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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.
Posted in Analysis, Engineering, Engineering & Science Chat, heat, heat sink, Heat Sink Attach, heat sink clip, heat sink design, Heat Sink Material, Heat Sinks, heat transfer, Manufacturing, materials, Thermal, Thermal Analysis, Thermal Design, thermal electronic coolers, thermal management, Thermal Research
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.
Figure 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.
Figure 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]
Table 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
increases.
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].
Figure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2].
Figure 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.
Figure 7. Conceptual Design Schematic of a Flat Heat Pipe [1].
Figure 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.
Figure 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.
References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid Liquid/Air Cooling System, IEEE, Semiconductor Thermal Measurement and Management Symposium 2006.
Passive Thermal Management Technology for Fuel Cells
Part 1: Control Valve Tests
Last Month’s Qpedia featured part 1 of the two-part article series, “Passive Thermal Management Technology for Fuel Cells”, which discussed control valve tests. The article discussed tests performed by Burke et al, which demonstrated that the passive thermal control of fuel stacks is possible. They found that using either the electronic proportional control valve or the thermostatic valve can effectively control the fuel cell stack temperature. Read Part 1 in the December 2013 Issue by downloading it at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues-Content/114.aspx
Part 2: Passive Cooling Plates
The second part of “Passive Thermal Technology for Fuel Cells” is featured in this month’s Qpedia, focusing on passive cooling plates. The author discusses the importance of fuel cells by illustrating a description of Burke’s study using passive thermal management technology to regulate the PEMFC stack temperature. This study demonstrated that passive thermal control of fuel cell stacks is feasible and that the passive removal of heat is controllable and provides the highly uniform thermal environment desired for fuel cell operation. This revolutionary thermal control approach was shown to reduce the components and parasitic power compared to the traditional pumped loop thermal control approach.
Download the current Qpedia Thermal eMagazine to read the full article.
The life expectancy of most products is estimated at some point prior to their introduction. Reliability analyses are an integral part of the design cycle of a product. In all reliability calculations, temperature is the key driver. The predicted life span from these calculations is often the deciding factor for introducing the product or investing more resources in redesign.
The questions that linger are: to what level of accuracy can we determine the temperature magnitude, and what is the impact of temperature uncertainty on the predicted reliability (i.e., the expected life of the product)?
When a system is operating, it incessantly experiences temperature and power-cycling. Such fluctuations, resulting from system design and operation or from complex thermal transport in electronic systems, create large bandwidths in temperature response. Whether it happens in the course of an analysis or a compliance/ stress testing, we often overlook the accuracy by which temperature is measured or calculated. Yet to truly obtain an adequate measure of a systems reliability in the field, such temperature data is essential.
To demonstrate the impact of temperature on reliability, consider the two models commonly used in practice. The Arrhenius model [1], often referred to as Erroneous, is perhaps the most broadly used model in the field. Equation 1 shows the reaction rate (failure rate) k and the acceleration factor AT. KB is the Boltzmann constant (8.617 x 10-5 eV/K) and Ea is the activation energy. All temperatures are in Kelvin. Activation energy depends on the failure mechanism and the materials (for example, 0.3 – 0.5 for oxide defects, and 1.0 for contamination).
[1]
The second model, Eyring, often referred to as More Erroneous, is shown by Equation 2.
[2]
The data shows that the uncertainty band is between 7 to 51%. These numbers by themselves are alarming, yet they are commonly encountered in the field. In either case, Stand-Alone or Device-In-System, being able to accurately determine the temperature or air velocity in a highly three-dimensional thermal transport environment is not a task to be treated casually.
To measure the impact of such uncertainty on the reliability prediction, it’s best to calculate its impact on the Acceleration factor AT.
Let us consider the case when:
T1 = 40oC
T2 = 150oC
Ea = 0.4 eV
kB = 8.6×10-5 eV/K
This results in AT = 48. Now, let us impose a 10% and 35% uncertainty on the temperature measurement of T2. Table 1 shows the result of this error on the acceleration factor.
Table 1 clearly demonstrates how a small degree of uncertainty in temperature measurement can negatively impact the Acceleration Factor and, thus, the reliability predictions where AT is often used. The first row shows the correct temperature. The second row shows the result of a 10% error in temperature measurement (i.e., 165oC instead of 150oC). The last row shows the impact of a 35% error (i.e., 202oC vs. the 158.6oC that the device is actually experiencing). The end result of this error in measurement is a 230% error in the Acceleration Factor.
One may think such an error is rare, but the contrary is true! In a simple device-case-temperature measurement, the temperature gradient could be in excess of 20oC from the die to the edge of the device. Or the air temperature variation in a channel formed by two PCBs could exceed 30oC. Of course, there are variations due to geometry, material and power dissipation that are observed in any electronics system. If we add to these the effects of improperly designed instruments, the combination of physical variation and the instrument error could certainly be detrimental to a products launch.
Longevity and life cycle in the market are keys for a products success. Therefore, to determine system performance, a reliability analysis must be performed. Since time is of the essence, and first-to-market is advantageous, the quickest reliability prediction models (analysis in general) will continue to be popular. To make such models, the use of Equations 1 and 2, or others more meaningful, must include accurate component and fluid temperature data. Measurement is heavily relied upon for temperature and air velocity determination. It is imperative to employ instruments designed for use in electronics systems with the highest level of accuracy and repeatability. High-grade instruments with quality output will enhance the reliability of the product you are working on.
SUMMARY
Small errors in temperature and air flow measurements can have a significant effect on reliability predictions. The origin of these errors lies in the measurement process or the use of inaccurate instruments. The former depends on the knowledge-base of the experimenter. That is why a good experimentalist is even a better analyst. You must know where to measure and the variations that exist in the field of measurement. Electronics system environments are notorious for such variations. It is repeatedly seen that, in one square centimeter of air flow passage between two PCBs, you can have temperature variations in excess of 30oC. Therefore, measurement practices and instrument selection must address these changes and not introduce further errors because of inferior design. Besides its design, an instrument’s construction and calibration should not introduce more errors. Accurate and high-quality instruments are not only essential for any engineering practice, their absence will adversely impact reliability predictions of a product at hand. No company wants to have its products returned, especially because of thermally induced failures.
References:
1. Klinger, D., Nakada, Y., and Menendez, M., AT&T Reliability Manual, Van Nostrand Reinhold, 1990.
2. Azar, K., The Effect of Uncertainty Analysis on Temperature Prediction, Therminic Conference, 2002.
Posted in Instrument, thermal management, Thermal Research, Thermal Science