CFD Simulation of Air Flow and Thermal Behavior of Electronics Assembly for Outdoor Environments

CFD simulation shown natural buoyancy air flow across the PCB and chips within plastic enclosure for outdoor environments
Analysis: 
CFD

A new high-power wireless system was being developed for a pole-mounted outdoor application. The complete system would be enclosed within a plastic clamshell and then mounted outside on utility poles or on the side of buildings. Our clients were concerned about solar heating of the enclosure and of course, the internal heat buildup due to the power dissipation by the electronic assembly. As CFD consultants to this project, our objective was to ensure the highest possible thermal efficiency given the low heat transfer rates from the interior through the plastic enclosure and out to ambient air.

The CFD simulation took into account natural convection within and external to the electronics enclosure and the conjugative heat transfer between the ASIC’s and the PCB. Chip lead temperatures were back calculation from the analysis. Our CFD consulting expertise with electronic assemblies allowed us to interpret the CFD results in a logical manner and thereby avoid “false positives” or one might say – unnecessarily over conservative predictions.

The biggest challenge to this CFD work was the diurnal cycle since the plastic enclosure acted almost as a black body to the solar radiation. Although internal fans were considered to alleviate the predicted heat buildup between PCB’s, it was reasoned that by mounting the device vertically, a strong natural convection pattern could develop to self-cool the device during hot days.

Given the CFD simulation predictions, the electronics assembly was fabricated and tested. The CFD model was shown to accurately predict its thermal behavior and the device is in production. On our side as CFD consulting engineers, the validation of a simulation model is a big thank you.

CAD geometry of plastic clamshell enclosure used in the CFD simulation of the electronics assembly

Figure 1:  CAD geometry of plastic clamshell enclosure used in the CFD simulation of the electronics assembly

CFD simulation model of internal electronics assembly with thermal flow shield and chips and printed circuit boards (PCB)

Figure 2:  CFD simulation model of internal electronics assembly with thermal flow shield and chips and printed circuit boards (PCB)

Within the plastic enclosure, natural convection was simulated to capture the thermal gradients driven by the power-dissipating chips on the PCB. The CFD simulation showed that the majority of the heat transfer was driven by natural convection and not through PCB conduction.

CFD simulation results showing the thermal gradients in the system and the CFD driven natural buoyancy air currents within the plastic enclosure

Figure 3:  CFD simulation results showing the thermal gradients in the system and the CFD driven natural buoyancy air currents within the plastic enclosure

 

CFD simulation results for the electronic assembly showing velocity vectors and temperature gradients

Figure 4: CFD simulation results for the electronic assembly showing velocity vectors and temperature gradients

These CFD results were crucial in the interpretation of the thermal flow behavior within the system, as CFD consultants, our understanding of the flow pattern led us to avoid the use of internal fans and place internal baffles to direct the air flow toward the hotter chips.

CFD temperature profile over the plastic clamshell enclosure documenting that the internal electronics assembly was not overheating

Figure 5: CFD temperature profile over the plastic clamshell enclosure documenting that the internal electronics assembly was not overheating

Summary: CFD results from this simulation were used to successful drive their design to a lower cost solution, that is, no internal fans. This device has gone into production and the results have been validated as correctly indicating the internal temperatures of the electronic package.

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Siemens PLM Software
Livermore Software Technology Corporation
Applied CAx
Sherpa Design