules can get hot due to their use of lasers to transmit data. Even though the popular QSFP28 provides lower power dissipation than earlier transceivers – abut 3.5W, the QSFP28 factor has also allowed a significant increase in port density.

Newer microQSFPs can dissipate even more heat. microQSFP interconnects fit more ports (up to 72) on a standard line card, saving significant design space.

Fig 2. Air Gap Locations Shown in Thermal Specifications Feature on QSFP. Top: QSFP at the Inside Edge of a Cage, Bottom: QSFP Section Showing Typical Internal Layout. [2]

The performance and longevity of the transceiver lasers depend on the ambient temperature they operate in and the thermal characteristics of the packaging of these devices. The typical thermal management approach combines heat dissipating fins, e.g. heat sinks, and directed airflow.

Fig 3. Test set-up of different heat sink designs on QSFP28 connector cages. (Advanced Thermal Solutions, Inc.)

Recently, Advanced Thermal Solutions, Inc. (ATS) tested a variety of pin and fin-style heat sinks for their comparative cooling performance on a standard QSFP connector cage. For this setup, an even amount of heat was provided to each connector site via a heater block. Individual thermocouples measured the heat flux resulting with the different heat sink types.

A main goal of this test was how each of four heat sinks would perform while relying on airflow incoming from just one side. By the time it reached the fourth heat sink would the airflow provide enough conduction for adequate cooling? An image from this series of tests is below in Figure 4.

Fig. 4. 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 tests results showed that the denser the heat sink pins or fins on the sink closest to the incoming air, the hotter the farthest away QSFP will be. Thus, the best solution used heat sinks whose pin/fin layouts were optimized to work in the actual airflow reaching them.

This meant more open layouts closer to the air source, allowing more air to reach denser pin/fin sinks farther from the air. The non-homogeneous heat sinks allowed for a low, uniform temperature across the QSFP for the most effective function of the QSFPs’ lasers.

microQSFPs

Cooling solutions are different between QSFP28 designs and microQSFP installations. QSFP28 transceiver cooling is typically provided at multiple connector sites. microQSFP modules, e.g. from TE Connectivity, have an integrated heat sink in the individual optical module. Used with connection cages that are optimized for airflow, their heat is controlled in high density applications.

Fig. 5. Integrated Module Thermal Solution (Fins) on microQSFPs Provides Better Thermal Performance and Uses Less Energy for Air Cooling. [3]

Fig. 6. A Video Demo from TE Connectivity Shows 72 Ports of microQSFP Transceivers Units Running at 5W Each and All Kept Under 55°C Temperature Using 82 CFM Airflow. [4]

Finally, another factor affecting cooling performance is surface finish and flatness. Designers can reduce thermal spreading losses by keeping the heat sources close to the thermal interface area and by increasing the thermal conductivity of the case materials.

For QSFP, the size of the cage hole for heat sink contact given in the multi-source agreement (MSA) can be increased giving a reduction in the thermal interface resistance and therefore module temperature.

References:
1. FMAD IO, http://fmad.io/images/blog/20160612-100g-connectors.png
2. https://arkansashq.wordpress.com/2016/10/11/pluggable-optics-modules-thermal-specifications-part-2/
3. microQSFP, http://www.microqsfp.com/
4. TE Connectivity, https://www.youtube.com/watch?v=k_qNj-yAKz4

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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Posted in Qpedia Thermal eMagazine, SFP/QSFP, thermal management

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Applications of Vapor Chambers in Thermal Management of Electronics

Posted on August 1, 2017 | 1 comment

The use of vapor chambers in the thermal management of electronics has grown exponentially since Advanced Thermal Solutions, Inc. (ATS) first wrote seven years ago about their ability to spread heat uniformly across the base of a heat sink, reducing the spreading resistance and enhancing the heat sink’s heat transfer capabilities when applied to high-powered components.

In a two-part series published originally in 2010 and based on an article from Qpedia Thermal eMagazine entitled, “Vapor Chambers and Their Use in Thermal Management,” it was explained that “a vapor chamber (VC) is basically a flat heat pipe that can be part of the base of a heat sink. It is vacuumed and then injected with just enough liquid (e.g. water) to wet the wick.” [1]

Similar to heat pipes, “The heat source causes the liquid to vaporize on the evaporator side. The resulting pressure increase in this area forces the vapor into the condenser side, which is the base of the heat sink. Here, the vapor transfers the heat to the heat sink, and it then condenses back to liquid. The liquid is pumped back to the base through the capillary action of the wick structure.”

In Fig. 1, two heat sinks are shown. One has a solid base and the other has a vapor chamber in its base and it is clear from the temperature distribution that the vapor chamber spreads out the heat across the base and distributes heat to a larger portion of the heat sink.

As the original article explained, “The very high equivalent thermal conductivity of the vapor chamber has spread the heat uniformly, leading to more efficiency from the heat sink.”

Vapor Chambers

Figure 1. Schematic View of Heat Sinks with (a) Solid Base and (b) Vapor Chamber Base. [1]

The second article runs through some of the equations that define the effective thermal conductivity of the wicking structure inside the vapor chamber and the impact that changing the wick material can have on its efficiency.

“This article shows that while a vapor chamber presents exciting technology, some calculations should be made to justify its use,” it continued. “In some situations, a solid copper block might provide better thermal performance than a vapor chamber. To use a vapor chamber instead of solid copper must be justified, for example, to reduce weight.

Another issue with vapor chambers presented by the article was that “some vapor chambers have a power limit of 500 watts. Exceeding this value might cause a dry out, as with a heat pipe, and could increase the vapor temperature and the pressure. The increase in internal pressure can deform the VC surfaces, or cause leakage from the welded joints.”

The study of vapor chambers has developed in the past seven years and, although some of the same issues remain, they are now thinner and lighter than ever and engineers are finding many new ways of incorporating them into cooling systems. Vapor chambers are now frequently used in applications ranging from hard drive disk cooling, PC cooling (not just for gamers and overclockers, but also for office computers), graphic card cooling, server cooling, high heat flux chips (IGBT and MOSFET), LED, and in consumer products (particularly mobile devices such as cell phones and tablets).

In addition to the benefits explained above, vapor chambers are critical in applications where height is limited, which is an increasing problem in today’s era of miniaturization, and where power densities are high. Vapor chambers are also important in applications where there are hotspots, where weight is a concern, and where there is a high ambient temperature or low airflow.

Hard Drive Disk Cooling

Several manufacturers in the hard drive market have turned to vapor chambers because of increases in spindle speed. In the past, many manufacturers and designers limited the thermal management of hard drives to using the aluminum case as a heat sink to dissipate the excess heat from the device, but as drives began working at 7,200 RPM and higher another option was required to ensure the reliability and longevity of the drive. [2]

A 2013 study that was published in International Communications in Heat and Mass Transfer explored the use of vapor chambers to cool hard drives in personal computers. The researchers found that adding vapor chambers to the cooling system could reduce the hard drive temperature by as much as 15.21%. [3]

Gaming, Overclocking, Personal Computing

The gaming and overclocking community has turned towards liquid cooling in recent years, as evidenced by a recent survey from KitGuru that showed 51% of its readers had already or would shortly be using liquid cooling for their personal computers. [4] While there is a trend in that direction, just under half (49%) of the respondents were also sticking with convection cooling options and many companies are incorporating vapor chamber technology in elaborate cooling devices (many with fans and heat pipes) for the PC market.

Cooler Master has introduced the V8 GTS CPU Air Cooler, which strongly resembles a car engine and has a horizontal vapor chamber and eight heat pipes. [5] The vapor chamber spreads the heat evenly from hotspots in the CPU and the heat pipes draw that heat into the tower’s heat sink.

Vapor Chambers

The Cooler Master GTS V8 has a distinct car engine look and uses vapor chambers, heat pipes, and heat sinks to cool PCs. (Cooler Master/YouTube)

ID Cooling has introduced several products that boast vapor chamber technology, including the HUNTER, and FI (which stands for Finland) Series CPU coolers. [6] Even gaming systems have gotten into the act with the recently announced, high-powered Xbox One Scorpio expected to include a vapor chamber array as part of its thermal management. [7] Microsoft’s announcement that it was using vapor chambers in Project Scorpio was not surprising because of the technology’s ability to fit into the tight confines of the gaming system.

Microsoft’s Project Scorpio introduced a new, higher-powered gaming system that required an array of vapor chambers to keep it cool. (Microsoft)

Also, the increasing capabilities and power of next-generation graphics cards has led to a trend in the industry to use vapor chambers as part of a package to cool these components. Nvidia is one of the biggest names in graphic cards and for both the Titan X and the GeForce GTX 1080 (each launched in 2016) vapor chamber are used with a blower to dissipate the increased power of the devices. [8]

It is not only the gaming community that is benefiting from vapor chamber cooling. Hewlett Packard (HP) has also explored using vapor chambers for multiple purposes. HP released a white paper last year about using 3-D vapor chambers in its Z Coolers to enhance their thermal efficiency as well as reduce the acoustic impact of the fans. [9] Also last year, HPE Labs released a study of vapor chambers for cooling multiple chip modules dissipating 250 W and operating temperatures up to 45°C and found that “VC (vapor chamber) performs better for: high power, power density, off center or asymmetric heat sources.” [10]

Server Cooling

Much like in graphic cards or gaming systems, vapor chambers are increasingly used in server cooling applications because their size and weight allows them to fit into tight spaces, particularly in applications with high component density. For example, Rugged has released an M120 1-U server rack that includes vapor chambers to spread the heat evenly and high-speed fans to pull the heat out of the system. [11]

A study by Aavid Thermacore from the 2007 ASME InterPACK Conference explained that in blade processors that need to dissipate 100-300 W with heat sinks lower than 30 mm, vapor chambers could be used as the base of the heat sink to improve effective spreading and improve performance by 25-30%. [12] Radian’s Intel Skylake heat sink that is intended for server chips installed in a 1-U chassis put this into practice with a vapor chamber in its base that enhances the effective thermal conductivity of the stamped aluminum fins. [13]

Radian’s Intel Skylake heat sink uses a vapor chamber in the base to evenly spread the heat and improve the heat transfer through the fins of the heat sink. (Radian)

For more on the topic of vapor chambers as heat sink bases, read https://www.qats.com/cms/2017/07/26/vapor-chambers-solid-material-base-high-power-devices.

LED Cooling

A more recent development in the use of vapor chambers is their inclusion in LED packages. A 2016 study from the 37th International Electronic Manufacturing Technology Conference outlined the use of vapor chambers along with finned heat sinks in the thermal management of LED to enhance the thermal performance and provide a “more economical” process than making the heat sink larger or using more expensive materials. [14]

Advanced Cooling Technologies (ACT) also released a case study about cooling high-powered LED applications, such as ultraviolet (UV) cutting devices, which said, “Vapor Chambers are an important tool in LED thermal management, since they act as flux transformers, spreading the high input heat flux over the entire surface of the vapor chamber. This allows the heat to be removed from the vapor chamber by conventional cooling methods.” [15] ACT added that it developed C.T.E matched vapor chambers that allow for direct bonding with the LED and “dissipate heat fluxes as high as 700 W/cm2 and 2kW overall.”

Vapor chambers are also being used in automotive LED applications to prevent failures by spreading the heat quickly from the source. A study from the 2011 International Heat Pipe Symposium found that a vapor chamber with distilled water dropped the LED temperature from 112.7°C to 80.7°C, reduced thermal resistance by 56%, and reached steady state faster than conventional systems. [16]

Mobile Devices

The most obvious market for vapor chambers is mobile devices. Last fall, the news was filled with stories about Samsung cell phone batteries reaching thermal runaway and airplane passengers being forced to turn off the phones for concern about a midair fire. With their thin design and low weight, vapor chambers can be used to spread the heat quickly from batteries or high-powered processors in phones, laptops, tablets, etc. and reduce the risk for catastrophic failures.

A 2016 study from the International Journal of Heat and Mass Transfer described vapor chambers being used to reduce hotspots to improve the comfort of users, which is a problem unique to mobile devices. [17] The researchers proposed a “biporous condenser-side wick design” that “facilitates a thicker vapor core, and thereby reduces the condenser surface peak-to-mean temperature difference by 37% relative to a monolithic wick structure.”

A recent story from EE Times noted that the combined shipments of mobile devices was expected to decline in 2017, marking the third straight year of reduced shipments [18], but with companies expending resources to develop 5G technology there is still a need for superior cooling options moving forward and vapor chambers appear to be a perfect fit in mobile thermal management systems.

References
[1] “Vapor Chambers in Thermal Management”, Qpedia Thermal eMagazine, Sept. 2007.
[2] http://www.pcguide.com/ref/hdd/op/packCooling-c.html
[3] P. Naphon. S. Wongwises, and S. Wiriyasart, “Application of two-phase vapor chamber technique for hard disk drive cooling of PCs,” International Communications in Heat and Mass Transfer, January 2013.
[4] https://www.kitguru.net/components/cooling/andrzej/majority-of-kitguru-readers-now-planning-on-liquid-cooling/
[5] http://www.coolermaster.com/cooling/cpu-air-cooler/v8-gts/
[6] http://www.idcooling.com/Product/series/category_parent/25/name/AIR%20COOLING
[7] https://www.youtube.com/watch?v=RE2hNrq1Zxs
[8] http://www.pcworld.com/article/3102027/components-graphics/nvidias-monstrous-new-titan-x-graphics-card-stomps-onto-the-scene-powered-by-pascal.html and https://www.nvidia.com/en-us/geforce/products/10series/geforce-gtx-1080/
[9] http://www8.hp.com/h20195/v2/getpdf.aspx/4AA6-1205ENW.pdf?ver=2.0
[10] https://www.labs.hpe.com/techreports/2016/HPE-2016-85.pdf
[11] http://www.coresystemsusa.com/filedata/prod/443m120_1u_rugged_rackmount_computer.pdf
[12] http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1595384
[13] https://www.radianheatsinks.com/intel-skylake-heatsink/
[14] K.S. Ong, C.F. Tan, K.C. Lai, K.H. Tan, and R. Singh, “Thermal management of LED with vapor chamber and thermoelectric cooling,” 37th International Electronic Manufacturing Technology Conference, 2016.
[15] https://www.1-act.com/led-thermal-management-case-study-cte-matched-vapor-chamber/
[16] Ji Won Yeo, Hyun Jik Lee, Soo Jung Ha, et al., “Development of Cooling System of LED Headlamp for Vehicle Using Vapor Chamber Type Heat Pipe,” 10th International Heat Pipe Symposium, November 2011.
[17] GauravPatankar, Justin A.Weibel, and Suresh V.Garimella, “Patterning the condenser-side wick in ultra-thin vapor chamber heat spreaders to improve skin temperature uniformity of mobile devices,” International Journal of Heat and Mass Transfer, October 2016.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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Vapor Chambers and Solid Material as a Base for High Power Devices

Posted on July 26, 2017 | Leave a comment

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Microelectronics components are experiencing ever-growing power dissipation and heat fluxes. This is due to dramatic gains in their performance and functionality. To cope with the heat issues of tomorrow’s technology, more efficient cooling systems will be required. It should be noted that as computer systems continue to compact, the components adjacent to the processors are experiencing an increase in power dissipation.

As a result, ambient temperatures local to the microprocessor heat sinks have increased, and temperatures in excess of 45°C have been reported. [1] Improvements are needed in all aspects of the cooling solution design, i.e., packaging, thermal interfaces, and air-cooled heat sinks. The article discusses the use of vapor chamber technology as a heat spreader to help cool high-power devices.

Introduction

Spreading resistances exist whenever heat flows from one region to another in a different cross-sectional area. For example, with high performance devices, spreading resistance occurs in the base plate when a heat source with a smaller footprint is mounted on a heat sink with a larger base plate area. The result is a higher temperature where the heat source is placed.

The impact of spreading resistance on a heat sink’s performance must not be ignored in the design process. One way to reduce this added resistance is to use highly conductive material, such as copper, instead of aluminum. Other solutions include using heat pipes, vapor chambers, liquid cooling, micro thermoelectric cooling, and the recently developed forced thermal spreader from Advanced Thermal Solutions, Inc. (ATS).

In the case of vapor chambers (VC), the general perception has been that phase change technologies provide more effective thermal conductivity than solid metals. The spreading resistance of the base for both solid metal and conventional VC heat spreaders is defined as:

Vapor Chamber (1)

Where Ts [°C] is the temperature of the hottest point on the base, and Tb,top [°C] is the average temperature of the base top surface. [1]

Vapor Chambers

Figure 1. Schematics of a) Heat Pipe and b) Vapor Chamber [2], with c) Photo of Vapor Chambers. [3]

Table 1 shows the thermal conductivity of different materials in spreading the heat at the base. Heat pipes and VC emerged as the most promising technologies and cost effective thermal solutions due to their excellent uniform heat transfer capability, high efficiency, and structural simplicity. Their many advantages compared to other thermal spreading devices are that they have simple structures, no moving parts, allow the use of larger heat sinks, and do not use electricity. This article’s emphasis is on vapor chambers.

Is a heat pipe considered a material? Should we include vapor chambers in this table?

The principle of operation for VC is similar to that of heat pipes. Both are heat spreading devices with highly effective thermal conductivity due to phase change phenomena. A VC is basically a flat heat pipe that can be part of the base of a heat sink. Figure 1 shows the schematics of a typical heat pipe and VC. [2]

A VC is a vacuum vessel with a wick structure lining its inside walls. The wick is saturated with a working fluid. The choice of this fluid is based on the operating temperature of the application. In a CPU application, operating temperatures are normally in the range of 50-100°C. At this temperature range water is the best working fluid. [3]

As heat is applied, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it condenses, releasing its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action, ready to be vaporized again and repeat the cycle.

The capillary action of the wick enables the VC to work in any orientation, though its optimum performance is orientation dependent. The pressure drop in the vapor and the liquid determines the capillary limit or the maximum heat carrying capacity of the heat pipe. [4] For electronics applications, a combination of water and sintered copper powder is used. [2]

A VC, as shown in Fig.1 (b), is different from a heat pipe in that the condenser covers the entire top surface of the structure. In a VC, heat transfers in two directions and is planar. In a heat pipe, heat transmission is in one direction and linear.

The VC has a higher heat transfer rate and lower thermal resistance. In the two-phase VC device, the rates of evaporation, condensation, and fluid transport are determined by the VC’s geometry and the wicks’ structural properties. These properties include porosity, pore size, permeability, specific surface area, thermal conductivity, and the surface wetability of the working fluid. [5] Thermal properties of the wick structure and the vapor space are described in the next section.

Effective Thermal Conductivity

Wick Structure

Heat must be supplied through the water-saturated wick structure, at the liquid-vapor interface, for the evaporation process to happen. With water and sintered copper powder, the water becomes a thermal barrier due to its much lower thermal conductivity compared with the copper. [2]

There are several ways to compute the effective thermal conductivity of the wick structure.

For parallel assumption:

(2)

For serial assumption:

(3)

For sintered wick structure, Maxwell gives: [2]

(4)

Chi gives: [2]

(5)

Where:

(6)

In the equations above, Kl and Ks are the thermal conductivities of water and copper, respectively, ε is the porosity of the wick, rc and rs are the contact radius (or effective capillary radius) and the particle sphere radius, respectively.

Table 2 shows a comparison of effective thermal conductivity (W/m°C) for the wick using equations 2—5. It appears that Equation 5 gives a more realistic value. This is also the typical value used in Vadakkan et al. [6]

Table 2. Effective Thermal Conductivity for the Wick Structure.

Vapor Space

Effective thermal conductivity for vapor chambers used in remote cooling applications has been derived from Prasher [4], based on the ideal gas law, and from the Clapeyron equation for incompressible laminar flow conditions.

(7)

Where Hfg is the heat of vaporization (J/Kg), P is pressure (N/m2), ρ is density (kg/m3), d is the vapor space thickness (mm), R is the gas constant per unit mass (J/K.Kg), μ is the dynamic viscosity (N.s/m2), and T is the vapor temperature (°C).

As shown in Equation 7, effective thermal conductivity is a function of thermodynamic properties and vapor space thickness. Larger vapor space thickness reduces the flow pressure drop, and thus increases the effective thermal conductivity. Note that the effective thermal conductivity is relatively low at low temperatures. This has significant implications for low heat flux applications or start-up conditions [2].

Drawbacks

There are a few drawbacks to using a VC instead of solid copper. Some VC have a power limit of 500 watts. Exceeding this temperature might cause a dry out and could increase the vapor temperature and pressure.

An increase in internal pressure can deform the VC surfaces or cause leakage from the welding joints. Other factors to be addressed include cost, availability, and in special cases, the vapor chamber’s manufacturability.

When to Use a Vapor Chamber

The early design stages are when to decide if it makes sense to use a heat pipe/VC instead of copper or other solid materials to better spread heat. To predict the minimum thermal spreading resistance for a VC, a simplified model was developed by Sauciuc et al. [1]. Their model assumes that the minimum VC spreading resistance θsp is approximately the same as the evaporator (boiling) resistance θev.

(8)

Here, hev [W/m2K] is the boiling heat transfer coefficient and Aev [m2] is the area of the evaporator (heat input area). It is also assumed that the boiling regime inside the VC is nucleate pool boiling. This is a conservative assumption, since in reality the spreading resistance in a VC is greater than just the boiling resistance. If the spreading resistance calculated from this simplified model is higher than that of a solid copper base, then a VC should not be used. [1]

The boiling model is based on Rohsenow’s equation for nucleate pool boiling on a metal surface, and is given by: [7]

(9)

Where μf is the dynamic viscosity of the liquid, hfg is the latent heat, g(ρf – ρg) is the body force arising from the liquid-vapor density difference, σ is the surface tension, cp,f is the specific heat of liquid, Cs,f and n are constants that depend on the solid-liquid combination, Prf is the liquid’s Prandtl number, and ΔT = [Ts – Tsat], which is the difference between the surface and saturation temperatures.

It can be seen that the heat flux is mainly a function of fluid properties, surface properties, and the fluid/material combination, and that superheat is required for boiling. For electronics cooling applications, it is widely accepted that water/copper is the optimum combination for VC fabrication [1].

The evaporator heat transfer coefficient definition is:

(10)

The ratio of phase change spreading over copper spreading can be estimated for the base of conventional rectangular heat sinks using Rohensaw’s equation and conventional modeling tools, Figure 2 from [1] shows the relationship of this ratio versus base thickness (solid metal heat sink only) for different footprint sizes. The heat input area is kept constant for this plot. This figure shows that for spreading resistance ratios greater than 1.0, the ratio decreases with increasing condenser size.

This implies that the VC type base is better situated for larger condenser sizes. The figure also indicates that ratio 1.0 occurs at greater base thickness for larger condensers. For example, with a 200×200 mm footprint, a VC would outperform a corresponding copper base heat sink (with a thickness of 10 mm or less). However, with a 50×50 mm footprint the sink’s base thickness would have to be less than about 2.5 mm for the VC to make the same claim. [1]

Figure 2 also shows that there is a “worse case point” when comparing the thermal performance of a VC and a solid copper base heat sink. This is identified by the maximum in the curve for the 50×50 mm footprint at a base thickness of 10 mm. At this point the spreading resistance ratio is at its largest value, which indicates the worst performance for the VC (when compared with the corresponding solid copper base). In general, there will be a maximum base thickness (dependent on heat source size and footprint) in considering a VC base.

Unless weight is a major concern, with a base thickness above this maximum, a VC base should not be considered. Conversely, for a heat sink base thickness below this maximum, a VC base is a viable option.

Figure 2: Ratio of Phase Change Resistance (Rohensaw’s Equation) Versus Solid Metal Resistance. [1]

Summary

Although a VC enhances heat spreading through high effective thermal conductivity, some modeling needs to be considered early in the design stage. Because a VC is a liquid filled device, cautions need to be exercised in its deployment in electronics. The dry out or loss of liquid due to poor manufacturing will render the VC as a hollow plate, thus adversely impacting device thermal performance.

In some situations as shown earlier, a solid copper base might provide better spreading of heat without the potential pitfalls of a VC.

References:
1. Sauciuc, I. Chrysler, G., Mahajan, Ravi, and Prasher, Ravi, “Spreading in the Heat Sink Base: Phase Change Systems or Solid Metals?”, IEEE Transactions on Components and Packaging Technologies, December 2002, Vol. 25, No. 4.
2. Wei, X., Sikka, K., Modeling of Vapor Chamber as Heat Spreading Devices, 10th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronics Systems, 2006.
3. Wuttijumnong, V., Nguyen, T., Mochizuki, M., Mashiko, K., Saito, Y., and Nguyen, T., Overview Latest Technologies Using Heat Pipe and Vapor Xhamber for Cooling of High Heat Generation Notebook Computer, Twentieth Annual IEEE Semiconductor Thermal Measurement and Management Symposium, 2004.
4. Prasher, R, A Simplified Conduction Based Modeling Scheme for Design Sensitivity Study of Thermal Solution Utilizing Heat Pipe and Vapor Chamber Technology, Journal of Electronic Packaging, Transactions of the ASME, 2003, Vol. 125, No. 3.
5. Lu, M., Mok, L., Bezama, R. A Graphite Foams Based Vapor Chamber for Chip Heat Spreading, Journal of Electronic Packaging, December 2006.
6. Vadakkan, U., Chrysler, G., and Sane, S., Silicon/Water Vapor Chamber as Heat Spreaders for Microelectronic Packages, IEEE SEMI-THERM Symposium, 2005.
7. Incropera, F., Dewitt, D., Bergman, T., and Lavine, A., Introduction to Heat Transfer, Wiley, Fifth Edition, 2007.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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