Streamlining Thermal Design of PCBs

Reading time ( words)

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.


Suggested Items

Excerpt: The Printed Circuit Designer’s Guide to…Flex and Rigid-Flex Fundamentals

06/20/2018 | Dave Lackey and Anaya Vardya, American Standard Circuits
The design process is arguably the most important part of the flex circuit procurement process. The decisions made in the design process will have a lasting impact, for better or worse, throughout the manufacturing cycle. In advance of providing important details about the actual construction of the flex circuit, it is of value to provide some sort of understanding of the expected use environment for the finished product.

Faster Board Speeds Demand Constraint-Driven Design

06/19/2018 | Ralf Bruening, Zuken
Using powerful constraint techniques can be a double-edged sword. While the design process is made much safer by including constraints, it is all too easy to over-constrain the design and make it impossible to complete routing and placement. Even paper design guidelines can make products uneconomic to produce unless a great deal of engineering knowledge is applied during the design.

Making the Most of PCB Materials for 5G Microwave and mmWave Amps

06/13/2018 | John Coonrod, Rogers Corporation
Ready or not, 5G is coming, and it will require the right circuit materials for many different types of high-frequency circuits, including power amplifiers. 5G represents the latest and greatest in wireless technology, and it will be challenging to design and fabricate, starting with the circuit board materials, because it will operate across many different frequencies, such as 6 GHz and below, as well as at millimeter-wave frequencies (typically 30 GHz and above).

Copyright © 2018 I-Connect007. All rights reserved.