Electronics Industry Business Unit Manager
Icepak Product Manager
The best way to model electronic designs in thermal-management software uses minimal component detail. Too much detail produces simulation models large enough to bring workstations to their knees and answers come too late to be useful. Simple models, on the other hand, provide useful information fast, especially early in the development phase when designs are still flexible and there’s time to pursue promising alternatives.
The best approach is to start with a simple system-level model to determine a general layout. Then gradually increase the model’s complexity as the design stabilizes. Recent advancements in CFD-based software have made it easier and faster to develop relatively simple system-level designs.
Thermal management software also holds the potential to dramatically streamline the thermal management process. Simulations let designers investigate more alternative designs than they could with traditional methods such as hand calculations, spreadsheets, or physical models.
Software tuned for thermal analysis of electronic packaging differs from general-purpose CFD codes in its use of software “objects,” devices that model vents, openings, fans, resistances, or heat sources. When a model needs a particular component, users simply enter its characteristics such as size, thermal conductivity, or specific heat into a macro program rather than modeling it from scratch as is required with general-purpose CFD software. The resulting object becomes the thermal component.
Don’t get fancy
Modeling electronic equipment with every possible detail greatly increases the time required to solve the model. One company, for example, generated a 600,000-cell CFD model to resolve basic cooling issues in an enclosure. The large model took several weeks to solve on a midrange workstation with 128-Mbytes RAM. By the time the computer reached a solution, the design changed, making the results useless. A smaller model, with 30,000 to 50,000 cells, provided enough accuracy to resolve design questions on component placement, fan sizing, and vent location. The simpler model let designers modify and solve different component arrangements in less than 30 min.
System-level models can show temperature, pressure, and velocity at every point in the cabinet. What’s more, quickly evaluating multiple designs makes it possible to determine a design’s sensitivity to changes in design parameters.
Recent thermal-management software aimed at the special problems of cooling electronics dramatically reduces the time and cost of solving thermal-management issues. Several case histories show how the software can be applied to different products.
System models lead to quick solutions
The problem of dissipating heat from a typical desktop computer in a tower enclosure shows how a system-level model helps early in design phases of electronic packaging. The original design specified 300 W of heat from sources including a CPU, power supply, hard-disk drive, motherboard, PCI cards and DIMMs. The baseline design incorporated a large vent, fans for the CPU and power supply, but no forced cooling of the PCI cards. System-level analysis showed an intolerable maximum temperature of 500°C in the cards. This called for a new component arrangement.
At this stage it’s easy to change the vent configuration and relocate fans. A simple change surprisingly showed that just reducing the size of the vents and moving them to rear of the card area dropped the maximum temperature to 200°C, according to the software. Analysis revealed that smaller vents directed air over the cards. The larger vent generated no such airflow.
A second configuration retained the smaller vents, eliminated a CPU fan, and added a system exhaust fan to change the direction of flow in the area of the PCI cards. This reduced the maximum temperature to 140°C. Finally, engineers modified the configuration further by adding a baffle to channel flow. Analysis showed this approach increased resistance to the system exhaust fan, actually dropping the flow rate through the PCI cards and pushing the maximum temperature up to 150°C. Consequently, designers went back to the previous case.
At this point the system-level model has done its work. Modeling efforts can now turn to component-level designs to evaluate individual device temperatures and consider the effect of other cooling options, such as heat spreaders and heat sinks. The completed component-level model can be incorporated into the system-level model to determine the impact of any changes on the overall design. n
Blocks and plates represent cards, fans, drives, and power supplies in a system-level model of a workstation. Software such as Icepak from Fluent Inc., lets packaging engineers identify and fix the hot spots. Such software usually includes macro programs for generating the blocks that accurately represent real components.
© 2010 Penton Media, Inc.