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Manage thermal optimisation during PCB design

Posted: 09 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:computational fluid dynamics? CFD? simulation software? response surface optimisation? PCB?

In the traditional industries in which computational fluid dynamics (CFD) has been employed to investigate product performance, such as aerospace, nuclear, and automotive, design times are relatively long, and safety and reliability have been prioritized over cost and performance.

Thermal design of electronics in these industries is also influenced by these issues. The focus is more on reducing component temperatures by some safety margin to well below their rated value to increase their lifetime. Effort is expended on building redundancy into the cooling system, so that if a fan fails, the system can still operate well within specification and that the fan can be replaced while the system is in operation.

In high-volume consumer electronics, cost and performance are the key drivers. The pace of change is such that design times are compressed to just a few months from conceptual design to production. Minimising product unit cost is a key part of the design activity.

This translates into a need to explore the design space to ensure the most cost-effective cooling solution is chosen by considering the effect of all aspects of the design such as package selection, PCB layout, board structure, and enclosure design including fan size, location, and vent positioning for example.

This unique and overriding requirement of rapid analysis and design space exploration has given rise to the development of electronics-cooling specific CFD software from the late 1980s to the present day. These solutions leverage different CFD technology to that of traditional body-fitted CFD to deliver a first result faster, as well as much faster turnaround for subsequent design iterations.

With the latest CFD technology, modifications to the thermal model, including geometric changes, meshing, solution, and result post-processing can be automated, freeing up valuable engineering time for higher value activities. This technology can be used to optimise functionality by allowing engineers to run multiple scenarios early in the design process.

For example, the command centre in the CFD 3D simulation software enables engineers to run a design of experiment (DOE). They can run different variables at the same time to run many simulations, and the software will mathematically figure out the best distribution of input variables to consider, greatly reducing the number of required simulations.

Response surface optimisation (RSO) uses the results of the DOE to mathematically estimate the optimum values of input variables that minimise a cost function, such as junction temperature of a component. In addition to providing the optimal design inputs, RSO provides information on the sensitivity of the design inputs to the design goal, allowing the engineer to focus only on the parameters that effect the thermal design.

Heatsink optimisation for cPCI card
The following is an example of how to construct and analyse a CFD model of a cPCI card with two components that have heatsinks. CompactPCI is a very high-performance industrial bus based on the standard PCI electrical specification in rugged 3U or 6U Eurocard packaging. The goal is to optimise the heatsinks for components with reference designators U7 (upstream) and U8 (downstream) subject to the following three separate cost functions:
???Case 1: Minimise the mass of heatsink, affects their cost, smaller and lighter are better.
???Case 2: How low can component temperatures go?
???Case 3: How can differences between components minimised because electrical function works better if operating at same temperature?

We will simulate the PCB with the following environmental parameters. The orientation with respect to flow is shown in figure 1.
???Elevation: Sea level
???Ambient Temperature: 55C
???Upstream Velocity: 400 ft/min
???Slot Pitch: 0.8 in

Figure 1: The orientation with respect to flow.

With a board layout as shown in figure 2, the PCB is defined as:
???Stack Up: 2S2P
???PCB Dimensions: 100 mm x 160 mm
???PCB Thickness: 1.6 mm
???Total Power: 22.5 W

Figure 2: Board layout.


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