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

Transcription

1 Keywords: CFD Simulation, Computational Fluid Dynamics, CFD conjugate heat transfer, CFD natural convection, CFD PCB simulation, CFD electronics enclosures, CFD ASICS, thermal predictions of electronic circuit boards, CFD simulation of chip temperatures and leads, electronic assembly within plastic enclosure, CFD simulation of solar diurnal heating Main Graphic: Caption: CFD simulation shown natural buoyancy air flow across the PCB and chips within plastic enclosure for outdoor environments Page 1

2 CFD SIMULATION OF AIR FLOW AND THERMAL BEHAVIOR OF ELECTRONICS ASSEMBLY WITHIN PLASTIC ENCLOSURE FOR OUTDOOR ENVIRONMENTS 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. 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. 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 CFD analysis. Our CFD consulting expertise with electronic assemblies allowed up 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 CFD 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 CFD simulation model is a big thank you. Page 2

3 Figure 1: CAD geometry of plastic clamshell enclosure used in the CFD simulation of the electronics assembly Page 3

4 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. Page 4

5 Figure 3: CFD simulation results showing the thermal gradients in the system and the CFD driven natural buoyancy air currents within the plastic enclosure Page 5

6 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. Page 6

7 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 CFD results have been validated as correctly indicating the internal temperatures of the electronic package. Page 7