by: Prabhu Sathyamurthy, Fluent Incorporated
In the not very distant past, thermal management was an afterthought in product design. The amount of heat generated by systems did not warrant decisions based on thermal priorities, and problem areas could be addressed in the prototype phase. But today's high-power devices generate considerable amounts of heat, and the drive for miniaturization puts further stresses on the design process, leading to a higher likelihood of product reliability and safety problems. Because of this, thermal considerations now frequently drive the design process. Therefore, it is extremely important to make sure that thermal design starts when all the other design processes startat the beginning of the project.
We asked three experts in the field of thermal design to share their timesaving tips. Participating in this discussion were Dr. Vivek Mansingh, Executive Vice President and General Manager of Applied Thermal Technologies, a thermal consultancy business; Chris Chapman, Computer Industry Manager of Aavid Thermal Products, a manufacturer of cooling products; and Dr. Prabhu Sathyamurthy, Marketing Manager for Icepak, Fluent's computational fluid dynamics (CFD) software for thermal modeling.
Mansingh contributes his many years of experience as a thermal design consultant and his ordered methodology to help organize your thinking. Chapman offers some important factors to consider when choosing hardware. Sathyamurthy defines the critical role that thermal modeling software can play in compressing the design cycle.
Mansingh asserts that the essential first step is to understand the most important factors driving the product design. The project team needs to define which factors are critical. In the case of notebook computer design, is it to be the lightest, the smallest, or the fastest notebook computer? What are the cost parameters? From the hardware standpoint, what kind of space is available? Chapman feels that it is almost impossible to formulate a plan for cooling without knowledge of the requirements and parameters of the final product.
The best method for defining the design drivers comes from teamwork. If there are written specifications, they should be included in the plan, but the most powerful tool is discussion and collaboration between all parties involved in the design.
Once the drivers are understood, an overall approach should be considered. Our experts unanimously agree that you should start with a system-wide viewpoint. Focusing on individual components at this stage could be a mistake. You can waste a lot of valuable time working around parameters that have already been mandated on the component level when you find that they do not fit into the overall design to your satisfaction. Even though all the data may not be complete, it is vital to begin conceptualizing and analyzing as soon as possible. Start working with the available information and make assumptions based on possible ranges of power. Waiting until exact figures are available and all calculations have been made simply wastes too much valuable time.
From his years teaching and lecturing, Mansingh has formulated a method for breaking the thermal management process down into phases, which he teaches to students attending his thermal management seminars. This step-by-step methodology will help you to organize and streamline the entire process.
The trick to both efficiency and success is in knowing how far and to what detail to carry your analysis at each stage of the development. According to Sathyamurthy, good design practices are essential. There is more than one solution for a problem, but only one best solution. Sometimes the most obvious solution is not the best, so focusing on the design drivers and taking the time and effort to think through the whole concept will usually result in time and cost savings as well as increased product reliability.
As the selection of individual system components begins, Chapman emphasizes the importance of working concurrently to investigate two or three IC packaging and heat-sink attachment methods. Try not to limit yourself to one scenario. The production attachment method may differ from the prototype attachment method due to time-to-market constraints.
When selecting different IC packaging, cost and performance are critical. Less expensive packaging typically has a higher internal thermal resistance, which requires a larger heat sink. Since electronic products must also meet shock and vibration requirements, a large BGA-packaged heat sink must be secured into the system to be stabilized. The designer must trade-off IC cost, heat sink size, and circuit layout in order to develop a heat-sink attachment that is robust, acceptable to circuit routing, and cost effective. The ideal solution is easy to manufacture and does not require special tools for installation.
Ultimately, analyzing pre-production considerations, such as where you are producing and selling your product, will result in time savings. Choosing a partner who can design and manufacture your thermal solution is critical, but it is also important to know if this partner can produce and sell in the same markets where you do business. A good thermal partner is able to understand the full scope of your product and your marketplace, and can bring the solution to production for a global market, which will result in time and money saved in the long run.
Before computer modeling was available, the only way to find out whether a heat removal method worked was to build a physical prototype and measure it. Now we can use computer-aided thermal modeling to avoid the necessity for expensive, reiterative physical prototyping. When you use an optimized computer model as the blueprint for your physical prototype, you'll probably only have to build once.
However, Sathyamurthy and Mansingh caution us to use the modeling process judiciously. A detailed model of a questionable design wastes time and effort. Do a quick design using relevant details only to narrow down your options. A simple model could consist of a processor, a board, an enclosure, and a fan, for example. Use the software to test whether the fan can cool the processor. If it cannot, it certainly can't perform adequate cooling when the disk drive and other components are added.
There are a few computer-aided thermal modeling software packages available to choose from. Remember that the software should make your work easier, so look for a program that will help you arrive at the optimal solution quickly and easily. Look for one that is easy to use and can accurately model systems of the complexity you are designing. Pre-programmed components, such as fans and heat sinks, allow you to build preliminary models quickly. The ability to import CAD images helps with more detailed modeling. The CFD software allows you to see airflow patterns and temperature "hot spots." After you move the critical components to a cooler location, the software helps verify the success of your choice, and points out other factors to consider. This iterative process can take place very quickly, resulting in an optimized solution in a compressed time frame.
Once you've optimized the design using computer modeling, what's left is to make it a reality. Because you saved time with computer modeling, you will probably have more leeway for fine tuning the physical prototype. For example, a design might include two fans to cool a circuit, and the model shows that they are providing sufficient cooling for the design. With the extra time gained from proper design planning, the design might be improved so that only one fan is necessary. Eliminating one fan saves money as well as increasing the reliability of the end product. Waiting to begin the process until the electrical design is complete, or working in an unorganized fashion usually results in a solution driven by the necessity of completing the task rather than by the best solution for the given design requirements.
Rapid prototyping and cost analysis are necessary to provide a marketable solution. If the components of your thermal solution take a month to prototype, you have wasted up to one-third of your design time. Rapid turnaround on heat-sink prototypes allows you to test the subassembly before it becomes a potential bottleneck for the whole project.
A thermal specialist might know solutions that, when considered up-front, can save you the time and expense of going down the wrong path. Thermal expertise at the system level can be found through thermal consultancies such as Applied Thermal Technologies. Heat-sink manufacturers, such as Aavid Thermal Products, can assist with optimizing airflow and design of custom heat sinks, for example. These specialists will work with you to define the requirements and constraints of your project. Together, you can set forth an orderly process to solve the challenges.
To get your product to market faster, approach thermal management with a clear, organized process. Conceptualize several possible scenarios and settle on the most promising from which you can create simple models to try out various solutions. Narrow it down to what appear to be the best alternatives and make detailed models for the whole system. Once you have found a version that meets all the design requirements, work to refine it into the best possible design. Following these guidelines will allow for rapid and efficient prototyping and get your product to market quickly.
Aavid Thermal Technologies, Inc., One Eagle Square, Suite 509, Concord, NH 03301. Tel: (603) 224-1117; Fax: (603) 224-6673