Thermal materials: Pliable, compressible thermal-interface materials can also play a role in keeping notebooks cool. The aluminum plate is located on top of the chassis under the keypad, while the IC is mounted to the bottom of the chassis. Tolerance stack-up between the CPU and the chassis creates a variable gap in the thermal-conduction path. By compressing gap-filling, compliant, interface materials between the CPU and the chassis-mounted heatsink, thermally conductive material replaces the air gaps. Doing this ensures good thermal contact between the CPU and the heatsink. Even though the compliant interface material isn't as thermally conductive as the commonly used 5-mil-thick pad, it also isn't subject to the tolerance mismatch that might occur with that pad. If the pad is too thin for the space, there will be no contact. If it's too thick, it will put pressure on the IC.
A custom cooling solution for one high-end notebook design is shown in Figure 1. The large, stamped plate connects directly to the keyboard, providing 2 to 4 W of passive cooling. Though not shown, the gap-filling thermal-interface material creates a thermal pathway from the CPU to both the stamped plate and the heat pipe. The heat pipe conducts heat to a low-cost heat exchanger, where directional airflow from the custom, low-profile fan can take it out of the system. Efficient component placement minimizes cost.
Modeling: With the addition of heat-generating ICs other than the CPU, the thermal issues become much more involved. Thermal solutions for these more complex notebooks typically cost two to three times that of a low-end notebook. The standard, 3-mm heat pipe isn't large enough to dissipate the heat generated in these systems. By using a thermal modeling package, such as Icepak from Fluent Inc., Lebanon, N.H., the designer can optimize board layout to take advantage of the natural thermal convection and directional airflow provided by a fan.
Thermal modeling can predict the heat generated by each component in a particular system and model the effectiveness of the thermal solution. Such programs are capable of taking existing CAD files and determining the airflow direction and velocity at any point in the notebook. They also calculate thermal resistance, offering an indication of thermal transfer and cooling.
The same tools can help develop an optimal heatsink for the notebook. The heatsink design can be modeled in terms of volume resistance. Then, the modeling package can analyze each face of the heatsink for the free-area ratio characterizing the amount of air that can flow past the face. Together with the thermal resistance calculations, volume resistance is a powerful tool to optimize heatsink design for a specific application in the shortest amount of time.
By modeling different low-cost and passive thermal solutions, the designer can reduce the overall cost of the thermal solution. Without modeling, the designer will need to prototype the system and try "what-if" scenarios in hardware. Often, "what-if" scenarios are difficult or impossible to cover with a single prototype. So, additional prototypes must be built and tested. Although modeling increases up-front design time, it allows for multiple "what-if" analyses—typically reducing system test time and often eliminating the need for redesign.
In systems with multiple heat generators, designing a heatsink to address several hot components in close vicinity may minimize the height of the heatsink itself. Now, the layout and heatsink design phase must occur simultaneously. The complexity of the thermodynamics with two heat generators makes thermal modeling of the new heatsink imperative. Without it, the design-cycle time increases. Much more time is spent in prototype testing. It's also easy to miss a critical heat-generating component without thermal modeling. Any shortcuts to these processes can result in missed data.
Figure 2 shows a layout in which the heatsink effectively cools the CPU. Yet it also shows significant heat transfer from the CPU to the hard drive. Unless the layout is modified, the hard drive will be subject to frequent failure due to the additional heat load.
In a layout that restricts airflow, there must be a strategy to move heat out. In the case of Figure 3, the CPU will suffer from reliability issues caused by excessive heat.
Figure 4 shows a strategy where the heatsinks pipe the heat to a localized area where active airflow can pick it up and eliminate it. The temperature chart shows that this strategy is effective for the chosen layout and component mix.
These examples display how thermal modeling identifies thermal problems before the hardware is built. Such modeling exemplifies how the lowest-cost solutions, such as layout and passive heatsinks, can be tested and compared with more expensive solutions to generate the most robust, reliable package.
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