Understanding thermoelectric analysis of chips

Electromagnetic phenomena can change a device’s thermal properties and so it is important to combine thermal and electrical analysis when simulating chips or designing electrical bushings, writes Bruce Klimpke.

As computing power and software efficiency increases, the demand for more complex simulations grows alongside the need to improve reliability and costs. One of the most notable demands is the interdisciplinary approach, coupling physics from different disciplines. 

In reality all electromagnetic phenomena change a device’s thermal distribution and properties. For many applications the effect is minimal and can be ignored.

For others, neglecting the coupling will significantly reduce the realism of the design or render the final product useless. Therefore, it is essential to combine thermal and electrical analysis when simulating chips or designing electrical bushings.

With every electrical system the flow of current through a conductor generates heat. The heat produced can prove significant in the design of a product and in some cases be the dominant factor.

To solve this, look at the electrical conduction and, once current paths are known, the volume heat density can be calculated directly through the ohmic losses, providing the required heat source for a thermal analysis.

Whether using 2D or 3D models, simulation programs require all the thermal material properties to be known as well as appropriate boundary conditions, such as applied temperature.

While general thermal analysis programs can be used to model everything from chips and motors to high voltage equipment, there are programs which have two solvers on the same package, namely a boundary element and a finite element.

There are two distinct levels of difficulty involved in solving a design’s combined electrical and thermal problems. The first is where the temperature distribution within the device has a negligible effect on the electrical solution.

For this, solve the electrical problem once and use subsequent ohmic losses as an input for the thermal analysis. The thermal or temperature distribution within the device is readily calculated. So, when designing a chip, if the electrical properties are not significantly changed by the temperature only one electrical and thermal analysis is required.

The second refers to more advanced simulations, requiring many solutions to be generated from the electrical and thermal programs. For example, if the material’s electrical conductivity is strongly dependent on the temperature, a further electrical analysis would be required after the thermal analysis, changing the heat distribution from the original.

Thus, an iterative procedure occurs between the electrical and thermal analysis, until the electrical and thermal parameters converge – typically required in high voltage bushings.

Solving electrical and thermal problems are mathematically the same for static problems but the practical implementations are quite different. For most practical thermal analysis, radiative effects and forced convective heat transfer are introduced into an equivalent heat transfer coefficient, used on the surface of the part being analysed.

Most heat or temperature modelling is undertaken by analysing the temperature within the part of interest with effects on the nearby volumes simply neglected.

However, solving electrical problems is quite different. Not only is the designer interested in the electric field within the part, but also the field in the air space surrounding the part. Even if the electric field calculation is only required within or on the part, the entire volume about the part has to be modelled.

Solutions requiring combined physics disciplines are best obtained by simulation using both finite and boundary element software. The advent of 64 bit personal computers with memory of 32Gbyte permits the solutions of coupled electrical and thermal analysis that would have been impractical for full 3D simulation on a 32 bit machine. 

Multiple core processors, and the software enhancements which follow the level of realism of coupled field analysis, are truly expanding this technique.

Author is Bruce Klimpke, technical director at Integrated Engineering Software

www.integratedsoft.com

Tags: chips, design, EDA

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