All electronic components have defined boundaries of operation where they can be expected to operate reliably. For semiconductors, voltage, current, power, and junction temperature are key specifications which should not be exceeded, but the can sometimes be a bit difficult to understand or control. Key is to understand why devices heat up as they do, what mechanisms are responsible for the temperature rise of the component, how hot they will get during operation, and how to cool them down in the most efficient way practical.
John Escallier discusses these issues, including sufficient thermal theory to understand the various hardware tradeoffs encountered when designing the mechanical layout of a circuit, and the effect those tradeoffs will have on the overall thermal reliability. All examples and calculations are for simple geometric models, and the analysis is at a reasonable level to allow the designer the options of simple hand calculations, or a more rigorous spreadsheet setup. John has developed an Excel spreadsheet to illustrate this, and the spreadsheet will be made available for download at the linearaudio.net website.