Editor’s note: Like most industrial machinery, wind turbines rely on electronic controls and devices for peak performance. This article, from analysis software firm Mentor Graphics provides 10 tips for streamlining PCB thermal design, a general component in the controls and devices. A link to the full paper is below.
Why is PCB thermal design important?
Many aspects of a PCB’s performance are determined during detailed design, e.g. making a trace a specific length for timing reasons. Timing issues are also affected by temperature differences between components. Thermal issues with the PCB design are largely ‘locked in’ during the component (i.e. chip package) selection and layout
phases. After this point only remedial actions are possible if components are found to run too hot. We advocate a top-down approach starting at the system or enclosure level to understand the flow environment for the electronics, which is critical for air-cooled electronics. Assumptions made about the uniformity of the airflow in early design that subsequently proves unachievable can have a disastrous impact on the commercial viability of the product and meeting the market window.
Optimizing the thermal layout
The golden rule is to start early and start simple. The mechanical engineer responsible for the thermal integrity of the product should aim to provide as much useful feedback as possible to the electronic engineers to guide the design, about the thermal impact of their choices, especially during early design.
From the mechanical engineer’s perspective, at the PCB level this entails helping with package selection and the best positioning of components to utilize system air flow for cooling. Inevitably both layout and package selection are driven primarily by a combination electronic performance and cost considerations, but the consequences of those choices on thermal performance should be made as clear as possible, as temperature and cooling also affect performance and cost.
1: Start preplacement – prelayout
There is a lot of work that can be done well before layout is completed within the electrical design flow. Indeed, any influence thermal considerations have on the design need to be factored in before this point. A lot of work can be done with a simple representation of the enclosure to provide information about the air flow profile over the board.
You can start by simply smearing the total board power over the total board surface. This will give you a temperature map that will give an indication of any hot regions that are caused by a maldistributed air flow, and the enclosure-level air flow should be optimized ahead of the PCB design. For this you can treat the board as a block with an isotropic conductivity of between 5Wm-1K-1 and 10Wm-1K-1. The results at this stage will be quite insensitive to the value chosen.
A word of caution – components inject heat locally into the board, so the heat flux density into the board below a component will be higher than the average for the board. As a result, the local board temperature will be higher than that predicted in the simulation, so at this stage the board temperature should not be used to try to estimate component temperatures. To do that the model needs to be refined.
If the board temperature at any point is close to the maximum component case temperature, then it is very likely that this limit will be exceeded once the component heat sources are represented discretely. This may be expected for example if one or more components are known to require a heatsink.
For the rest of the paper:
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