PCB thermal design developments: new 3D thermal quantities help designers address thermal problems as they arise.

Author:Blackmore, Byron

Electronics thermal management is the discipline of designing electronics systems to facilitate the effective removal of heat from the active surface of integrated circuits to a colder ambient environment. In doing so, heat passes from the package both directly to the surrounding air and via the PCB on which the IC is mounted. The PCB and, to a lesser extent, the surrounding air thermally couple the various heat sources.

Heat coupling increases as components and PCBs become smaller and more powerful. Designers must take remedial action to bring all components within their respective thermal specifications, but this step is becoming more challenging and constrained, even when preventative measures are taken early in the design process.

For the past 20 years, computational fluid dynamics (CFD) techniques have provided 3D thermal simulations that include views of the air-side heat transfer that predict component junction and case temperatures under actual operating conditions. Designers routinely use these predicted temperatures to judge thermal compliance simply by comparing the simulated temperatures to maximum rated operating temperatures. If the operating temperature exceeds the maximum rated value, there will be at least a potential degradation in the performance of the packaged IC, and at worst, an unacceptable risk of thermo-mechanical failure.

Simulated 3D temperature and flow fields provide detailed and useful information, but give little physical insight into why the temperature field is the way it is. Examining heat flux vectors can yield some insight into the heat removal paths. But the heat flux vector direction and magnitude data do not provide a measure of the ease with which heat leaves the system. Nor do they provide insight as to where and how the heat flux distribution might be better balanced or reconfigured to improve performance.

How easily heat passes from the various sources to the ambient will determine the temperature rise at the sources and all points in between. Heat flow paths are complex and three-dimensional, carrying portions of the heat with varying degrees of ease. Paths that carry a lot of heat but offer large resistances to that heat flow represent bottlenecks. A redesign can relieve these bottlenecks, permitting heat to pass to the ambient more easily and reducing temperature rises along the heat flow path all the way back to the heat source. In addition, there may be unrealized opportunities to introduce new heat flow...

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