Thermal simulation is a fiequently needed task in the design of integrated power packaging. Thermal simulation is used in the design of new devices, in the design of the placement of the dissipating elements on the chip and for the design of the packages. Thermal simulators can help to find the best mounting solutions for the devices if they have to operate in thermally strained conditions. Thermal simulation is usually done today with the help of Finite Element Method (FEW based simulator programs. These are general-purpose expensive simulators, where the general usability comes together with complicated, and usually difficult to leam and difficult to use user interfhces, and these programs are relatively slow and inaccurate. To overcome all these problems we have developed two fast and easy to use 2D and 3D thermal simulator programs SUNRED[ 1], and THERh4AN,. In the recent development the first goal was to create user fiendly tools, that design engineers are happy to use in their thermal design tasks, since the tools are fast enough to provide answers to their thermal questions practically immediately. As a result of this feature the designers can study the effect on the thermal behavior of all modifications in the geometry of their structure or in the boundary conditions - immediately on their computer Screen.