REBECA-3D (REliable Boundary Element Conductive Analyser in 3 Dimensional) is the software developed by EPSILON INGENIERIE that simulates thermal transfers driven by conductive exchanges. It uses the Boundary Element Method (B.E.M.). It is at the same time a tool of design and a tool of modelling. It gives more accurate and reliable results than classical methods. It allows to carry out a great number of parametric studies with very few calculations.The main bases of this three-dimensional software are given hereafter in order to demonstrate its performances. Firstly, REBECA-3D is applied for an electronic example (in collaboration with ALCATEL ESPACE). Secondly, it is used in a new methodology to automatically compute conductive conductances becoming integrated in the thermal chain.