In this paper a 3D conduction model and its possible calculation methods are presented. Our 3D model is based on the thermal (central) node theory. It means that all elements of the circuit contain a central node, to which all of the thermal mass of that area of the assembly is assigned . The model can describe and compute the conduction effects and the thermal distribution during reflow soldering at the level of discrete components. Three calculation methods of the 3D conduction model (discrete calculation, half-continual calculation and continual calculation) were presented. The calculation methods were compared according to the computing time, the difficulty of the implementation and the accuracy limits. Finally a method (conduction orientation) was presented which can define the dominant direction of conduction in a system or in a part of the system. The ability of this method was also discussed.