A new approach to the dynamic thermal modelling of semiconductor packages

This paper describes a novel approach to the dynamic thermal characterisation of semiconductor packages. Conceived as an extension of the variable angle model for multilayer structures, the thermal capacitance of each layer is evaluated at the same time and for the same volume used to calculate the thermal resistance, which guarantees the coherence. Because this volume or lump is a function of system geometry, materials and boundary conditions, the model can be said to include 3D aspects. The thermal resistances and capacitances obtained from the model, each pair representing a layer of the structure, are used as input to an electric circuit simulator to obtain the transient response of the package. Moreover, some common assumptions usually made when interpreting the transient thermal response of semiconductor packages are revised and discussed in an attempt to clarify the scene. Particularly, the log-of-time derivative of the step response is shown to give all the information about the dynamic response when the characteristic time constants of the system are widely separated. In that particular case, otherwise very common in practice, a novel analytical method for the obtention of the characteristic time constants and associated amplitudes is presented and their results evaluated for different industrial power packages. (C) 2001 Elsevier Science Ltd. All rights reserved.