Product Performance Drives the Design
Identify Several Potential Designs
Preliminary CFD Modeling
Detailed Modeling and Optimization
Component-Level Design and Heat Sink Selection
Final Analysis and Prototype Testing
In the quickly changing, highly competitive world of notebook computer design, any decrease in time-to-market is worth its weight in marketshare. Cooling the increasingly power-hungry integrated circuits in these designs is a major technical challenge, especially in light of the constraints inherent in notebook design, such as limitations on weight and battery size.
In the last article, we introduced a six-step thermal design methodology that can reduce time-to-market. This article will look at notebook design through the six phases of this methodology.
The notebook computer market is serving many different users with many a variety of requirements. In some cases, the notebook functions as the user's primary computer, and includes lots of processing power, high-end graphics capability, and extended communications. Since this notebook spends considerable time on the user's desk, weight is secondary to the system's capabilities. In other cases, the portability of the notebook is its primary feature; when the notebook becomes part of the traveler's briefcase, size, weight, and battery life influence the design. Just as performance drives selection of the microprocessor, performance must also drive the thermal design.
The portability of a notebook computer adds to the heat-removal problem. The user must be able to handle the product without getting burned. The computer must be capable of operating with the lid closed without creating internal hot spots, which could compromise reliability. The heat removal cannot be obstructed when the notebook is operating while resting in someone's lap.
To gain a competitive edge, many notebook computers are upgradeable with drop-in components. Usually, components used to upgrade the notebook will use more power than the components they replace, increasing the amount of heat to be removed. In this scenario, allowances must be made in the preliminary design stage for these upgrades as well as for the addition of peripherals, such a CD-ROM, to which the user needs easy access.
The design team must fully understand all the product performance goals, so every part of the design works together. This principle, which is important in any design process, becomes indispensable with the limitations and quick turnaround required in the design of notebook computers.
Once the product goals have been set, the entire design team begins to lay out the basics of the notebook. Each component in the system has a maximum temperature specification that must be accommodated. The average microprocessor, for instance, requires a temperature less than 85°C to continue to function properly. The maximum temperature for a hard drive with greater than 2.4 Gbytes capacity can be as low as 55°C. Based on the power dissipated by the major components of the system and their maximum temperature specifications, the thermal engineer can carry out a preliminary analysis to generate the basic chassis design.
For low-power systems, dissipating 18 watts or less, the thermal designer can look at cast and fan heat sinks to cool the microprocessor. An aluminum plate on the back of the keyboard dissipates additional heat without increasing the size of the notebook. Initial calculations should indicate whether this system will require a fan.
Mid-power range notebooks, dissipating around 25 watts, will need more sophisticated thermal solutions. A 3-mm diameter heat pipe, for example, can move as much as 20 watts from one part of the system to another. For this range of power, the thermal designer can evaluate the need for a fan and whether the aluminum plate on the back of the keyboard will get too hot if all the system heat is piped to it through heat pipes.
With high-power systems, dissipating more than 30 watts, at least one fan will be required to cool the notebook. A combination of heat pipes, metal plates, and extruded heat sinks have to be evaluated. At this phase, it is important for the thermal designer to be creative. A novel thermal design at this stage will not increase the time-to-market, and could result in more efficient heat removal and, therefore, optimized thermal performance of the notebook computer. This type of creativity becomes riskier and riskier as the system design becomes fixed.
Once the thermal designer has reached a basic configuration and has several system options that appear to work on paper, the thermal analysis continues with preliminary CFD (computational fluid dynamics) models. These initial, rough thermal models allow a quick overview of the system's heat-removal requirements using the maximum temperature ranges for the critical components and the system as a whole.
At this stage, the thermal engineer can begin specifying vent and fan locations and the optimum location for heat-sensitive devices. By working with the mechanical and electrical members of the design team early, before all the components have been identified and their locations fixed, the thermal designer ensures that the best thermal solution can be implemented. A CFD software package, such as Icepak by Fluent, Inc., which is dedicated to thermal design, allows rapid modeling of different thermal solutions, providing a significant aid to the design engineer.
Once all the components have been identified, the thermal designer can create a detailed model of the system. The detailed model often includes refinements of board and component layout, and vent location and size, for example. Automatic meshing capability allows the CFD system to accurately model even the most complex shape.
Once the model is available, "what-if" scenarios take little time. The designer can test various configurations quickly to optimize thermal cooling long before a piece of hardware becomes available.
Figure 1 shows a possible preliminary thermal model for a notebook. To remove heat from the CPU, a heat pipe connects to thermal interface material, which attaches to the microprocessor package. The pipe moves the heat to a heat sink that is placed directly in front of the system fan for optimum cooling.
Figure 2 shows a thermal plot analysis of this configuration. The heat pipe/heat sink setup effectively cools the CPU. The hard drive, however, is around 120°C, well over the recommended limit.
Using the CFD software to solve for airflow can help determine why the hard drive is so hot. The resulting analysis in Figure 3 shows that no airflow reaches the drive.
This is the ideal time to solve this potential problem. The designer can model a more effective heat sink, evaluate a second or larger fan, consider ducting the airflow through the system past the drive, or even model a change in placement of the hard drive. A simple, cost-effective solution that takes up little space is the addition of an aluminum plate that extends from the drive to the heat sink. The plate carries heat from the drive to the heat sink, where the system airflow can remove it.
CFD is a useful tool in developing component-level thermal solutions as well. Many heat-sink manufacturers, such as Aavid Thermal Products, use CFD modeling to develop heat sinks ideal for a specific notebook application. Because of the complexity of heat sink shapes, meshing capability becomes even more important at this phase of design.
Figure 4 shows the final thermal solution for the notebook discussed above. A copper plate has been added to improve cooling to the VGA chip, PCMCIA, and dc-dc converter. This solution allows upgraded components to be used in the future.
If the thermal designer for a notebook computer begins the design early, the prototype phase is merely confirmation that the model was made correctly. Although notebook computers represent a significant challenge due to size and weight restrictions, as well as performance demands, a sound thermal design strategy will ensure the product meets or exceeds specifications reliably and repeatedly.
After the working prototypes are available, the last step is thermal characterization, where thermocouples verify that all components are within thermal specification. Following the strategy outlined in this article will reduce design time because several alternative cooling strategies will have already been explored by the time the system characterization phase is reached. At this point in the design cycle, the cost of designing the product is much lower than traditional design strategies because fewer prototypes are required. Although fewer physical prototypes were used in testing, the final solution is based on analysis of many more potential solutions, it is probably closer to the optimum design.
Aavid Thermal Technologies, Inc., One Eagle Square, Suite 509, Concord, NH 03301. Tel: (603) 224-1117; Fax: (603) 224-6673.
Christopher Chapman, global computer industry manager,is a veteran in the surface mount components industry, bringing to Aavid Thermal Products more than seven years of experience in product engineering and product management of thermal components. Mr. Chapman formerly served as a product engineer at Aavid and as a sales application engineer for Aavid in California's silicon valley. In his current position as computer industry manager, Mr. Chapman is responsible for the development and marketing of cooling solutions for computer applications. Drawing on Aavid's broad base of cooling technologies, including surface mount components, extrusions, die castings, and liquid cooling, Mr. Chapman aids in the development of tailored, cost-effective solutions as well as off-the-shelf thermal solutions.
Mr. Chapman earned his bachelor's of science degree in chemical-environmental engineering from the University of New Hampshire, where he has frequently returned to give graduate and undergraduate lectures on heat transfer and microelectronics cooling.. He has published several technical papers in nationally recognized electronic trade journals and has spoken at national electronics trade shows such as Portable by Design, the International Society for Hybrid Microelectronics (ISHM), Semi-Therm, and the Electronics Industries Forum of New England.
Dr. Vivek Mansingh, executive vice-president and general manager of Applied Thermal Technologies, Inc., has been involved with research and development in electronics cooling for 15 years. When Aavid Thermal Technologies established Applied Thermal Technologies as a consultancy focusing on design and development of thermal solutions, they selected Dr. Mansingh as its leader.
Dr. Mansingh, an author of more than 30 technical papers who holds five patents, is a contributor to the "Handbook of Reliability," published by McGraw-Hill. He has taught electronics cooling courses at a number of companies and conferences in Europe, North America, India, and Japan, as well as on college campuses at the University of California, Berkley; San Jose State University; and Lehigh University in Pennsylvania, where he was previously a member of the faculty.
Prior to the formation of Applied Thermal Technologies, Dr. Mansingh was director of marketing and sales and a senior member of the technical staff at Fujitsu Computer Packaging Technologies, San Jose. He also served as project leader and senior scientist at Hewlett-Packard.
Dr. Prabhu Sathyamurthy, icepak marketing manager of Fluent, Inc., leads the company's efforts in the development of electronics cooling applications through the promotion of Icepak CFD software.
His background encompasses projects as varied as the cooling of gas turbines to the modeling of flow in hard disk drives. To his credit are more than 20 papers and articles, which have appeared in leading journals or have been delivered at technical conferences.
Dr. Sathyamurthy earned his bachelor's of science degree in engineering in India. He received his master of science and Ph.D. in mechanical engineering from the University of Minnesota.