BACK-OF-THE-ENVELOPE CALCULATION
When a formal thermal analysis is performed, the goal is to provide a complete understanding of how heat is both formed and moved throughout the system. However, a simple back-of-the-envelope calculation may be quite sufficient in the early stages of the development process.
The idea is to get a rough feeling of just how hot things are going to get after throwing the power switch. Another way to look at it is that you’re preventing the inadvertent reduction of the mean time between failures by letting a device or the system overheat.
Once you perform the calculation, you should have a basic understanding of the level of sophistication needed for your thermal-management scheme. That is, are you looking at adding a simple heatsink to your bill of materials, a more exotic solution requiring a heatpipe, or some cuttingedge solution that uses a combination of heat spreading, forced air, and even new materials? Even if you can get away with something simple like adding thermal vias, it’s much better to know up front and plan for it than getting burned later.
So how do you perform a back-of-theenvelope thermal analysis? According to Byron Blackmore, electronics cooling engineering supervisor for Flomerics Inc., one of the first numbers to crunch is the total power density on both surfaces of the board. “This can be determined by calculating the total power dissipation divided by the surface area,” he says.
Blackmore also provided a rough rule of thumb by indicating that if your calculation reveals your design will dissipate more than 1.5 W/in.2, you need to start thinking about additional measures to keep heat from creating downstream issues.
Paisner also chimed in with some guideline numbers. “One of the key determining factors for additional action is temperature,” she says. Up to 85°C is acceptable, and 85°C to 100°C is probably okay, but proceed with caution. However, additional measures typically will be needed at 100°C and higher. Of course, in addition to the absolute temperature, you should worry about how the temperature changes as system conditions change.
How do you get there? “Take the maximum power dissipation of each component at the highest temp the board will run at and divide by the surface area, and then repeat for the other side of the board,” says Blackmore. Then, you must research the thermal resistance (e.g., θJA) and multiply by expected power dissipation to determine temperature rise above ambient. Now, compare that number to the maximum rated temperature for the component.
Note that the θJA listed is for “stale air” and must be taken with a grain of salt, especially if you plan to have air moving through the system. Some datasheets may list the thermal resistance at a given airflow rate above the part (e.g., θJMA). Obviously, if your design is pushing one of these limits, you probably need to consider additional thermal-management measures, and it may be time to think about simulation software.
These calculations may be sufficient for a given design, especially if you have a lot of leeway in regards to the system chassis. So when may additional thermal analysis be required?
“Optimally, you would like to do thermal analysis twice: once after the EE has a rough idea of the board size and components that will be used, and later when a preliminary route has been performed,” says Rosato. Again, depending on your system, you may need to consider a much more accurate simulation using thermalanalysis software at this post-layout point (Fig. 1).
LAYOUT AND CHASSIS CONSIDERATIONS
Thermal analysis must be performed early and often. Some designers may even want to consider it before going after a patent, because if a product will fail due to a thermal problem, what’s the point? But other factors impact the system design.
“[Systems] engineers must understand how different materials interact with various package sizes and types,” says Paisner. “Companies like Lord Corporation work with customers to develop new materials to meet thermal requirements.”
She used Apple’s Mac Air notebook as an example of a product with significant design challenges, because designs like that likely don’t have room for large heatsinks or other cooling technologies. As a result, the limitations of an extremely small form factor can be overbearing unless you’re willing to spend some serious cash for exotic thermal solutions.
Continued on page 4
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