When designing a PCB, thermal issues are often locked in at the point of selecting and laying out the chip package for the board. After that, only remedial actions are possible if the components are running too hot. Assumptions made about the uniformity of the airflow in these early design stages can mean a disaster for the commercial viability of a PCB if those assumptions are incorrect. A different approach is needed to improve reliability and to optimize board performance.
This approach starts with considering the overall flow environment, which is especially critical for good operation of air-cooled electronics. Start early, keep it simple, and focus on collaboration between mechanical and electrical design. If you are the mechanical engineer responsible for the thermal integrity of the product, you can provide as much useful feedback as possible to the electronic engineers about effects their choices will have on the thermal issues in their PCB design.
This collaboration entails advising with package selection and the positioning of components to best use system airflow for cooling. Layout and package selection usually are determined by electronic performance and cost; however, temperature and cooling can inevitably affect operation and cost so the consequences of design choices on thermal performance should be made as clear as possible, as early as possible.
Before Placement and Layout
First step is to optimize the enclosure-level airflow. Begin with a simple representation of the enclosureto provide information about the airflow profile over the board. Smear the total board power over the total board surface to get a temperature map indicating any hot regions caused by badly distributed airflow. You can treat the board as a block with an isotropic thermal conductivity between 5 and 10. The results at this stage will not be affected by whichever value you choose in that range.
Do not use the board temperature to estimate component temperatures at this stage because components inject heat locally into the board, which means that the heat flux density into the board below a component is higher than the board average. If the temperature of any section of the board is close to the maximum component case temperature, when you later refine the model to represent the component heat sources discretely, the component temperature limit will then likely be too high.
Make a Best-Guess Estimate for Component Power
Make a best-guess estimate of the individual power budgets for the main heat dissipating components that will be used in the design, and the approximate size of those packages. Then you can describe these packages as footprint heat sources in your simulation, which will smear the remainder of heat uniformly over the board surface.
The system architect will already have some idea of what key components will be required, where they will need to be positioned, and their size, etc. For example, some components may be used that were selected for another product or retained from the previous generation product.
To read this entire article, which appeared in the December 2017 issue of The PCB Design Magazine, click here.