Thermal Modelling and Optimisation of Power Circuits

Citation

Pitt, K. (1998), "Thermal Modelling and Optimisation of Power Circuits", International Journal of Numerical Methods for Heat & Fluid Flow, Vol. 8 No. 3, pp. 381-382. https://doi.org/10.1108/hff.1998.8.3.381.1

Although there are well known analogies between the conduction of electricity and the conclusion of heat, the practical differences are enormous due to factors such as surface effects, two or three dimensional heat flow paths compared with wires and materials parameters difficult to measure accurately. The simple analogy and common sense are often helpful for a first order look to see whether there will be any thermal design problems. However, as the authors point out, due to the very small sizes of heat sources and their close proximity, an accurate estimate of the thermal regime is essential before commitment to design and manufacture of microelectronic structures. Unfortunately, every case is likely to be different and general solutions are rarely available. Their work has specialised in thick film substrates and hybrids, but can be extrapolated with care to other types of structure such as printed circuit boards and integrated circuit dice. Inevitably in work on heat transfer there is a very large mathematical content to the book, as far as possible the results are related to empirical measurements. Two pages of symbol glossary help the reader to find a way through the mathematics.

A short introduction puts the reader into the context of microelectronic structures giving the likely power densities in monolithic structures and typical surface temperatures to be observed. An introduction to hybrid circuits reminds us of the materials used and their thermal parameters. Almost 50 per cent of the book is taken up by the main theoretical aspects which describe a two‐ and three‐dimensional stationary thermal model of a hybrid system. The authors have used both analytical and numerical techniques and compared their applicability and performance. Their examples include encapsulated and unencapsulated circuits and forced and free convection. Some of the convection coefficients chosen are arbitrary, but could easily be changed to suit different structures and materials. This chapter includes an excellent section on the use of numerical methods in thermal analysis. Numerical and analytical approaches are then compared.

Chapter four is crucial to the objectives of the authors. A section on infra‐red thermography gives the experimental basis with which the computed results can be compared. These results are used to determine the heat transfer coefficients in systems. A brief sub‐section on laser interforemetry gives a useful idea of how convection flow can be visualised around heat sources on a board or substrate.

Chapter five discusses the importance of thermal feed back in the calculation and understanding of heat distribution in substrates using TOPO SPICE. This is a hybrid of proprietary and industrial programs. Non‐steady state, and slow change conditions are introduced here. Examples are given of the value to the system designer of using this kind of approach.

Chapter six looks at the optimisation of microcircuit constructions, reminding the reader of the reasons for its importance. Where possible analytical and thermographic results are compared. An heuristic approach to design is also proposed. However, perhaps the most interesting idea is the use of neutral networks in place of conventional computer programs. The results show considerable promise, not least in the marked reduction in computing time for repeat computations once the network has been fully taught.

There is also a short concluding chapter, two mathematical appendices, a flow chart for TOPO SPICE and a useful subject index.

Thermal design of current and planned microelectronic structures is absolutely essential to the viability of products, both in performance and long term reliability. The trend towards both high packing density and higher speed of signal propagation makes design aspects which could in the past almost be ignored now become a critical path. Although there is no universal route to a good and trustworthy thermal design, the authors point out a route they have chosen. They try to show what may be possible and what problems ought to be avoided. Regardless of the specialisms of some of the work, reading through this book will have a salutary effect on designers and make them realise that thermal design must be taken seriously. It is the kind of text which is essential for researchers in the field who will have the expertise and background to make use of the many interesting points proposed by the authors.