Category Archives: Thermal

ATS maxiGRIP and superGRIP Heat Sink Attachments

Advanced Thermal Solutions John O’Day and Len Alter showcase the patented heat sink attachments maxiGRIP and superGRIP. With its patented and discrete design, these heat sink attachments are well worth it for being your only choice for a cost-effective, high performing thermal solution.

What are Heat Pipes and What Characteristics Make Them Helpful for Electronics Cooling?

Heat Pipes, the Super Conductors
Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.

The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.

Figure 1. Schematic View of a Heat Pipe [1].

A heat pipe has three sections: the evaporator, adiabatic, and condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with
a liquid such as water. When the evaporator section (L ) is exposed to a heat source, the liquid inside vapor- izes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (L ). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section
of the heat pipe (La), the adiabatic portion, has a very small temperature difference.

figure2Figure 2. Pressure Drop Distribution in a Heat Pipe [1].

Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result
is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.

figure3Figure 3. Different Wick Structures

A typical heat pipe is made of the following:
1. Metallic pipe  The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.

2. Working fluid  Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the
operating temperature range.

3. Wick  The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance. Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.

Table 1. Heat Pipes with Different Structures and Operating Conditions [1]table1Table 1 shows experimental data for the operating temperature and heat transfer for three different types of heat pipes [1].

Certain factors can limit the maximum heat transfer rate from a heat pipe.

These are classified as follows:
1. Capillary limit
  Heat transfer is limited by the pumping action of the wick.
2. Sonic limit  When the vapor reaches the speed of sound, further increase in the heat transfer rate can only be achieved when the evaporator temperature
3. Boiling limit  High heat fluxes can cause dry out.
4. Entrainment limit  High speed vapor can impede the return of the liquid to the condense.

A heat pipe has an effective thermal conductivity much larger than that of a very good metal conductor, such as copper. Figure 4 shows a copper-water heat pipe and a copper pipe dipped into an 80°C water bath. Both pipes were initially at 20°C temperature. The heat pipe temperature reaches the water temperature in about 25 seconds, while the copper rod reaches just 30°C after 200 seconds. However, in an actual application when a heat pipe is soldered or epoxied to the base of a heat sink, the effective thermal conductivity of the heat pipe may be drastically reduced due to the extra thermal resistances added by the bonding. A rule of thumb for the effective thermal conductivity of a heat pipe is 4000 W/mK.

Figure 4. Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper Pipe [1].

Heat pipe manufacturers generally provide data sheets showing the relationship between the temperature difference and the heat input. Figure 5 shows the temperature difference between the two ends of a heat pipe as a function of power [2].

figure5Figure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2].

figure6Figure 6. Typical Round Heat Pipes in the Market.

There are many heat pipe shapes in the market, but the most common are either round or flat. Round heat pipes can be used for transferring heat from one point to another. They can be applied in tightly spaced electronic components, such as in a laptop. Heat is transferred to a different location that provides enough space to use a proper heat sink or other cooling solution. Figure 6 shows some of the common round heat pipes available in the market.

Flat heat pipes (vapor chambers) work conceptually the same as round heat pipes. Figure 7 shows a flat pipe design, they can be used as heat spreaders. When the heat source is much smaller than the heat sink base, a flat heat pipe can be embedded in the base of the heat sink, or it can be attached to the base to spread the heat more uniformly on the base of the heat sink. Figure 8 shows some common flat heat pipes.

figure7Figure 7. Conceptual Design Schematic of a Flat Heat Pipe [1].


figure8Figure 8. Commonly-used Flat Heat Pipes.

Although a vapor chamber might be helpful in minimizing spreading resistance, it may not perform as well as a plate made from a very high conductor, such as diamond. A determining factor is the thickness of the base plate. Figure 9 shows the spreading resistance for 80 x 80 x 5 mm base plate of different materials with a 10 x 10 mm heat source. The vapor chamber has a spreading resistance that is better than copper, but worse than diamond. However the price of the diamond might not justify its application. Figure 9 also includes the spreading
resistance from the ATS Forced Thermal Spreader (FTS), which is equal to that of diamond at a much lower cost. The FTS uses a combination of mini and
micro channels to minimize the spreading resistance by circulating the liquid inside the spreader.

figure9Figure 9. Thermal Spreading Resistances for Different Materials. [3] – ATS

Heat pipes have a very important role in the thermal management arena. With projected lifespans of 129,000-260,000 hours (as claimed by their manufacturers), they will continue to be an integral part of some new thermal systems. However, with such problems as dry out, acceleration, leakage, vapor lock and reliable performance in ETSI or NEBS types of environments, heat pipes should be tested prior to use and after unsatisfactory examination of other cooling methods.

1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc.,
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid Liquid/Air Cooling System, IEEE, Semiconductor Thermal Measurement and Management Symposium 2006.

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Announcing our ATS Electronics Cooling Webinars for Third Quarter of 2012

ATS, Advanced Thermal Solutions, Inc. will present technical webinars on electronic cooling topics in July, August and September 2012. Each of these free events will provide engineering-level training in a key area of modern thermal management.

Here are the different webinar topics and presentation times:

Using Thermal Interface Materials to Improve Heat Sink Thermal Performance

July 26, 2012 at 2:00 p.m. ET

To cool hotter components, engineers are using larger fans and heat sinks, and increasing surface areas. These hardware enhancements can add significantly to design costs. In many cases, cooling performance can be improved by using a higher performance interface material between the case and the heat sink. Participants will learn the importance of lowering thermal resistance using thermal interface materials, or TIMs, and the different kinds of TIMs available from the market.

Air Jet Impingement Cooling

August 23, 2012 at 2:00 p.m. ET

Ongoing increases in power in devices such as processors and IGBTs mean that higher capacity cooling methods are needed to remove excess heat. One such method is the jet impingement of a liquid or gas onto a surface on a continuous basis. Lab experiments at ATS have shown up to a 40% improvement in cooling achieved using this method. This webinar will explore jet impingement cooling theory, implementation and best practices.

LED Thermal Management in Commercial and Consumer Lighting Applications

September 27, 2012 at 2:00 p.m. ET

Excess heat directly affects both short-term and long-term LED performance. The short-term effects are color shift and reduced light output, while the long-term effect is accelerated lumen depreciation and thus shortened useful life. Participants will learn how to diagnose and solve thermal issues in consumer and commercial LED applications.

Each of these one-hour online tutorials will include detailed visuals, real world examples, instructions, definitions and references. Audience questions will be answered by the presenters during and after the presentation. One or more ATS PhD-level thermal engineers will be presenting live.

There is no cost to attend these ATS webinars, but virtual seating is limited. Registration is available online at, or by calling 1-781-949-2522.


Testing Thermal Interface Materials

Illustration: Parker Chomerics

Thermal interface materials, TIMs, provide the thermal pathway for transferring heat from components to heat sinks. At one time, most TIMs were simple, homogenous pads filled with thermally conductive fillers. But increasing power levels of processors and other components present a continuous need for improved thermal material performance. Today, a much wider range of TIMs is available, including phase change materials, compounds, and gap fillers.

When choosing a TIM, its essential to understand the testing methods to accurately determine the materials bulk thermal properties and in its performance.

The most common test is ASTM D5470: Linear Rod Method. This is the standard for measuring the thermal impedance of a TIM. Heat flow is carefully controlled through a test sample of a TIM. Typically, a heater is attached to an aluminum cylinder that has thermocouples arranged in series.

The thermocouples not only report temperature, but also the heat transfer through the known aluminum cylinder. Next, the interface material is compressed between the raised cylinder and an identical lower unit. Finally, a cold plate is attached to the bottom of the assembly to ensure the direction of heat transfer. The assembly can accommodate various material thicknesses and apply a range of pressure to the sample.

Another TIM test is laser flash diffusivity. Here, a small sample of interface material is subjected to a short pulse of laser energy. The temperature rise of the material is then recorded at a very high sample rate. Diffusivity is calculated using the equation shown below.

k = D/ρCp


k= thermal conductivity;

D = thermal diffusivity,

ρ = density of sample,

and Cp = specific heat.

The halftime of the sample is defined as the time between the start of the laser pulse to when the temperature of the back side of the sample has risen to half of its maximum value. The other variable in equation 1 is L, the thickness of the sample, which may be directly measured. Once diffusivity is known, it can be used in equation 2 to calculate thermal conductivity.

This laser flash method is very accurate as long as the density and specific heat are well known. However, it only measures thermal conductivity, as opposed to the ASTM standard which also measures thermal impedance. Thus, a key drawback to laser flash testing is that it doesn’t provide the contact resistance.

In comparisons of interface materials must be carried out by the user to provide meaningful results. Interface material testing procedures are different than heat sink testing methods. When testing several heat sinks it is possible to affix a thermocouple to the component’s case surface or to the heat sink itself and draw direct comparisons of performance. However, this approach will not work if the interface material is changed. To accurately compare interface materials, die-level temperature measurements must be taken, while the same heat sink is used in identical PCB and flow conditions.

New maxiFLOW Heat Sinks for Cooling DC-DC Converters

ATS now provides maxiFLOW heat sinks specially designed to cool eighth, quarter, half and full brick size DC-DC converters. The patented maxiFLOW heat sink design reduces air pressure drop and provides more surface area for more effective convection (air) cooling. The same ATS maxiFLOW technology is used in heat sinks cooling millions of BGAs and other PCB components,

The brick DC-DC converter heat sinks offer a range of fin patterns, directions and profiles to match different height and weight restrictions and airflow patterns. All of these heat sinks are protected with a gold anodized finish.

Each heat sink is provided pre-assembled with a layer of Chomerics T766 Thermflow phase change thermal interface material to enhance heat transfer from brick to heat sink. All of these heat sinks also come with three sets of screws in lengths of 5, 6 and 8 mm for varied attachment situations. The heat sinks pre-drilled hole patterns fit all major DC-DC converter designs.

DC-DC converters are circuits which convert direct current (DC) from one voltage level to another. They are extensively used in electronic devices serving communications, computing, data storage, health care, industrial equipment, instrumentation and test and measurement. Heat sinks are typically required to keep the converters running within safe operating temperatures.