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Minimise heat in PCB with intelligent simulation

Posted: 26 Aug 2015     Print Version  Bookmark and Share

Keywords:thermal simulation  PCB  EMC  heatsink  Conductive vias 

Component temperature limits need to be determined. In some cases it can be advantageous to specify more expensive components – perhaps those with ceramic rather than plastic packages. These can withstand higher temperatures, conduct heat more effectively, and may avoid costly and space-hungry heatsinks or fans.

Some component packages need to be cooled with a heatsink. The heatsink can be a specific component, but it could also be a copper pad on the PCB, or the wall of a metal enclosure or EMI screen.

Conductive vias that run between the layers of a board can be effective for transferring heat away from a component. Even when these are not needed for electrical reasons, additional ones may be added simply to move heat away from a component. If feasible, filling vias will further improve heat transfer.

The 'copper pour' function in EDA tools is used to create large copper areas – usually ground planes that ensure all components are at the same reference potential. The extensive use of ground planes improves signal integrity, and also minimises the amount of etching fluid used during manufacturing because less copper needs to be removed from the board. Large areas of electrically and thermally conductive copper are potentially useful as heatsinks, spreading the heat away from components rapidly to minimise hotspots.

Increasing the number or thickness of copper layers is another technique that can be used improve the thermal performance of PCBs. In some high power applications, including LED assemblies, metal core boards provide dramatically lower thermal resistance than traditional FR4 types.

If the design does demand the use of heatsinks, it pays to consider their location carefully. You need to ensure there is enough space around components, and consider what stresses the type of heatsink may impose on the board. If the heatsink is fixed to the board via mounting holes, remember that the coefficient of thermal expansion of FR4 will be much lower than that of aluminium, so physical stresses will be created.

How components are mounted to heatsinks, and how heatsinks are affixed to other thermally conductive surfaces (such as metal enclosures), also needs to be considered. Transferring heat through air – particularly slow moving air – is not very effective. Transferring heat through a rigid thermal interface material or thermal paste gap-filler can be much more efficient.

Improved simulation techniques for understanding heat transfer through a PCB
To be most effective, thermal simulation should be done at an early stage in PCB layout design. It can be too late, or too expensive, to redesign boards when thermal management issues are discovered.

A PCB is a complex part, with many layers and hundreds of traces, holes, and vias. This is a challenge for thermal simulation, as small objects can lead to impractically long processing times for a simulation tool's mathematical solvers. So, the goal is to reduce the complexity of PCB traces without reducing simulation accuracy.

As mentioned previously, thermal simulation software products traditionally over-simplify the PCB layers: either by specifying a uniform planar isotropic conductivity, or by specifying a percentage copper for each layer. This means that the variation in copper coverage on layers is not considered in the simulation, with significant impact on the accuracy of results.

The most recent thermal simulation tools take a different approach. They allow the copper traces, vias, and holes to be imported through industry standard file formats such as IDF, IDX, XFL, and Gerber, and they can even import images. Component sizes, positions, and other key data are also imported, along with PCB outlines. The board designer can then either solve the traces in full detail using a powerful unstructured grid, or intelligently simplify the copper traces.

The tools then use the trace and via information to calculate the thermal conductivity for each region – or grid cell – of the PCB. Approaching simulation in this way allows the board designer to see how the layer design impacts the thermal design without increasing the simulation times. In short, this method enables an accurate simulation of the thermal performance of each region of the PCB without explicitly including its complex traces. For example, the 6SigmaET thermal simulation tool uses this methodology.

Conclusion
Thermal simulation of PCBs, when undertaken early enough in the design process, can deliver substantial mechanical, electrical, and commercial benefits to electronic and electrical products. Simulation tools are becoming much smarter and easier to use. They are delivering more accurate simulations that board layout engineers can rely on to make their working lives more productive.

About the author
Tom Gregory is a product specialist for 6SigmaET at Future Facilities in London, UK. He is a trained electronic engineer with a Master's degree in Electronics and Computer Engineering from the University of York, UK. He has previous experience designing and testing electronics for the automotive and defence industries, he now supports engineers across the electronics industry. He works on simulation consulting projects and training for both the electronics and data centre industries. He has also shared his experience and knowledge with the wider industry through white papers and conference presentations, including Semi-Therm in California and the Electronics Design Show in the UK.


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