Tag Archives: thermal research

iMAPS New England 38th Symposium & Expo – May 3, 2011 The Largest Regional Microelectronics Symposium

The New England Chapter of IMAPS has assembled an impressive Technical Program for their 38th Symposium and Expo being held May 3, 2011 at the Holiday Inn Conference Center, Boxborough MA.

Representing a wide range of Microelectronic disciplines, an Exhibit Hall featuring 60 Companies and 300-500 Participants from North America and beyond.

For details on Exhibiting or Sponsoring, e-mail: HarveyS@imapsne.org

Compilers Come to the Rescue in Thermal Management

Generally when we think of thermal management here at ATS, we think of it in terms of hardware. We consider heat pipes or heat sinks. We look at air flow and it’s direction. We consider materials and if their thermal conductivity is sufficient for a project or not. But that’s not the only part of the equation. Software is as well. And not just software for thermal modeling, but in terms of controlling semiconductors to reduce the thermal load in a system.

The key here is temperature aware compiling for VLIW (very long instruction word) processors. By properly grouping instructions in a CPU, compiler developers can help reduce the overall thermal load in a system. They do this by grouping as many instructions as possible in parallel, thereby minimizing the CPU workload and it’s peak temperature. There are a couple of techniques that have been published in research papers that we want to make our readers aware of.

The first is the technique of TempIB or Temperature Aware Binding Technique. This approach was developed by Benjamin Carrion Schafer, Yongho Lee, and Taewhan Kim at the School of Electrical Engineering and Computer Science Seoul National University, Korea. This technique:

effectively binds the instructions executed in parallel to the coolest possible functional units for a given fixed schedule. It generates, for each instruction in a scheduled instruction word, a priority queue of the coolest functional units that can execute the instruction, and rebinds it to the coolest possible unit, considering the temperature as well as the power consumed by the instruction.

Tests by the team show that temperature can be lowered almost 13% using this technique in thermal management. You can read their paper at this link: “Temperature-Aware Compilation for VLIW Processors“.

Another approach by researchers Madhu Mutyam, at the International Institute of Information Technology, Hyderabad, India and Feihui Li, Vijaykrishnan Narayanan, Mahmut Kandemir, and Mary Jane Irwin at Pennsylvania State University uses

a compiler-based approach to make the thermal profile more balanced in the integer functional units of VLIW architectures. For balanced thermal behavior and peak temperature minimization, we propose techniques based on load balancing across the integer functional units with or without rotation of functional unit usage.

You can read their work at this link, “Compiler-directed thermal management for VLIW functional units

Using software in this fashion may not be the standard approach to thermal management, but, adding it into the mix of strategies for electronics cooling, it can clearly help the overall goal.

When is a book on thermal management more useful than using the internet?

photo of qpedia book seriesWe consume a lot of information here in our labs at ATS.  From researching information, to documenting information, through finding suppliers we use the Internet a lot.  And who doesn’t? There are hundreds of millions of people who use the Internet everyday for a wide variety of tasks for work and personal use.

And yet, despite all the value and ease by which the Internet makes information finding easy, ATS has published a book series on thermal management. Why is that?

Well, we’ve got a few reasons as to why we did this and one request, which we’ll get to in a minute.

As for why ATS has published a three volume set (with more books in the waiting) on thermal engineering the answer is simple, convenience.ATS’s three volume book set is effectively a yearly anthology of the articles from our thermal engineering journal, QPedia. Putting all the articles in one place, in one book, for a given year’s QPedia gives our readers the ability to easily access a complete years worth of QPedia Thermal eMagazine reference articles in one space. When you are in a time crunch, wading through a sea of results on Google or Bing might lead you in the wrong direction, open a QPedia book instead and you are likely to find your answer faster and it will be from a technically competent source. Now to our request,

Take a look at our books , you’ll be glad you did. They are chalk full of authoritative and respected articles on thermal management. No junk science and no dead ends searching the net. To see our book series just click to the ATS Thermal eBook Site for a look

Why Temperature Management is Critical

Temperature management of electronics matters and today is even more critical for the proper operation of systems. As Andre Pope of Mentor Graphics noted in his blog post, “How thermal testing can help increase reliability of electronic systems?

According to a study of the US Air Force Avionics Integrity Program from about two decades ago failure of electronics systems in about 55% of all cases was due to thermal issues. This figure is being quoted since then. Actually, I have my personal evidence for this from my own history of using a PC at home. My actual PC at home broke down twice. The first breakdown was something I could smell and see – there was a smoke signal issued by the machine: the controller IC in its hard-drive electronics simply burnt out. The second computer breakdown I had was when my 7 years old no-name PC stopped working completely. In this case I was not sure if it was a real hardware failure. I suspected that either the BIOS has forgotten its contents or my well known operating system gave up after 7 years of service. So, the ratio is 50-50.

Thus, making sure that we eliminate half of the possible failures is really very important. With such an action we can double the reliability of our systems. The question is, how one can achieve reducing the number of thermal problems? Of course with thermal-aware design. By doing so, we can eliminate our own failures.

In short, reliability is directly affected by temperature! Or, in mathematical terms:

Thermal  management of electronics formula: heat transfer is a function of temperature

Why thermal management of electronics is important formula 2: stress

thermal management of electronics formula 3: activation energy

Thermal management of electronics symbol definitions for math formulas

This answers the “why we should” care about thermal management certainly, but, why is thermal management such a challenge? Well, at ATS our team has identified five reasons:

  • Higher Frequency Circuits result in high heat flux
  • Smaller Packaging hence, high speed circuits tighter placement and higher density
  • Low Acoustic Noise Requirements Force chassis designers to utilize low-to-medium (200-400 LFM) speed air flow
  • Circuit Designers Determine Component Placement Sometimes resulting in less-than-optimal air flow at the PCB level
  • EMI Shielding Because of High Frequency Results in constricted area for air passage

I’m sure each of us could identify with at least one of the challenges we’ve listed above. And, those challenges – the raising of the bar if you will – is what we need to focus on to provide successful thermal management.

One of the ways we work with engineers is to educate them on thermal solutions; one tool we have for that is our Qpedia Thermal Journal, its published online once month, you can get the current copy, free, from our engineering staff to you by visiting, Qpedia Current Issue

Thermocouples for Thermal Analysis: what they are and how they work (part 2 of 2)

In part 1, we covered the basics behind what thermocouples are and how they work. In part 2, we’ll cover how thermocouples can be made and know when to select the right thermocouple for your project.

Thermocouples can be made of any two dissimilar metal wires, and their emf voltage depends on the composition of the chosen metals. However, what makes thermocouples so popular is that the materials used to construct them are restricted and their output emf’s have been standardized. Certain materials and combinations are better than the others, and some have basically become the standard for given temperature ranges. Table 1 lists some of the available thermocouples in the U.S. market.

Thermocouple Type Material Composition Temperature Range Uncertainity Color Code
T Cu vs. Constantan -250 to 350°C Greater of 1°C or 0.75% Blue-Red
K Chromel vs. Alumel -200 to 1250°C Greater of 2.2°C or 0.75% Yellow-Red
J Iron vs Constantan 0 to 750°C Greater of 2.2°C or 0.75% White-Red
R Platinum vs. Platinum-13% Rodium 0 to 1450°C Greater of 1.5°C or 0.25% None Established
S Platinum vs. Platinum-10% Rodium 0 to 1450°C Greater of 1.5°C or 0.25% None Established
C Tungsten 5% Rhenium vs. Tungsten 26% Rhenium 0 to 2320°C Greater of 4.5 °C to 425°C, 1% to 2320°C None Established
E Chromel vs. Constantan -200 to 900°C Greater of 1.7°C or 0.5% Purple-Red

Table 1: Different types of thermocouples

Selecting the right type of thermocouple for an application depends on many factors. These include sensitivity, temperature range, corrosion resistance, linearity of output voltage, and cost. For example, types R and S are relatively expensive and are not sensitive. However, they perform well at high temperatures up to 1768°C and are resistant to a number of corrosives. Type C thermocouples are suitable for higher temperature applications, but they are relatively expensive and corrode easily in an oxidizing environment. A Type T thermocouple is inexpensive and very sensitive, but will corrode at temperatures above 400°C. Type K is very popular for general use, relatively inexpensive, reasonably corrosion-resistant, and can be used at high temperatures, up to 1372°C. K-type thermocouples also have provide relatively linear output as compared to the other types [2].

The actual magnitude of the thermocouple emf is very small, and is in the order of few millivolts. At a given temperature, Type E has the highest output emf among common types, but this voltage is still measured in millivolts. The sensitivity of thermocouples is also relatively low. For instance, the voltage change per degree Fahrenheit from 38 to 93°C is only 36 microvolts. As a result, thermocouples require accurate and sensitive measuring devices and cannot be used for temperature changes of less than about 0.1°C. Traditionally, expensive voltage balancing potentiometers were used to measure emf. Today, a high quality digital voltmeter is sufficient [3].

The National Institute of Standards and Technology (NIST) has developed standard calibration curves for determining temperature based on the measured emf voltage. These data represent the output emf of thermocouples when an ice cold junction is used, and are incorporated in the memory of most DAQ systems.  Unfortunately, the temperature-voltage relationship of thermocouples is nonlinear and curve-fitted using polynomial functions. Obviously, the higher the order of the polynomial function, the higher the accuracy of temperature reading. The polynomial function should only be used inside the temperature range of the thermocouple type and should not be extrapolated. To save computational time, a lower order polynomial fit can be used for a smaller temperature range.

Thermocouple wires come in variety of sizes. Usually, the higher the temperature, the heavier should be the wire. As the size increases, however, the time response to temperature change increases. Therefore, some compromise between response and life may be required.

Thermocouples can be connected electrically in series or in parallel. When connected in series, the combination is usually called a thermopile, (whereas there is no particular name for thermocouples connected in parallel). A wiring schematic of a thermopile combination is shown in Figure 3.

Thermocouples example diagram

Figure 3 – Series-connected thermocouples forming a thermopile

The total output from n thermocouples will be equal to sum of the individual emf’s. The main purpose of using a thermopile rather than a single thermocouple is to obtain a more sensitive element. Parallel connection of thermocouples is used for averaging.

 

References:

  1. Omega Temperature Handbook, Omega Engineering, Inc. 2nd edition.
  2. Wheeler, A. and Ganji, A., Introduction to Engineering Experimentation, Prentice Hall Inc., pp. 240-246, 1996.
  3. Beckwith, T., Marangoni, R., and Lienhard, J., Mechanical Measurements, Fifth Edition, Addison Wesley Longman, pp. 676-685, 1993.