Thermal analysis during the design process is an essential step towards the achievement of high reliability in modern high density hybrid and integrated circuits. Thermal analysis is also essential for modern, high density PCBs. Traditionally, a solution of the thermal problem is obtained by either the method of finite differences or the method of finite elements. Both methods, however, require a fine 3-D partition of the substrate, leading to large systems of linear equations the solution of which demands substantial computing power provided by number crunching machines and/or powerful computer work-stations.The widespread use of personal computers, however, dictates the development of new approaches to the thermal problems so that a design engineer can solve them in reasonable time with a PC. A new treatment of steady-state thermal analysis combined with a layout editor is proposed in this paper, which makes use of the analytical solution for the temperature distribution in a single-layer substrate with heat sources on the surface, and having an isothermal bottom surface. In this way the mathematical complexity of the problem is dramatically reduced allowing the thermal analysis of Multi Chip Modules (M.C.M.s) and complex hybrid circuits with the use of a PC/XT or compatible.