Design for Thermal Issues

Unit 4: Managing thermal design

So far you will have seen something of the increasing need for managing the thermal aspects of the design, and have gained an appreciation both of where heat is generated and of the deleterious effect that unremoved heat may have on system performance and reliability. But clearly it is not enough to accept the possible impact of over-temperature without doing something about it, and this task has to be carried out in a structured way if it is to be effective. So, before we examine the detail of how heat is managed, in this Unit we are taking a overview of how the thermal design process is managed.

Unit contents

Introduction

You will be aware of a continuing trends towards smaller, lighter, low-cost and high-reliability designs of increasing complexity, and of the consequent emphasis on both ever-higher packaging density and increased power per unit volume:

But a further complication arises from the fact that product development cycle times for many electronics systems have shrunk from a few years to a few months. So the approach often previously taken, of building a product, testing it, and then dealing with any major thermal issues, no longer meets the requirements of the design schedule. In any case, build and test is both an expensive procedure and one that really only works when designs have a considerable thermal margin.

This combination of higher thermal density and time constraints creates a significant challenge for the thermal designer. Fortunately, progress in computational heat transfer and fluid flow simulation techniques, and major improvements in computing capability, have provided fast, accurate and cost-effective ways of predicting thermal behaviour and optimising design to enhance thermal performance, even at a system level. Such tools, in conjunction with limited validation testing, are rapidly replacing the older build-and-test approach to design.

However, not only do the power and schedule trends have implications for the tools and methods that are used, but they affect both the way in which the thermal designer interacts with the rest of the design team, and the timing of the interactions.

The place of thermal analysis in electronic design

Managing thermal risk

The ultimate goal of system thermal design is not the prediction of component temperatures, but rather the reduction of thermally-associated risk to the product. This risk, inherent to today’s high-density electronic systems, is manifested by compromised designs that do not meet projected schedules due to unforeseen thermal and/or reliability issues.

In the past, the thermal designer’s role was seen as one of predicting temperatures and ensuring that reliability limits were met for products. However, the role of an effective thermal designer is now much more than that, and is at its most critical and most useful in the early conceptual stages of product design. At this stage, a key function is to challenge specifications that are often used merely for convenience or for legacy reasons, and to uncover the true requirements, which in many cases may turn out to be more stringent than was initially anticipated.

Source information

Much of the discussion in the early part of this unit is derived from the paper Effective Thermal Design for Electronic Systems by Belady and Minichiello, which contains a rationale for starting thermal design early in product development and for using different levels of estimation and simulation at appropriate stages during the process.

More information on their project, and some photographs of the application can be found at Thermal Design Methodology for Electronic Systems (PDF file, 1,095KB).

 

Figure 1 illustrates this concept. Rather than wait until the later part of the development cycle (Figure 1a), the designer should be an integral part of the design right from the start (Figure 1b). The point these curves make is that intuition or simple hand calculation early in the design process is sometimes worth more than detailed CFD analysis at the end.

Figure 1: A comparison of approaches used in risk management for thermal design

A comparison of approaches used in risk management for thermal design

Source: Belady and Minichiello

Thermal design is the process by which engineers use temperature and airflow predictions to uncover potential risk areas for the system engineering team, the ultimate goal of the thermal design effort being to provide optimal designs that meet or exceed projected schedules and component requirements.

A key point here is that the thermal design engineer is an integral part of a systems engineering team (an example of which is shown in Figure 2), and it is it is important for the engineer both to identify the key players in the team and to understand the basic features and requirements of the product.

Figure 2: A typical system engineering team

A typical system engineering team

Source: Belady and Minichiello

Key information

It is critical that the thermal engineer develop feasible solutions in a timely and reasonably accurate manner. Many tools exist to assist during the design process, including heat transfer correlations, Flow Network Modelling (FNM), Computational Fluid Dynamics (CFD), as well as experimental measurement techniques. But the key to efficient and comprehensive thermal design is not necessarily to choose the “best” tool, but rather to use a range of tools in a way that gets an adequate answer quickly, so that the systems engineering team can make timely design decisions.

 

Figure 3 shows the impact the thermal engineer can make on the product. The further to the left that the issues and solutions are identified, the more positive the impact upon the product and product schedule. However, at some point, the thermal engineer can have a negative impact (further to the right) by identifying problems late and requesting changes when the design has become relatively fixed. This is not where the thermal engineer wants to be, as it conflicts with good design practice and the ultimate goal of Design for Excellence.

Figure 3: The thermal designer’s impact on the product schedule

The thermal designer’s impact on the product schedule

Source: Belady and Minichiello

[ back to top ]


Thermal design strategies

The thermal design task is not just to interact appropriately with the many players in the systems engineering team, but also to give specific advice on thermal design strategies. We have seen in Unit 2 that components heat both themselves and each other, and that there is potential for some parts of the assembly to exceed the temperature at which they will become unreliable. However, we can use conduction and convection (and to a lesser extend, radiation) to maximise life by minimising both temperature and temperature gradients. Typically we will do this at all levels, from thermal enhancement of individual component performance, through component and board layout to enclosure design. Ways in which board and component temperatures can be reduced include:

The practicalities of these methods will be dealt with primarily in Units 13 to 16, although there will inevitable be references elsewhere.

it is important to recognise the role of the thermal engineer in recommending appropriate choices, bearing in mind the overall aim of the design exercise, which is to produce a product that will meet the end-user requirements at the maximum possible profit. This means not incurring unnecessary cost! Tony Kordyban has pointed out the value of thermal simulation in allowing the engineer to target just those aspects of the design that could potentially cause non-compliance; there is no value in adding extra cooling, or in making the board larger to reduce the thermal challenge, if neither are required by the components. Worse, adding cost and/or size may make your product uncompetitive and unattractive to the purchaser.

Quote

“Reducing component temperature costs something; not only directly (heat sinks and fans and making holes in chassis costs money), but in other facets of the product design. One board may be split into two to spread out heat. Two devices might share a load to prevent overheating when one could perform the same function by itself. Clock speed might be reduced to keep power dissipation low. Some product concepts may not appear feasible because they are too hot to fit in the allotted space . . . there are many trade-offs to be made when playing the component temperature game.

“In an ideal design world, I don’t try to reduce component temperatures as much as possible. I want them to be as hot as allowable because that means that other facets of design have been given as much freedom to be optimised as possible. Simply put, don’t force components to be cooler than they have to be.

“On a typical circuit board that we make, about 95% of components dissipate little or no heat, and they are way below their operating temperature limit. The other 5% are the ones that keep me in business.”

Tony Kordyban, Hot Air Rises and Heat Sinks

 

[ back to top ]


Implementing thermal analysis in product development

A thermal design methodology

The framework of most electronic thermal design methodologies was first introduced in 1997 (by Biber and Belady) and later evolved into what is now known as the “Enhanced Product Design Cycle”. According to this method, the development cycle consists of three distinct phases:

concept development

detailed design

hardware test

During each phase, the most applicable and expedient thermal design tools should used. This ties in with a common-sense view that different types of thermal evaluations will be required at the different phases of a project. For example, at the quotation or concept phase, there is a need for a quick approximation of the thermal behaviour of a device; in the early design phase a tool is needed that can make time-efficient estimates for comparing different thermal designs; when the product becomes better defined, there will be a wish for more detailed thermal simulations, to identify potential hot spots and to evaluate different ways of ‘disarming’ these; finally, experimental measurement techniques will be needed to verify the accuracy of the design predictions. The tools available include:

The Biber and Belady methodology promotes a ‘fluid’ process (Figure 4) in which the predictions of tools used in the first phase are compared with subsequent predictions as the design enters the next phase and new tools are adopted. In this way, each step is validated and the thermal designers can test their intuition and the assumptions of their earlier work.

Figure 4: Enhanced product development cycle concept development

Enhanced product development cycle concept development

Source: Belady and Minichiello

Note that there is an overlap between the approaches, but no substitute for three elements of initial estimate, more detailed simulation, and verification of the estimates and simulations. We will be looking at the techniques involved in Units 8, 9 and 11, so only a brief description is necessary at this stage:

Flow Network Modelling (FNM) is a generalised methodology for calculating system-wide distributions of flow rates and temperatures in a network representation of a cooling system. Practical electronics cooling systems can be considered as networks of flow paths through components such screens, filters, fans, ducts, bends, heat sinks, power supplies, and card arrays. FNM employs the overall characteristics of these components, obtained from handbooks, vendor specifications or in-house testing, instead of attempting to calculate a detailed distribution of velocity and temperature within each element. As a result, FNM is very fast in terms of model definition, computation, and examination of results and is especially useful at the concept stage.

Computational Fluid Dynamics (CFD) studies the dynamics of elements that flow. Using CFD, a computational model is built that represents a system or device that is to be analysed. A CAD system then applies the fluid-flow physics to this virtual prototype, and the software outputs a prediction of the fluid dynamics. A sophisticated analysis technique, CFD not only predicts fluid flow behaviour, but also the transfer of heat, mass (such as in perspiration or dissolution), phase change (such as in freezing or boiling), chemical reaction (such as combustion), mechanical movement (such as an impeller turning), and stress or deformation of related solid structures (such as a mast bending in the wind).

Finite Element Analysis is a complex set of algorithms that uses finite element mathematical methods to solve problems in structural simulation, modelling and analysis. Aerospace and automobile designers, civil engineers, defence contractors, manufacturers of electronic equipment and university research teams use several variants of FEA. Generally, FEA is used to determine stress, deformation, heat transfer, magnetic field distribution, fluid flow, and other continuous-field problems that would be impractical to solve by other methods.

Grid generation systems. One of the main issues that needs to be solved for CFD calculations is the simultaneous transfer and simplification of CAD data to CFD input. Grid-generation software that will pre-process CAD files to create inputs for CFD solvers is available both as standalone tools or integrated into the thermal design package, so that the importation of design data from other sources is largely transparent to the designer.

[ back to top ]


Concept development phase

The concept development phase is the initial stage in the product design cycle. At the start of this phase, the product concept is in its infancy. This phase is characterised by rapidly changing product layouts and requirements as representatives from the systems engineering team meet to discuss requirements and to develop new ideas. Here, the goal of the thermal designer is to analyse scenarios thoroughly, yet rapidly, as well as offer design suggestions for improvement in real time. This phase concludes in a single product layout that meets or exceeds all requirements. At the conclusion of this phase, the thermal designer will have:

One of the most overlooked yet powerful tools that the seasoned thermal designer has is his or her intuition. In a matter of seconds, the experienced designer can draw on past experiences to provide a solution that may not be wholly accurate but is, in fact, good enough. These responses allow real-time sorting of many concepts and offer many benefits for system engineering teams. In order to do this, the thermal designer needs to develop a sense of intuition, which necessitates constant validation by using tools.

Based upon their ease of use, quick solve times, and limited required input data (usually geometry and fluid data only), the common tools used in the concept phase are generalised correlations for heat transfer and fluid flow (that is, hand calculations), spreadsheets (such as heat sink design optimisers), and Flow Network Modelling (FNM) techniques. FNM tools can be as simple as solving a network of resistances (both thermal and fluid) either with hand calculations and spreadsheets or by using commercially available tools.

In an early stage of product design, when the company is trying to win an order for developing a certain product, hand calculations can be performed to provide a quick estimation of the thermal performance of the product. Given the total dissipated power, a rough estimation of the mechanical design of the device, and the boundary conditions applied, resistance networks representing the different heat transfer paths within the system can be modelled. The Thermal Resistance Network (TRN) gives rather accurate predictions of relative heat transfer efficiency when comparing different design suggestions.

When TRNs get too complex – many thermal resistances in parallel leading to areas of different temperatures is the most common case – there is an option to use Microsoft Excel for handling the extensive number of equations to be solved simultaneously. Spreadsheets can handle advanced mathematical and engineering functions and provide graphics capabilities for displaying results.

A tool that could be used for more detailed analyses, still in the early phase of product design, is Flow Network Modelling (FNM). FNM is a generalised methodology for calculating system-wide distributions of flow rates and temperatures in a network representation of a cooling system, although the technique does not give the detailed level of results needed for predicting component temperatures. Practical electronics cooling systems can be considered as networks of flow paths through components such screens, filters, fans, ducts, bends, heat sinks, power supplies, and card arrays. To be able to use FNM, empirical correlations of the impact on the flow from these components are a prerequisite. FNM techniques are easy to master as long as the 2-D air flow paths can be clearly defined. In some cases, 2-D flow path prediction is not possible and advanced techniques such as CFD or testing may be warranted.

However, in order to quickly estimate average temperatures of sub-systems and compare the heat transfer efficiency of different designs, FNM is a useful tool. Regardless of whether FNM-specific software or spreadsheets are used, solutions are gained within seconds. This is extremely important in this phase of the development cycle.

[ back to top ]


Detailed design phase

Once a single layout has been agreed upon, the detailed design phase of work begins in earnest. The thermal designer must now focus on identified areas of thermal risk within the product. Here, thermal analyses become more detailed (and time consuming), while results become more refined. Experimental measurement and construction of mock-ups of system-critical areas may be required for input into models or to gain information concerning product areas that are difficult to model. The ultimate goal of the thermal designer is to dig deeply into critical areas and to offer design suggestions, which can facilitate an optimal product design that meets or exceeds the project schedule and reliability goals.

Tools commonly used in the electronics industry for detailed thermal design are Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) solvers. Typically, these types of analyses require longer setup/solve times and more detailed input data. Three-dimensional (3-D) modelling or simplification of existing 3-D Computer Aided Design (CAD) models may be required. User experience is required for the most accurate results. Furthermore, based upon system size and complexity, empirical sub-system flow resistance data may be acquired using flow-bench testing of fans, heat sinks and subsystems. CFD solvers marketed for the electronics industry afford designers the ability to perform heat transfer calculations in addition to fluid flow solutions.

The main issue with CFD is that extensive calculation times are required when the detail level gets high. At present, much effort is put on developing systems for transferring CAD-data directly to the CFD software, including automatic simplifications for speeding up the CFD calculations.

The main use of Finite Element Analysis (FEA) has traditionally been analysis of mechanical stress and deformation, although conduction heat transfer can also be included in the calculations. FEA analyses may be required to solve for component, interconnect or board temperatures once global airflow rates or heat transfer coefficients have been calculated.

Software packages are also available which combine FEA and CFD solvers.

[ back to top ]


Hardware test phase

Once product prototypes are available, the hardware test phase begins. The goal of the thermal designer here is to measure critical components and areas of risk experimentally within the product to verify the design. At this point, there should be no surprises. Additionally, measurements are compared to estimates in order to ‘calibrate’ or fine tune earlier models (calibrated models can be used in future studies). These comparisons are used to determine the accuracy of initial predictions and to help the designer develop “thermal intuition”.

While the most common tool for temperature measurement is certainly the thermocouple, additional tools, such as thermistors, resistance temperature detectors (RTDs), thermochromic liquid crystals, thermopiles and infrared imaging techniques are also available. Air velocity measurement is also valuable during this phase. Classic hot-wire anemometers provide the most precise measurements (that is, speed and direction), but can be fragile, difficult to calibrate, and expensive. Newer ‘rugged’ hot-wire multi-channel probes that measure speed and temperature may also be used. There is more about this in Unit 7.

[ back to top ]


Risk management

Figure 5 illustrates the importance of using the right tool at the right time. The red curve represents the risk level throughout the development cycle if only intuition, simple hand calculations and spreadsheets are used prior to testing the hardware. The green curve shows the risk level throughout the development cycle if a complete suite of tools is used. Note, in this case, the methodology approaches the ideal risk curve as a result of using the modelling technique that matches the design’s fluidity.

The key to minimizing the risk early is to use the tool that gets a reasonable answer most quickly. Note that the green curve shows that the same risk level can be reached in about a quarter of the time shown for the orange curve, which implies that an engineer using the blend of techniques proposed can be four times more productive.

Figure 5: Using the right tools for risk management

Using the right tools for risk management

Source: Belady and Minichiello

[ back to top ]


Summary and classification of thermal tools

Classification Tool Features
Hand calculations Experience, intuition Straightforward, fast evaluation at concept stage.
Thermal Resistance Network (TRN) Accurate, rapid solutions.

Renders a good overview of the thermal influence of different designs.
Spreadsheets Microsoft Excel Handles advanced mathematical and engineering functions.

Macros, Graphics capabilities.

Data table formatting.

Efficient for solving TRNs.
Flow Network Modelling (FNM)

Computational Fluid Dynamics (CFD)
MacroFlow


Coolit, Electronic Systems Cooling (ESC), Flotherm, Hotbox, Icepak, MacroFlow, UNIC, E-cool, Thermal Desktop, Fluent, ANSYS


Multiphysics CFD & FSI
Design of different types of air- or liquid-cooled electronics systems.

Quick evaluation of system-level thermal design.

Flow constraints important. Component and Board level modelling also possible.

No component temperature predictions – only system level.

Detail-level simulations and analyses of heat transfer and fluid flow.

Extensive calculation times when modelling with high level of detail.
Finite Element Analysis (FEA) ANSYS, FemLab (MatLab) Detail-level analysis of primarily conduction cooling. Thermal stress analysis
Grid generation systems ICEM CFD, TrueGrid Pre-processor for meshing CAD data for using a vast number of CFD or FEA solvers
Table 1: Thermal tool classifications and vendors

[ back to top ]


A revised product development cycle

At different phases of the design, different types of thermal evaluation are necessary; early on it is useful to be able to compare different designs quickly from a thermal perspective, whereas a more detailed simulation may be needed as the product gets better-defined. At the earliest stages, hand calculation may be appropriate, building a simple model based on the total power dissipation and a projection for what will be the mechanical design, with boundary conditions applied. Where the calculation becomes more complicated, spreadsheets are helpful, although simplifications still have to be made. At a more complex level, Flow Network Modelling (FNM) is another estimating tool that can be carried out by hand or with a spreadsheet, although it is also available in software implementations. A generalised way of looking at flow rates and temperatures in a network of flow paths through the components of a system, FNM is a global approach, not able to give the detail that may be needed at a component level.

All these tools use ‘analytical’ (mathematically-exact) calculations, made possible by severe simplification. In order to get more realistic answers, we must take a more detailed view of the situation, and use more complex tools. Frequently there will be no exact solution, so the program will need to use numerical methods to reach an approximation of the answer. Two basic types of tool are available for this, hand calculation, helped by a spreadsheet where necessary, and different types of modelling of which Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) are used most commonly.

We will be describing more about the methods for making first-order approximations in Unit 9, so that you are aware of the likely errors of such approaches and their limitations. However, as software becomes more available, and competent computers are available for modest cost, on many occasions a designer will go straight for a modelling solution. It is this approach that we will be taking in Unit 11, showing how simulation is a useful tool for comparing different designs and assessing the extent to which thermal good practice guidelines are being followed. Of course, for some applications at the very start of a design process, it might be more appropriate to use a black box model accompanied by a simple calculation – there is no point in using a sledgehammer to crack a nut!

The Enhanced Product Design Cycle stressed the use of FNM, mostly because of the extended computing time needed for CFD when applied to the task of making early estimates of temperatures. However, the technological evolution that has brought thermal design to a greater importance in product development has also led to increased computing power in desktop PCs, so that CFD can now be used in the early design phase. The primary advantage of doing this is the opportunity it gives to evaluate thermal performance without using correlations of flow and heat transfer.

A revised product design cycle, including a quotation phase, while excluding the hardware test phase, is shown in Figure 6. This approach includes the types of tools described previously, but the use of Flow Network Modelling has been diminished, and the dominating tool in concept development and detailed design phase is now CFD, combined with Grid Generation Systems.

Figure 6: Revised product development cycle

Revised product development cycle

 

For the designer, using CFD early on has the obvious benefit of minimising the range of tools required to implement effective thermal analysis at all stages of product development. This represents not only a cost saving in terms of tool purchase and maintenance, but, more importantly, can result in considerable time savings and thus shorten the development cycle. This can result in a quicker time to market with all its associated advantages for the organisation.

Caveat

It cannot be sufficiently emphasised that thermal designers must remain aware of the need always to challenge the outputs from sophisticated software, and not accept its results in blind faith, without applying engineering judgement and common sense, and without comparing any predictions with estimates produced by other means.

 

[ back to top ]


The next step

From this point on, we hope that you will be managing more than one concurrent activity. As you will recall from the figure in the Module Overview, which is repeated below, we recommend that you look at the materials topics in parallel with your study of modelling, in both theory and practice, using first computational methods and then the Flomerics simulation software.

This parallel study is because it’s very important to remember that thermal management is not just building a model of reality and letting the software do the rest. The professional approach to thermal management is intimately bound up with the practicalities of extracting surplus heat, and making sure that the entire product can be made profitably and reliably.

Figure 7: General arrangement of the module

4817_u00_02_img.gif

[ back to top ]


Resources for this Unit

Both of these lists is in the order in which the material is referenced in the Unit text

Recommended supplementary material

Optional links and information

[ back to top ]