Tackle Thermal Design At the System Level

Jun 1, 2008 12:00 PM
By Kim Gauen, Applications Engineer, Freescale Semiconductor, Kokomo, Ind.

Simulation Software

Ultimately, the complexity of thermal modeling can lead users to commercially available thermal modeling software. Before using a set of thermal modeling tools, it's appropriate to consider the purpose of the modeling, the characterization data available to support those models and what a particular software program can and cannot provide (see the special Selecting Thermal Analysis Software section on bottom of page).

The most fundamental feature of a thermal analysis program is the numerical method used to solve the governing mathematical equations. This is the means by which the software resolves the governing mathematical equations, including the momentum and energy equations, coupled with the continuity (conservation-of-mass) equation.

The particular numerical method a program uses makes it more or less suitable for specific modeling tasks. The most obvious example is computational fluid dynamics (CFD) software. Like all viable thermal analysis software, it accounts for conduction and radiation. But CFD also predicts fluid flow, which is necessary to model convection.

If convection is a primary transport method in the system, CFD software is probably required. ANSYS is one well-known CFD thermal analysis software supplier and offers CFX, Fluent, Iceboard and Icepak. Flomerics, another well-respected vendor, provides FLOTHERM. CFD programs provide the ability to view and export images of a fluid's speed and direction. This feature helps to clearly illustrate the size and effectiveness of thermal plumes, which are likely to form above hot surfaces in the presence of natural convection.

Optimize the Environment

Reducing the operating temperature of semiconductor devices involves several techniques, many of which add little or no cost to the end module. Some possibilities include reducing the module's or IC's thermal load, reducing thermal impedance in the IC's immediate vicinity or using a continuous low-impedance path from the IC to ambient.

It is also possible to increase the surface area from which the heat exits the module or to use thick copper cladding, if allowable. Another option is to avoid the heating of pc-board traces and connectors. Also, consider the effects of pc-board and module orientation and, if possible, take advantage of thermal capacitance.

No matter which approaches or how many techniques are used, expect surprises. Monitoring the temperature at many points on a pc board or photographing the module and its harness with an infrared camera may reveal unexpected temperatures and misunderstood aspects of the thermal circuit. Finding these surprises provides valuable clues that can lead to a better understanding of the module's thermal behavior and, in the end, a more reliable product.

References

1. The equations used to calculate thermal resistance and impedance are usually found on the data sheet. For a more detailed explanation of the calculations, refer to Freescale's white paper online by searching BASIC THERMWP at www.freescale.com.

2. “Important Questions to Ask When Evaluating Thermal Analysis Software for Electronics,” Stokes Research Institute, www.stokes.ie/pdf/Questionnaire.pdf.

Selecting Thermal Analysis Software

Designers must carefully evaluate what a particular thermal analysis software program can and cannot do. These are some of the best uses:

• Provide tradeoff assessments before any hardware is built
• Uncover poorly understood thermal phenomena
• Speed development
• Assess test conditions that are difficult to create
• Accurately simulate behavior of simple structures
• Provide a means to estimate a system's response to power transients.

There are quite a few commercially available thermal modeling software packages. Each package claims its niche, and it is important to understand how they differ. Some of the differentiating features are^[2]:

• Cost, including hardware and maintenance fees

• Training required for competency

• Simulation speed

• Ability to model all three modes of heat transfer, which for convection requires the ability to model fluid flow

• Ability to model responses to time-varying power waveforms

• Ability to import files from other CAD packages

• Method of managing boundary conditions

• Ability to use a multilevel uniform and nonuniform-nested mesh (ability to create a body-shaped mesh is a plus)

• Ability to link thermal models to models in other domains

• Inclusion of a software library that contains common thermal elements such as heatsinks, heat pipes, enclosures and pc boards

• Ability to view/export simulation results in a generally accepted format

• Customer support, including technical literature

• Numerical method used to solve the governing mathematical equations.

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