Light-weight components are of crucial interest for all industries producing moving masses. The aim of reducing weight is closely followed by high production efficiency and component performance. Friction stir welding (FSW) offers the possibility to produce welds in light metals with the advantages offered by fusion welding but in addition they also give a significant reduction in distortion and improve metallurgical transformations, with inherent improvement in performance. The development of FSW has been advanced in the industrial sector typically characterized by large companies. The risks associated with this technology transfer into main stream industry however is still high due to a lack of fundamental understanding over the mechanics of material transport and the influence that tool design and process parameters have on ultimate joint properties. FSW encompasses complex phenomena related with plastic flow deformation resulting from the stirring of the workpieces materials. The development of a process model would significantly contribute towards a better understanding of joint formation and resultant mechanical properties. STIR is a thermal analytical model for 2D and 3D cases when FSW similar and dissimilar materials. The model allows simulation of asymmetric heat field developed below the tool shoulder due to the composition of the rotation and linear speeds, and the hot-to-cold welding conditions, considered in the establishment of the thermal field generated resulting from all the energy sources, i.e., viscous and interfacial friction dissipation. The iSTIR application field is reviewed and the advantages and limitations of the model are discussed. The analytical formulation is described and an application sample based on a friction stir weld produced under cold conditions for butt-welded AA6056-T4, in 3.9 mm of thickness is evaluated.