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Miniaturization of electronic/mechanical systems is achieved by packing different functional components into tight space. The heat transfer analysis for such systems has to deal with geometrically complex heat transfer paths, and it needs to be done quickly in response to every design alteration. This paper proposes a concept that aims at reduction of analysis load on the packaging designer. The proposed scheme is composed of two major steps. First, a computer program "configuration generator" is used to generate possible geometric configurations of heat transfer paths in a systematic manner. Second, temperature solutions for these configurations are compressed into "fast estimate" formulas that free the packaging designer from the need to perform involved heat transfer analysis. This approach is illustrated using a topic of heat spreading on the planar substrate as an example. Miniaturization of systems also raises another issue for the thermal design; that is the coupling between the system configuration and the overall heat dissipation to the environment. A model situation is considered where heat diffuses from a zone on the system shell and to the environment by natural convection and radiation. In the parametric domain spanned by the thermal conductivity of shell material and the system's characteristic length there is a zone where the system-level heat transfer is sensitive to the system's configuration. Such characteristic length is around 1 cm for systems encapsulated in plastics, 3-10 cm for those in ceramic and alloy shells, and 10-40 cm in copper or aluminum clad systems.