Category Archives: How To

Understanding how to attach heat pipes into an assembly to cool electronics

In this video, we show how to attach heat pipes into an assembly. This 8-minute video covers:
– Mechanical press fit, thermally conductive epoxy, and soldering
– Grooving the base
– Some best practices

If you work with heat pipes in nearly any application, this video will be helpful to your work. See the video “How to Attach Heat Pipes into an Assembly

==> ATS’ large offering of heat pipes includes 350+ round and flat heat pipes, you can learn about that on our web site at https://www.qats.com/Products/Heat-Pipes

==> Check out our YouTube channel for more engineering education and how to videos: https://www.youtube.com/user/heatsinks

==> Have questions on the use of heat pipes in your application? Email us to talk to an engineer: ats-hq@qats.com

How to Calculate the Loads for a Liquid Cooling System

This article presents basic equations for liquid cooling and provides numerical  examples on how to calculate the loads in a typical liquid cooling system. When exploring the use of liquid cooling for thermal management, calculations are needed to predict its performance. While it is often assumed that a liquid coolant itself dissipates heat from a component to the ambient, this is not the case. A closed loop liquid cooling system requires a liquid-to-air heat exchanger. Because of its structure, several equations must be calculated to fully understand the performance and behavior of a liquid cooled system.

Cold plates bring localized cooling by transferring heat to a liquid that dissipates into the ambient or a secondary liquid. ATS cold plates cool high-powered electronics, IGBT modules, lasers, motor devices, automotive components, medical equipment, and other applications where liquid cooling is needed. Their internal, mini-channel fin structure enhances the surface area to maximize heat transfer with low pressure drop characteristics and provides uniform surface temperature.

Engineering How-To: Attaching Heat Pipes into an Assembly

Heat pipes are commonly used for cooling electronics by transporting heat from one location to another. They may part of a system that cools a certain very hot component, but they are used, typically in multiples, to bring cooling to electronic assemblies. Here are some common attachment methods used when assembling heat pipe-based cooling applications.

Press Fitting

First, we look at a cooling system where several heat pipes are integrated with a series of cooling metal fins. As shown, the fins may be mechanically press fit over the heat pipes resulting in a structure like that in Figure 1.

At this finned end of the assembly the heat transfers from pipe to fins where it dissipates to the air. These fins are typically stamped from sheet metal and the holes stamped through as well. When they’re properly sized, the fins press fit tightly on the raised heat pipes. The heat transfer is normally very good. To optimize thermal transfer, the fins can be soldered to the pipes, but press fitting into tight holes should provide more than sufficient performance.

Soldering

The other ends of these heat sinks are soldered into grooves in an aluminum plate. (Figure 2) This is an aluminum plate and the heat pipes are copper. In order to solder we need to nickel plate the aluminum. Then we add solder paste into the grooves and then the heat pipes are inserted into the grooves.

The solder paste is usually a low temperature solder paste, typically based on tin bismuth alloys with melt temperature of about 138°C. That’s important because you really can’t bring the heat pipes to more than 250°C or else the water in the heat pipes will boil and the heat pipes will burst. So, during the assembly process you would put the solder paste into these grooves, then insert the heat pipes, and then clamp it with some sort of fixture to maintain the contact.

Then the whole assembly will go through an oven to reflow the solder paste. The reflow oven will precisely control the temperature of the air inside and will also have some kind of circulating fan so that the part heats evenly and quickly. Temperature control in the oven is critical to avoid exceeding the max temperature of the heat pipes. Other reflow methods for heating up an assembly might include a soldering iron, torch or hot air gun. But these methods can be risky and difficult. It is difficult to heat the part evenly and to control the temperature that the heat pipe is being exposed to.

Thermal Epoxies

In a prototype environment you might turn to an epoxy for attaching heat pipes to assemblies. There are number of thermally conductive epoxies available. Their thermal conductivity ranges from 1 to 6 W/mK. When a heat pipe is epoxied into an assembly, the bond line is so thin that it really doesn’t make too much of a temperature difference, even when compared to solder. There might be a few degrees difference which is usually acceptable in a prototype when you’re in testing mode and are aware that there could be a temperature difference of a few degrees. That’s easily calculated from the specs on the epoxy.

To begin the epoxying process, first you either mix your epoxy or use a mixing tube. You apply a thin layer into the groove and then insert the heat pipe. The grooves shown here are for heat pipes that are pre-bent and fit very precisely. Once in place, a flat plate that goes on top and is clamped down during the epoxy curing period.

In the example here, the epoxy has room temperature cure. Once the heat pipes are in and clamped down, the assembly can be conveniently left for a time at room temperature for the epoxy to cure. For a shorter time, the assembly can go into an oven at a high temperature – not a soldering temperature, but still hot enough that it will accelerate the cure time.

When embedding heat pipes into a surface a good practice is to machine the grooves slightly deeper than the heat pipes are. Then, you can create a fixture that is like a negative of this plate with raised areas where those heat pipes. Such a fixture will press the heat pipes down into those grooves. After they’re epoxied or soldered in the assembly the heat pipes and base will be at the same height for optimum thermal contact.

In this kind of application, flat heat pipes should be used. They can maximize the surface contact area where there are hot components. And in applications where the components do not come in direct contact with the pipe it’s often easier to use round heat pipes. This is because round heat pipes are easier to bend and have slightly better thermal performance than the flat heat pipes. So whenever possible we use the round heat pipes, but when they are embedded into a surface and they have contact with the components then we use the flat heat pipes.

The above article is taken from a descriptive video by Advanced Thermal Solutions, Inc. that you can find on the ATS YouTube page at: https://www.youtube.com/watch?v=I5CQsBWKtOg

“Heat Sink Selection Made Easy” Free Technical Webinar on June 13

Advanced Thermal Solutions, Inc. (ATS) will present Heat Sink Selection Made Easy, a free technical webinar for engineers involved in the thermal management of electronic components. The hour-long webinar begins at 2:00 ET on Thursday, June 13.

The heat dissipation needs of todays components are more challenging than ever. Choosing the right heat sink the first time is essential. With so many application requirements and heat sink options, this can be a daunting task, but it is made easier by having an informed approach.

In this webinar, attendees will learn the importance of system airflow and its impact on heat sink design; attachment methods and how to solve thermal and mechanical design challenges; and how to make the right off-the-shelf or custom heat sink choice for your application and budget.

Presenting Heat Sink Selection Made Easy is Dr. Kaveh Azar, president, CEO and founder of Advanced Thermal Solutions. Dr. Azar is an active participant in the electronics thermal community and has served as the organizer, general chair and the keynote speaker at national and international conferences sponsored by ASME, IEEE and AIAA.

How to View the June 13th Webinar:

• The webinar starts at 2PM ET and will be available for 24 hours, until 2PM ET Friday the 14th.
• ATS felt that this approach would help engineers in other time zones to be able to watch the webinar.

• Today’s webinar speaker, Dr. Kaveh Azar, is happy to take your questions via email.
• Please send email to ats-hq@qats.com and write in the email’s subject, “Heat Sink Selection Webinar Question”

Some Basic Principles of Wind Tunnel Design

Wind tunnels generate uniform air flows, with low turbulence intensity, for thermal and hydraulic testing. These devices have been around for more than a century, and are used in many industries, including aerospace, automotive, and defense. They also play a key role in electronics thermal management. Wind tunnels are made in different shapes and sizes, from just 30 cm long to large enough to contain a passenger airplane. But the basic idea behind all wind tunnels is universal.

There are two basic kinds of wind tunnels. One is the open type, which draws its air from the ambient and exits it back to the ambient. This kind of wind tunnel provides no temperature controls. The air follows the ambient temperature. The second type of wind tunnel is the closed loop wind tunnel, whose internal air circulates in a loop, separating it from outside ambient air. The temperature in a closed loop wind tunnel can be controlled using a combination of heaters and heat exchangers. Air temperatures can be varied from sub-ambient to over 100oC. Figure 1 shows a schematic of a closed loop wind tunnel.

In general, closed loop wind tunnels are made with the following sections:

1-Test section

2-Settling chamber

3-Contraction area

4-Diffuser

5-ÂBlower assembly

6-Heater/heat exchanger assembly

Figure 1. Schematic of an ATS Closed Loop Wind Tunnel.

A good quality wind tunnel will have a flow uniformity of 0.5-2% and turbulence intensity of 0.5-2%. It should provide temperature uniformity within 0.1-0.5oC at the inlet of the test section [1].

To achieve uniform, high quality flow in the test section, the settling chamber and the contraction area are used to smooth the flow. The role of the settling chamber, which is upstream of the contraction area, is to eliminate swirl and unsteadiness from the flow. The settling chamber includes a special honeycomb and a series of screens. As long as a flows yaw angles are not greater than about 10o, a honeycomb is the most efficient device for removing swirl and lateral velocity variations and to make the flow more parallel to the axial axis [2]. Large yaw angles will cause honeycomb cells to stall, which increases the pressure drop and causes non-uniformity in the flow. For large swirl angles, screen meshes should be placed before the honeycomb. For swirl angles of 40o, a screen with a loss factor of 1.45 will reduce yaw and swirl angles by a factor of 0.7. Several screens are needed upstream of the honeycomb to bring the swirl down to 10o.

Using a honeycomb will also suppress the lateral components of turbulence. Complete turbulence annihilation can be achieved in a length of 5-10 cell diameters [2]. Honeycombs are also known to remove the small scale turbulence caused by the instability of the shear layer in front of them. This instability is proportional to the shear layer thickness, which implies a short honeycomb has a better ratio of suppressed turbulence to that generated.

Screens break up large eddies into smaller ones which decay faster. They lower turbulence drastically when several screens are placed in a row. Screens also make flow more uniform by imposing a static pressure drop which is proportional to velocity squared. A screen with a pressure drop coefficient of 2 removes nearly all variations of longitudinal mean velocity. Low open area screens usually create instabilities. In general, screens should have openings larger than 57%, with wire diameters about 0.14 to 0.19 mm. Sufficient distance is needed between multiple screens to stabilize static pressure from perturbation. This distance is typically a percentage of the settling chamber diameter.

The contraction area is perhaps the most important part of a wind tunnel’s design. Its main purpose is to make the flow more uniform. It also increases the flow at the test section, which allows flow conditioning devices to be at lower flow section with less pressure drop. Batchelor used the rapid distortion theory and estimated the variation in mean velocity and turbulence intensity [3]

A considerable number of shapes have been investigated for contraction, including 2-D, 3-D and axisymmetric shapes with various side profiles.

The shape of the contraction can be found using potential flow analysis. Consider the axisymmetric contraction shown in Figure 2 [4]

Figure 2. Schematic of an Axisymmetric Contraction [4].

The design of a wind tunnel is a lengthy process and, as shown above, it requires extensive knowledge and experience in both theory and construction. A novice might attempt to construct a tunnel, but considering the time spent, it might not be justified economically. Wind tunnel design also depends on economic and space constraints. Larger wind tunnels allow more space to have all the conditioning elements in place. A space-constrained wind tunnel must compromise some features at the cost of reduced flow quality, but can still be acceptable for practical engineering purposes.

References

1. Azar, K., Thermal Measurements in Electronics Cooling, Electronics Cooling Magazine, May 2003.
2. Bell, J. and Mehta, R., Design and Calibration of the Mixing Layer and Wind Tunnel, Stanford University, Department of Aeronautics and Astronautics, May 1989.
3. Batchelor, G., The Theory of Homogeneous Turbulence, Cambridge University Press, 1953.
4. Edson, D. and Joao, B., Design and Construction of Small Axisymmetric Contractions, Faculdade de Engenharia de Ilha Solteira, Brazil, 1999.