Electronics Cooling Best Practices

A major challenge in the Electronics industry is to ensure components operate at temperatures below their mandated limits. The challenge is further compounded by the relentless drive to reduce device size while increasing device power. As component power increases and packaging sizes decreases, designing to ensure adequate cooling becomes increasingly critical and challenging. By using Autodesk Simulation CFD early in the design process to incorporate an in-depth understanding of the flow as a part of the design, many of these challenges can be managed effectively.

This topic describes many of the best practices for System-level electronics thermal simulations. It begins with strategies and objectives that can be applied to any system-level application, depending on the design criteria. A classification system based on application is presented to help determine in which category a particular application falls. Analysis best practices are then presented for each category.

These techniques have been developed by Autodesk Simulation CFD application engineers, and applied to a great diversity of electronics applications. While these work for most applications, there are situations in which may require some customization.

These techniques are fairly conservative, and in some cases it may be possible to achieve convergence quicker by modifying them slightly. As experience is developed with a specific application, feel free to optimize these techniques to achieve results with the most efficient process possible.

Type of Electronics Analyses

There are three fundamental geometry classifications of electronics analysis: Component, Board, and System. Each has different objectives and analysis strategies:

Component

Autodesk Simulation CFD is well suited for simulating component-level analysis within a small system.

The objective of most mechanical and design engineers (typical Autodesk Simulation CFD users) is to understand the thermal performance of off-the-shelf components. The typical strategy is to enclose the component in a small air volume, and run as a small system. The techniques described below can be applied to such a small-scale system-level analysis.

Many electronic components come with specifications that specify the air velocity that must be maintained over the device to ensure adequate cooling. A powerful application of component-level analysis is to determine the resultant velocity over such components within a simplified version of the system. This is a general view of the process:

Board

In many board-level analyses, the objective is to assess the temperature rise within a PCB due to Joule heating (or potential difference). This involves modeling the traces, which is a difficult process as a CAD model of the trace layout is not usually available. The typical process is to migrate the date from an ECAD model to a CAD model to the Autodesk Simulation CFD analysis. This sort of analysis is usually of most interest to electrical engineers.

In other situations, the objective is to characterize the thermal performance of an individual board with components. The board is modeled in an air volume the width of the slot. This is a small system analysis, but the level of detail contained on the board and components is typically higher than the larger system analyses described below. The techniques described below can be applied to such a small-scale system-level analysis.The results from such an analysis can be inserted as a component sub-model within a larger system analysis.

System

System-level applications contain multiple components assembled within an enclosure or housing. They come in many different configurations, and can contain anywhere from a handful of components to hundreds. Depending on the application, systems can be completely sealed or contain large arrays of vents. Air moves through most systems as a result of buoyancy or of a forcing device such as an electric fan. Regardless of the geometric configuration or application, ensuring that components do not exceed their limiting temperatures is the primary objective. Several other objectives and design strategies are employed throughout various industries to achieve this goal.

System-Level Analysis Strategies

Before beginning any electronics analysis, the items discussed in the section should be evaluated to determine the correct approach. The objective of the project is an important consideration, and should be well understood before beginning because geometric and modeling assumptions are largely driven by the desired outcome. Another consideration is where in the design process the analysis performed. If conducted early, design-level geometry should be used. Design-level geometry is preferrable to production-level geometry due to reduced model complexity and improved agility to make changes based on analysis outcomes.

Basic Process and Objectives

There are several modeling strategies that are useful for many electronics applications. All of these techniques help to reduce the geometric complexity without sacrificing the critical flow and thermal aspects of the device.

The following questions should be considered for all system-level analyses:

  1. What is the environment surrounding the device? Are there structure or other components in proximity?
  2. Is the device cooled with a fan or does it rely on passive cooling (natural convection)?
  3. Is the device sealed or does it have vents?
  4. Are there devices within the system that can be simulated either using material devices or some other representation?
  5. Identify and understand the objective of the analysis.

There are many potential objectives for system-level analyses. Understanding them is essential, and will guide the analysis process. Potential objectives include:

Design Level vs. Production-Level Geometry

Sub-Module Representations

A powerful strategy for analyzing complicated systems containing many components is to use sub-module representations within the overall system model. Sub-modules that contain large amounts of geometric detail or occur in multiple instances are prime candidates for this approach. An example of a module is a channel between two boards within a large rack The benefit is that the amount of detail within the system model is greatly reduced, resulting in lower analysis times and greater design efficiency.

These representations are derived by characterizing their performance in separate, smaller analyses. The resultant flow and thermal behavior are applied to a geometrically simple representation of the module within the system model. This process is summarized:

  1. Create a CAD model of the component or module. For an individual module, construct a small air volume to surround it. Try to use a volume that is roughly the same size and shape as the footprint within the system model. In some cases, the "module" may already contain an air volume such as in an 1U rack sub-module of a cabinet assembly. In this case, ensure that the air volume is the same size and shape of the air channel in the 1U rack.
  2. Run the flow and thermal analysis to compute the pressure drop, heat output, and outlet temperature.
  3. In the system analysis, substitute a simple geometric form for the module. Assign a distributed resistance to the material to represent the pressure drop. Assign a heat generation boundary condition to represent the amount of heat dissipated by the physical device.
  4. The applied parameters may have to be adjusted in an iterative process to accurately represent the effect of the module.

Preliminary Design Approach

A good preliminary approach is to eliminate most design alternatives by approximating some of the physics within the analysis before modeling the complete external environment.

To do this, model the interior of the device, and simulate the external flow with a film coefficient boundary condition on external surfaces of the device. This eliminates the calculation of the flow surrounding the device, and provides a result much quicker than the full external domain analysis. The resultant temperature distribution will not be exactly correct, but it will often lead to the disqualification of problematic design elements. After the better design candidates are identified, a more rigorous analysis approach using a surrounding air environment can be employed to determine the actual temperature field.

The recommended film coefficient value for the external surfaces ranges from 5 W/m2K to 20 W/m2K. The former value simulates quiescent air with little to no movement. The latter represents more vigorous air flow, often attributed to higher temperatures. While it is difficult to know what value to use, it is far more important that the same value or values be used for all design alternatives to achieve consistent results.

Classification of system-level electronics analyses

Forced Cooling (Fan)

Passive Cooling