Thermal modeling and analysis of devices and MCMs ( modules) is becoming very important in recent times. In years past, most of the thermal effort was based on the design of devices, and thermal analysis was built into the circuit simulators such as SPICE. The rest of the modeling based on the electrical analogs of heat transfer was well understood, and could be done in a fairly simple way. Today the situation is quite complex. As more and more performance is demanded from semiconductors ( individually ) and from MCMs ( multi-chip modules ), and indeed entire products, such as cell phones, the heat per area is rising towards the 500 W/cm squared limit. This is a lot of heat, and it is very difficult to use the old methods to model and understand these problems. Therefore thermal modeling is attacking these issues in electrothermal dynamics by using newer CAD tools based on FEM ( Finite element methods ) and CFD ( Computational Fluid Dynamics). A number of new thermal modeling tools have appeared on the market. Some are reasonably priced and others are not. Again you get what you pay for! Heat flow is based on the conduction, convection and radiation of heat ultimately. Thermal CAD tools model these processes in their own proprietary way. We have used a couple of these tools and find them fairly, ( and I mean fairly ) complex. So practice is neccessary. However, the results obtained are within the 20% error band. The accuracy of results also depend on the skill of the user! More on this topic as time goes on. It is a fascinating subject.