Continue reading »]]> In part 1 of our 3 part series, “Sensors for Temperature Measurement and their Application”, we introduced various kinds of sensors  and discussed the linear and exponential relationships that temperature has in the operation of the electronic components.  In part 2 we’ll cover three specific sensor types: the resistor thermometer, thermocouple and diode transistor.  In part 3 of our 3 part series we’ll finish up and discuss infrared or radiation, flu0rescent detector, and liquid crystal.

1 — Resistance Thermometer

With these sensors, the resistance of the sensing element changes with temperature. The sensors come in two primary forms: thermistors (lightly doped semiconductors) and metal resistors. Equations 3 and 4 represent the relationships between resistance and temperature for these two sensors, respectively:

Figure 1 shows a surface-mounted RTD (resistance temperature detector) that can be installed onto a surface for temperature measurement:

Figure 1: Surface mounted RTD (photo courtesy of RDF Corporation)

 The following must be considered when using these types of sensors:

1. The sensor (resistor) must be in intimate contact with the test specimen — solder or careful epoxy is recommended.
2. The sensor must be placed in an isothermal region — constant temperature over the sensor.
3. The resistor power dissipation (if in voltage mode) must be minimized to not impact the problem.
4. This sensor is suitable for part-level measurement as it can be embedded directly on the die.

2 — Thermocouples (TC)         

These sensors are far and away the most commonly used devices in the field. Wide flexibility and broad availability enable their use for a variety of temperature measurements. TCs work on the principle that bringing together two wires of different elements or alloys produces a voltage as a result of temperature. Equation 5 provides the governing principle for TCs:

Table 2 shows some of the typical TC types that are used in electronics thermal measurement.

Table 2: Thermocouple Types and Their Respective Voltage Outputs [2]

Of the TC types shown above, E, J, K and T are the most commonly used. Many thermocouple meters on the market can use all of these sensors interchangeably. That’s because the voltage output of these TCs is in the same range; hence, the internal electronics can be designed to accommodate each of them.

There are some unique features about each sensor type that one needs to know. For example:

• E-type —  Though accurate, has a limited range.
• J-type —  Should not be used in a humid environment, since the iron component of the TC will oxidize, resulting in erroneous output.
• K-type —  Though widely used, the voltage output can be negatively impacted if the wire kinks.
• T-type —  Can be an effective heat transfer medium, because of its copper component, either as a fin or a conductor.

It is also important to note that thermocouples measure temperature at the point where the two wires are connected. The smaller the junction, the more precise the temperature that is read. A large TC junction will result in the temperature being averaged over its entire area. Multiple junctions, as shown in Figure 2, will have the same impact. In Figure 2, the multi-junction created as a result of twisting the wires prior to spot-welding the ends (the TC below), creates a significantly larger junction. Whether measuring surface or fluid temperatures, the number reported by this TC will report an average temperature over a 2-3mm junction length.

Thermocouple errors can be attributed to the following areas:

• Poor junction connection
• Galvanic action
• Thermal shunting
• Electrical noise
• Installation problem due to tester

 Figure 2: Single and Multi-junction Thermocouple Sensors [3]

Of the errors listed above, electrical noise is uniquely problematic, especially in today’s high frequency equipment. A TC can be used in a 4-wire format to resolve the electronic noise that may affect the reported temperature. Using a 4-wire thermocouple, as shown in Figure 3, we can measure temperature and electrical noise.

Let us consider a J-type thermocouple formed of Iron and Constantan. All four wires are spot-welded together to form the TC junction. The temperature can be read across any of the Iron and Constantan combinations (?), and the electronic noise can be read across either the two Irons or the two Constantans. Because two similar metals cannot create the Seebeck effect (convert thermal differentials to electric voltage), whatever signal is measured on these wires is the electronic noise in the measurement domain.

 Figure 3: Four-wire Thermocouple System for the Measurement of Electronic Noise and Temperature

Measuring surface temperature is always a challenging process. The following steps will help to increase the accuracy of such measurements:

• Keep installation size as small as possible.
• To reduce conduction errors, bring thermocouple wires away from the              junction, along an isotherm for at least 20 wire diameters.
• Locate the measuring junction as close to the surface as possible.
• To avoid changes in convective or radiative heat transfer, design the installation so that it causes minimum disturbance to any fluid flow or the least possible change in the emissivity of the surface.
• Reduce the thermal resistance between the measuring junction and surface to as low a value as possible.

3 — Diode or Transistor

Diodes and transistors are parts whose electrical properties are a function of temperature. Diodes are broadly used for temperature measurement, either as embedded sensors in functional devices or as a thermal test chips. Figure 4 shows one such thermal test chip for device-level simulation.

Figure 4.  Thermal Test Chip [3]

The following depicts the general considerations for usage of semiconductor materials for temperature measurement:

• Every semiconductor device has at least one electrical parameter that is a              function of temperature.
• Thermal test chips use the thermally sensitive parameter of semiconductor devices to measure chip junction temperature.
• Separate heating and sensing elements are usually used to avoid for electrical switching.
• Thermal calibration of the sensing device is necessary.
• Thermal test chips provide an effective means of measuring chip junction temperature in an actual package configuration.
• Use of materials is subject to availability/suitability for the intended package application.

We’ll conclude our series with part 3, addressing infrared thermography, optical probes and liquid crystal thermography

References:
1. Klinger, D., Nakada, Y., Menendez, M., AT&T Reliability Manual,
Van Nostrand Reinhold, 1990.
2. Azar, K., Thermal Measurement in Electronics Cooling, CRC Press,
1997.
3. Advanced Thermal Solutions, Inc., Tutorial Series, “Principles of
Temperature Measurement”.
4. thermVIEW™ System, product of Advanced Thermal Solutions, Inc.
5. White, F., Viscous Fluid Flow, McGraw-Hill, 3rd Ed., 2005.

If  you are in need of sensors for thermal measurement, click now to ATS’s sensor family.  Tired of using thermocouples that are finicky and breakable? Try ATS’s spot sensor.  It’s durable and cost effective.  Learn more by clicking to ATS Spot Sensor.   You can also email or call us with your questions on temperature measurement and one of our engineers will be happy to help you.  Email us at ats-hq@qats.com or call  us at 781-769-2800.

Continue reading »]]> We are kicking off our 2012 webinar series with the webinar, “What is The State of the Art in Thermal Management?” We’ll be holding it on January 19, 2PM EST.

It is a generally very well attended webinar, covering what new technologies emerged in 2012 in thermal management, which are useful, and which might not be worth checking. To register, click to our GoToMeeting registration page here: Registration for What is The State of the Art in Thermal Management?

Continue reading »]]> Today we start a three part series on on Temperature Measurement and Their Application.  There is an IT axiom that says, “garbage in, garbage out” and no where is that more true than in thermal analysis.  If you measure your data incorrectly, you’ll have no chance of getting the data you need to design the best thermal management solution for your application.

In today’s market, it is very rare to see electronic equipment that has not undergone extensive thermal evaluation, either by measurement or simulation. Inevitably, the temperature of the device junction or case, or the enclosure, has been measured to ensure that the system will operate to its intended specifications.  A quick look at the equations associated with stress in a lead wire, and with the acceleration factor used in reliability calculations, will show why temperature plays such an important role in electronics equipment [1].

Equations 1 and 2 clearly demonstrate the linear and exponential relationships that temperature has in the operation of the electronic components. Concurrently, simulation tools are used extensively in today’s thermal design. But, due to the complexity of the electronics packaging and composite nature of the materials used, the simulation data must be verified in order to ensure reliable data is obtained. In this article, we present different sensors and their application domains in electronics thermal management.

Table 1 shows six primary sensors used in temperature measurement:

Table 1.  Standard Temperature Transducers [2]

In part 2 of our three part series, we’ll start consider each sensor in detail, focusing on the resistor thermometer, thermocouple and diode transistor.

References:
1. Klinger, D., Nakada, Y., Menendez, M., AT&T Reliability Manual,
Van Nostrand Reinhold, 1990.
2. Azar, K., Thermal Measurement in Electronics Cooling, CRC Press,
1997.
3. Advanced Thermal Solutions, Inc., Tutorial Series, “Principles of
Temperature Measurement”.
4. thermVIEW™ System, product of Advanced Thermal Solutions, Inc.
5. White, F., Viscous Fluid Flow, McGraw-Hill, 3rd Ed., 2005.

If  you are in need of sensors for thermal measurement, click now to ATS’s sensor family.  Including our industry leading Candlestick Sensor.   You can also email or call us with your questions on temperature measurement and one of our engineers will be happy to help you.  Email us at ats-hq@qats.com or call  us at 781-769-2800.

Continue reading »]]>

ATS has expanded once again this week with our tie up with Weiss Company.  Weiss is a Canadian technical electronics sales organization.  We’re excited to have them on board frankly as they bring quite a bit to the Canadian market space.  But, what’s in it for our customers?

First, they are local to Canada.  So while ATS’ main headquarters are in Norwood, MA (just 20 miles south of Boston), we are close to Canada but not in Canada.  Weiss is there and that will make a big difference bringing ATS support to thermal engineers there.

Second, they aren’t “just” a sales organization.  They are composed of both field sales engineers, (FSE) and certified electronics technicians (CET).  What this means for our customers is that Weiss extends ATS’ support network right into Canada.  The Weiss company has a direct like to our factory and design team.  Customers of Weiss/ATS will have best possible support for their thermal management programs.

Third, they have been in the market for 40 years, so they are a stable company with an  understanding of the needs of our customer’s in Canada.

If you are in Canada, drop Ken Kong at Weiss Co an email and let him know ATS sent you, his email is kkong at jgweiss.com

Continue reading »]]> We’ve got a new distribution partner, Test Equipment Connection!   Test Equipment Connection is located in Florida, but sells worldwide to a variety of markets.  They sell both new and refurbished test equipment.  So if you need something, and your budget is stretched, these are the guys to call!

We had a chance to have a quick phone conversation with their Manager of Business Development, Phil Vogel last week.   Test Equipment Connection has been around since 1993.  They both buy and sell test equipment and provide full support.  He noted that thermal analysis and measurement is a growing need in the technology sector and we couldn’t agree more!   Our latest one day short course on thermal management was sold out!

So, if you need test equipment, such as spectrum analyzers or RF or microwave test equipment or ATS thermal test instruments, visit their web site at http://www.testequipmentconnection.com/ and contact them!

You can read more about it at our press release page.

Continue reading »]]> We’ve got another webinar on tap, this one for next week, December 15th, 2011 at 2PM EST.  Our topic this month is, “How to Design Out Your Heat Sinks with Smart PCB Thermal Design”.

It seems like a contradiction in terms doesn’t it?  Why would a thermal management company like ATS teach you to design out your heat sinks?  Wouldn’t we be better business people if we helped you design in more heat sinks?

Well, no.  And here’s why.

At ATS, we are the leader in Innovations in Thermal Management.  And for us, those innovations have to be useful.  There are many innovations in thermal management in the market but most simply aren’t truly practical to deploy except in a lab.

Here’s the link to join our free webinar, Thursday, December 15th, 2PM EST, “How to Design Out Your Heat Sinks with Smart PCB Thermal Design”.

And, many innovations are not products. They are know how, experience and understanding of the vectors involved in todays complex thermal management problems.   And that’s why ATS’s team would teach our customer’s how to design out their heat sinks.  Because we believe in innovations in thermal management that work.   Understanding how to optimize air flow with your PCB works.  Once  your optimization is set, then buy your heat sinks (hopefully from us!).

Can’t make our webinar?  Then visit the following links to reach important resources on this topic:

• Four Printed Circuit Board Thermal Management Stragegies
• ATS’s QooLPCB program for low cost supply of heat sinks for your PCB
• ATS’s PCB Thermal Design Services

Continue reading »]]> Qpedia, ATS’s monthly thermal e-magazine is ready for our followers to read!  We’ve got a number of terrific articles this month, including:

• How to Compare the Thermal Performance of Different Heat Sink
• Synthetic Air Jet for Electronics Cooling
• Waste Heat Recovery of Electronic Equipment
• Effect of Cross-Flow on Straight and Inclined Jets

The article, “How to Compare the Thermal Performance of Different Heat Sink” is one we want to highlight.  Some of the key points covered include:

• Effect of materials in comparing heat sinks
• Proper wind tunnel testing
• Data analysis and computation for tests

Qpedia’s current issues are always no cost to our readers.  Our Qpedia back issues are available at no cost for readers who register on qats.com

Continue reading »]]> In our Qpedia Thermal eMagazine we reported on whether or not thermal grease is a reliable thermal interface material.   When thermal greases are operated for an extended length of time the thermal interface resistance can actually increase.  The degradation mechanisms of greases are considerably different and more complicated to characterize than other thermal interface solutions.   In this article we explore the failure mechanisms of grease interface layers as well as reliability testing and results.

To read this Qpedia Thermal eJournal article in full, just click to this link:  Long Term Thermal Grease Reliability

To learn more about thermal interface material, join our free webinar on “Understanding and Choosing the Best Thermal Interface Materials to Improve Heat Sink Thermal Performance“, Thursday, November 17th, 2PM

Continue reading »]]> We’ll be having our November webinar, “Understanding and Choosing the Best Thermal Interface Materials to Improve Heat Sink Thermal Performance ” on Thursday, November 17th at 2pmEater.  There’s no cost to join in the conversation.

In the meantime we thought we’d list many of the current thermal interface companies in the market today to give you a “one stop” shop to source the right thermal interface material for your next project.   Here’s the list,  all links worked based on testing here at QATS.

• Fujipoly:  Thermal putty and gap fillers.  Don’t discount this stuff!  Worth a look for bridging the IC to the case without a heat sink. Also useful where you need to fill in spaces with odd shapes.  In some applications can be near to phase change material performance
• Chomerics:  One of our favs.  Phase Change Material (PCM) that works almost as good as grease but with none of the mess. You must have the proper pressure over time on the heat sink to the PCM to make this work well.  Obviously, we’d recommend our superGRIP or maxiGRIP for that task.
• Bergquist: Various types of thermal material.  Just announced a new phase change material type.
• 3M: Thermal tape, pads and epoxies
• AI Technology: Phase Change Material and Thermal Grease
• Laird Technology: Gap filler, Phase Change Material, Thermal Grease
• Honeywell:  Phase Change Material and the innovative printable thermal material good to 150 degress C.
• Shin Etsu: Thermal greases, thermal gels, Phase Change Material and more
• Dow Corning Thermal:  Various thermal interface materials in both pads and films
• Locktite Thermal:  I won’t kid you, we aren’t fans of thermal epoxies but in some cases you just have to do it.  Locktite has some nice products for that and we know several telecomm OEMs using them.
• NuSiL:  Offers low outgassing thermal interface material
• Indium:  Metal thermal interface materials

To learn more about thermal interface material, join our free webinar on “Understanding and Choosing the Best Thermal Interface Materials to Improve Heat Sink Thermal Performance“, Thursday, November 17th, 2PM

Continue reading »]]> To cool hotter components, engineers are turning to larger fans and heat sinks and increased surface areas. The downside is that these hardware changes add significant cost to the design. Alternatively, a cooling system’s performance can be improved just by using a better interface material to lower thermal resistance at the interface of the case and the heat sink. Participants will learn to overcome related thermal challenges by making simple and cost-effective changes in thermal interface materials.

Register for this webinar by clicking to our registration page here:  Understanding and Choosing the Best Thermal Interface Materials to Improve Heat Sink Thermal Performance

Continue reading »]]> We’re holding another webinar today, November 3rd, at 2PM EST “Tips, Tricks and Techniques for the Best Use of CFD for Heat Sink and System Modeling”  In our webinar we’ll cover what we do at ATS in our use of CFD as part of our overall thermal analysis work.   We think you’ll learn some of what we do so you can apply it to your own work.

You can register at no cost to you at this link:  https://www2.gotomeeting.com/register/738227666

Continue reading »]]> Data centers are the heart of cloud computing.  They are impressive computing systems in and of themselves.  But with so much CPU power generating so much heat how can thermal engineers architect them for optimal thermal management?  One solution is adaptive cooling.  Our thermal engineering team has written a white paper addressing adaptive cooling and it can be yours without cost or obligation, just click to this link: “ATS White Paper: Adaptive Cooling in Data Centers“

Continue reading »]]> One of the basic concepts of electronics cooling is effective transfer of heat from semiconductor devices to the ambient using heat sinks or other cooling technologies. The effectiveness of this approach depends on a system’s total thermal resistance, which is composed of discrete thermal resistances on the path of heat from the source to the ambient. One of these resistances is spreading resistance.

Spreading resistance occurs whenever a small heat source comes in contact with the base of a larger heat sink. The heat does not distribute uniformly through the heat sink base, and consequently does not transfer efficiently to the fins for convective cooling. Figure 1 shows a CFdesign^® simulation solution for such an occurrence. The spreading resistance phenomenon is shown by how the heat travels through the center of a heat sink base causing a large temperature gradient between the center and edges of the heat sink.

 Figure 1: CFdesign solution showing temperature distribution
at the base of a heat sink

Spreading resistance is an increasingly important issue in thermal management as microelectronic packages become more powerful and compact and larger heat sinks are required to cool these devices. In high heat flux applications, spreading resistance can comprise 60 to 70% of the total thermal resistance.

A good estimate of spreading resistance is required to manage heat effectively using conventional air-cooled heat sinks. There have been a number of theoretical and experimental studies to estimate spreading resistance. Two of the most notable methods belong to Yovanovich et al. [1] and to Gordon N. Ellison [2].

While these extensive studies cover all aspects of spreading resistance, they involve cumbersome infinite series and complicated coefficient terms.  (click the link to read the rest of our article here on our electronics cooling blog)

Fortunately, a simpler solution is provided by Lee et al. [3] that yield results very close to those of complicated methods and can be easily programmed in any spreadsheet software. The solution is based on a circular spreader plate and circular heat source. Thus, square spreader plates and heating sources must be converted into circular geometries as shown in Figure 2.

Figure 2: Transformation of a square spreader and
heat source into a circular geomety [4]

The transformation is based on the areas of the plate and the heat source being the same for both the square and circular geometries. So, the equivalent radii in the circular case are given by Equations 1 and 2:

The equivalent radii, r₁ and r₂, may then be used along with the thermal conductivity, k, of the spreader plate with thickness, t, to obtain the spreading resistance Equation 3 [4]:
Equation 3 estimates the thermal conduction spreading resistance from the maximum temperature of a heat source to the convective surface on the top of the spreader plate. The parameters that are required to evaluate this equation are defined in Table 1.

Table 1: Definitions of Terms Used in Spreading Resistance Equation 3

The accuracy of Equation 3 is tested in a study by R. E. Simons of IBM [4] which compares its results with those of the exact series solution developed by Ellison [2].

These calculations are based on a fixed heat input area of 10 x 10 mm on a square thermal spreader plate 2.5 mm thick and ranging in size from 20 x 20 mm to 40 x 40 mm. Two values of thermal conductivity were used, ranging from 25 W/m·K for a material such as Alumina, to 400 W/m·K for a material such as copper. Similarly, two values of heat transfer coefficient were used, ranging from 250 W/m²·K to 1000 W/m²·K. These values represent what could be achieved with low and high performance forced convection heat sinks, respectively [4].

A comparison of the results for the conduction/spreading thermal resistance, R_sp, reveals that the error for the simplified formula ranges between -2.1% to +4.8% over all parameters [4].

Spreading thermal resistance can be mitigated in a number of ways. One convenient and intuitive method is to simply increase the thickness of the base of a heat sink. However, one should realize that increasing the base thickness will always mean decreasing the height of the fins. A loss of the fins’ convective surfaces beyond a limit would offset the benefits of reduced spreading resistance. In estimating the spreading thermal resistance, the convective resistance must be evaluated at the same time to find the optimized thickness of the heat sink base.

Another way to lower the spreading thermal resistance is to use heat sinks made from materials with high thermal conductivity values. The most commonly used material is Aluminum due to its light weight, good conductivity, and ease of manufacturing. However, some applications require heat sinks with higher thermal conductivities. In these cases, Copper is often used because its thermal conductivity is twice that of aluminum. The main drawbacks of Copper are its high cost, weight, and difficulty to fabricate.

Figure 3 shows the effect of a heat sink’s base thickness and material on the spreading thermal resistance. To construct this figure, Equation 3 is used for a spreader plate 40 mm square, with a 10 mm square heat source. The convective heat transfer coefficient is assumed to be 27 W/m²·K. At first, as seen in the figure, thickening the base has a pronounced effect on the spreading resistance. However, this effect becomes less significant at subsequently higher thicknesses. On the material side, Copper heat sinks consistently have a spreading resistance about half that of Aluminum heat sinks. This is because the thermal conductivity of Copper is about twice that of aluminum.

Figure 3. The Effect of a Heat Sink’s Material and Base Thickness
on Thermal Spreading Resistance

In some high-heat flux applications where the heat sink needs to be considerably larger than a semiconductor die, thickening the base and/or use of copper heat sinks may not be adequate to compensate for the conduction losses in the base. In these instances, forced spreading devices need to be implemented. One device that can be very useful in reducing the component temperature is a heat pipe.

A heat pipe is a vacuum hollow tube that has a wick structure on its internal walls and has a small amount of coolant fluid [5]. The fluid becomes vaporized by absorbing the heat in the evaporative section of the heat pipe. Vaporized fluid will condense at the condenser section of the heat pipe. The condensed fluid is then drawn back to the evaporator side by capillary force of the wick structure and the cycle would continue. As a result, heat pipes if properly designed and manufactured, could have thermal conductivities of 10-50 times of a solid copper. Figure 4 depicts the internal structure of a heat pipe.

 Figure 4. Internal Structure of a Heat Pipe.

Heat pipes can be embedded in the base of a heat sink to enhance the heat spreading at the base of the heat sink. The heat pipe spreads heat effectively across the sink base before it gets distributed to the fins. The result is more effective convective cooling since the fins’ base temperature will be higher, providing more effective use of the fins at the outer edge of the heat sink. There are several methods for embedding a heat pipe in a heat sink base. They can be completely embedded in the base or surface-embedded inside grooves machined into the base of a heat sink. In either case, conductive thermal epoxy is used to reduce the interfacial contact resistance between the heat pipe and the heat sink.

Figure 5 depicts two ATS maxiFLOW^TM heat sinks with embedded heat pipes. One sink has a completely embedded heat pipe and the other sink’s heat pipe is surface-embedded.

Figure 5. Embedded and Surface-Embedded Heat Pipe Heat Sinks.

Flat heat pipes, or vapor chambers, also have wide applications for reducing spreading resistance. Similar to conventional heat pipes, they are implemented at the base of heat sinks to spread heat evenly through the sink. They are more efficient than cylindrical heat pipes because they’re able to transfer heat in two directions. In Figure 6, the right-side image shows the internal structure of a vapor chamber, which is essentially the same as a cylindrical heat pipe. The image on the left shows how the use of a vapor chamber heat sink helps transfer heat uniformly throughout the entire heat sink. However, many designers avoid the use of heat pipes and vapor chamber because of their reliability. Often times, with proper design of the heat sink the use of such devices can be eliminated.

Figure 6. Vapor Chamber Internal Structure and Performance [6].

Advanced Thermal Solutions, Inc. (ATS) has developed a solution to the spreading resistance problem: the Forced Thermal Spreader, FTS^TM. The FTS^TM is a relatively thin, rectangular chamber in which a combination of mini and micro channels transfer and distribute heat at a very fast rate. The coolant fluid is drawn and circulated inside the FTS^TM chamber by a mini pump whose power consumption is less than 2 watts. The coolant is pushed through the micro channels at the center of the FTS^TM, creating high heat transfer coefficient. Its flow is then redirected and passed through a number of mini channels, transferring heat as it exits the FTS^TM chamber.

Research conducted at ATS has shown promising results for the FTS^TM as a vital and effective method for reducing spreading thermal resistance. As a baseline for comparison, vapor chamber performance data was considered from a study presented by D. Copeland (Semi-Therm 2003). In this study, a vapor chamber with the dimensions 125 x 75 x 4.5 mm was used on a 17 x 17 mm heat source. It yielded a spreading resistance of 0.12 °C/W. By contrast, the FTS^TM that was tested was smaller than the vapor chamber at 80 x 80 x 5 mm, and used a smaller heat source, 10 x 10 mm. In testing the FTS^TM, convective conditions similar to those in the vapor chamber were created so the performance of the two could be compared. Testing under these conditions, the FTS^TM has yielded a spreading resistance of 0.03 °C/W, which is four times better than the one for the vapor chamber.

A CFdesign^® simulation of the FTS^TM was also run to compare with the experimental results. Figure 7 shows the solutions for two scenarios: one when no heat spreader is used and the other when the FTS^TM is applied.

Figure 7. CFdesign^® Simulations Comparing the Effects of the FTS^TM on Heat Sink Temperature Distribution.

Figure 7 clearly shows that with FTS^TM, the temperature throughout the heat sink on the left is uniform and substantially on the lower side (cool) of the temperature scale. As the right-hand image shows, when the heat sink is used alone, the result is a high temperature concentration at the component area and much cooler temperatures elsewhere in the heat sink. This huge spreading resistance would keep the heat from being dissipated through the base and consequently the fins.

As semiconductor chips are made with more compact packaging and higher power densities, spreading resistance is becoming the dominant part of their cooling system’s total thermal resistance. In order to stay within the limits of air-cooled capabilities, spreading resistance must be properly managed based on available techniques and technology. Before investing in exotic and expensive solutions, it is worthwhile to determine whether the problem can be solved by simply optimizing the heat sink design parameters or considering a heat sink with higher thermal conductivity.

References:

1. Yovanovich, M., Muzychka, Y. and Culham, J., Spreading Resistnace of Isoflux Rectangules and Strips on Compound Flux Channels, University of Waterloo, 1998

2. Ellison, G., Maximum Thermal Spreading Resistance for Rectangular Sources and Plates With Nonunity Aspect Ratios, IEEE Trans. Comp., Hybrids, Manufac. Technol., Vol. 26, No. 2, 2003.

3. Lee, S., Song, S., Au, V., and Moran, K., Constriction/Spreading Resistance Model for Electronic Packaging, Proceedings of ASME/JSME Engineering Conference, Vol. 4, 1995.

4. Simons, R., Simple Formulas for Estimating Thermal Spreading Resistance, Electronics Cooling, May 2004.

5. Advanced Thermal Solutions, Inc., Heat Pipes: Heat Super Conductors, Qpedia, July 2007.

6. Mehl Dale, Vapor Chamber Heat sinks Eliminate Hot Spots, Thermacore International, Inc.

]]> http://qats.com/cms/2011/10/26/how-to-control-spreading-thermal-resistance/feed/ 0 http://qats.com/cms/2011/10/24/how-to-optimize-your-heat-sink-and-more-in-this-months-qpedia-thermal-ejournal/ http://qats.com/cms/2011/10/24/how-to-optimize-your-heat-sink-and-more-in-this-months-qpedia-thermal-ejournal/#comments Mon, 24 Oct 2011 23:04:13 +0000 admin http://qats.com/cms/?p=3616

Continue reading »]]> Qpedia Thermal eJournal, our publication on all things related to thermal management and electronics cooling is hot off the presses!  This months Qpedia features articles on:

• Getting the Most Out of Your Heat Sink Design: An Overview of the Parameters Which Influence Your Design
• CFD Simulation of Jet Impingement
• Spray Cooling Electronics in a 1-U Chassis
• Transient Response of a PCM with Embedded Graphite Nanofibers

Get your copy right away, for no charge, by clicking to Qpedia Thermal eJournal

Continue reading »]]>

Thermal Conductivity:  A measure of the ability of a material to transfer heat. Given two surfaces on either side of a material with a temperature difference between them, the thermal conductivity is the heat energy transferred per unit time and per unit surface area, divided by the temperature difference [1].

Thermal conductivity is a bulk property that describes the ability of a material to transfer heat. In the following equation, thermal conductivity is the proportionality factor k. The distance of heat transfer is defined as ∆x, which is perpendicular to area A. The rate of heat transferred through the material is Q, from temperature T₁ to temperature T₂, when T₁>T₂ [2].

The path of heat from the die to the outside environment is a complicated process that must be understood when designing a thermal solution. In the past, many devices were able to operate without requiring an external cooling device like a heat sink. In these devices, the conduction resistance from the die to the board needed to be optimized, as the primary heat transfer path was into the PCB. As power levels increased, heat transfer solely into the board became inadequate. Much of the heat is now dissipated directly into the environment through the top surface of the component. In these new higher-powered devices, low junction-to-case resistance is important, as is the design of the attached heat sink.

To determine the importance of material thermal conductivity in a specific thermal management application (e.g. a heat sink), it is important to separate the overall thermal resistance associated with conduction heat transfer into three parts: interfacial, spreading, and conduction resistances.

• An interface material enhances the thermal contact between imperfect mating surfaces. A highly thermally conductive material, with good surface wetting ability, will reduce interfacial resistance.
• Spreading resistance is used to describe the thermal resistance associated with a small heat source coupled to a larger heat sink. Among other factors, the thermal conductivity of the base of the heat sink directly impacts spreading resistance.
• Conduction resistance is a measure of the internal thermal resistance in a heat sink as heat travels from the base to the fins, where it dissipates into the environment. In regard to heat sink design, conduction resistance is less important in natural convection and low air flow conditions, becoming more important as flow rates increase.

Common units of thermal conductivity are W/mK and Btu/hr-ft-^oF.

Figure 2. Thermal conductivity for silicon thin film [3].

In the electronics industry, the constant push for smaller size and faster speeds has considerably reduced the scale of many components. As this transition now continues from the macro- to micro-scale, it is important to consider the effects on thermal conductivity and not to assume the bulk property is still accurate. Continuum-based Fourier equations cannot predict thermal characteristics at these smaller scales. More complete methods, such as the Boltzmann transport equation and the lattice Boltzmann method, are needed [3].

The effect of thickness on conductivity can be seen in Figure 2. The material characterized is silicon, which is widely used in electronics.

Figure 2. Thermal conductivity for silicon thin film [3]

Like many physical properties, thermal conductivity can be anisotropic depending on the material (directionally dependent). Crystalline and Graphite are two examples of such materials. Graphite has been used in the electronics industry where its high in-plane conductivity is valuable. Graphite crystals have very high in-plane conductivity (~2000 W/mK), due to the strong carbon-to-carbon bonding on their basal plane. The parallel basal planes are weakly bonded to each other, however, and the thermal conductivity perpendicular to these planes is quite low (~10 W/mK) [4].

Thermal conductivity is not only affected by changes in thickness and orientation;temperature also has an effect on the overall magnitude. Because of the material temperature increase, the internal particle velocity increases and so does thermal conductivity. This increased velocity transfers heat with less resistance. The Wiedemann-Franz law describes this behavior by correlating thermal and electrical conductivity to temperature. It is important to note that the effect of temperature on thermal conductivity is non-linear and hard to predict without prior research. The graphs below show the behavior of thermal conductivity over wide temperature ranges. Both of these materials, aluminum nitride and silicon, are used extensively in electronics (Figures 3 and 4, respectively).

In the future, higher-powered processors with multiple cores will push the need for improved thermal conductivity even further. Therefore, it is worthwhile to also investigate other areas of research and development in thermal conductivity enhancement for existing material used in electronics packages. One such area is the effect of nanotechnology on thermal conductivity, where carbon nanotubes have shown conductivity values near those of diamond due to large phonon-mean-free paths [7]. Development of new materials, and enhancement of existing materials, will result in more effective thermal management, as device power dissipation is on the steady rise.

References:

1. Thermal Conductivity, American Heritage Science Dictionary, Houghton Mifflin Company

2. Moran, M. and Shapiro, H., Fundamentals of Engineering Thermodynamics, p 47, 1988

3. Ghai, S., Kim, W., Chung, P., Amon, C., Jhon, M., Anisotropic Thermal Conductivity of Nanoscale Confined Thin Films Via Lattice Boltzmann, Chemical Engineering, Carnegie Mellon University, Nov. 2006

4. Norley, J., The Role of Natural Graphite in Electronics Cooling, Electronics Cooling, August 2001

5. Slack, G.A., Tanzilli R.A., Pohl R.O., Vandersande J.W.,J. Phys. Chem. Solids 48, 7 (1987), 641-647

6. Glassbrenner, C. and Slack, G., Thermal Conductivity of Silicon and Germanium from 3°K to the Melting Point, Physical Review 134, 4A, 1964

7. Berber, S., Kwon, Y., and Tomanek, D., Unusually High Thermal Conductivity of Carbon Nanotubes, Physical Review Letters, Vol 84, No 20, pp 4613-4616, 2000

Continue reading »]]> In our Twitter feed we saw that ADI has a new 8-video series on thermo- couples.  It’s a great set of videos on this topic.  Well presented and informative, we’d recommend our reader’s put them in their bookmarks to check out.  Well worth your time.

You can reach the full series by visiting ADI’s site here:  ADI’s Thermocouple 101 Video Series

ATS has also covered thermocouples here on our blog. Click to this link to read our two part series, “Thermocouples for Thermal Analysis: What they are and How they Work“.

Here’s one of the first of ADI’s thermocouple series to check out!

Continue reading »]]> Give us 60 minutes and we’ll give you a cooler datacenter!  That’s right, our webinar on October 27, “Tools and Techniques for the Thermal Management of Small, Medium and Large Scale Data Centers ” will teach you:

1. How to consider your datacenter like a system for your thermal management strategy
2. Do you approach cooling a small, medium or large scale datacenter differently or the same?
3. Are you focusing on the most critical cooling goals for your datacenter or are you chasing the wrong goals and watching heat and power usage spiral?

We’ll answer these questions and more at our free Webinar, “Tools and Techniques for the Thermal Management of Small, Medium and Large Scale Data Centers ”

To join, click to the following link and you can sign up:   Reserve Your Spot at ” “Tools and Techniques for the Thermal Management of Small, Medium and Large Scale Data Centers “