Printed Circuit Board Thermal Modeling Without the Use of an Effective Thermal Conductivity

The complexity of the circuit traces on the layers of a typical printed circuit board (PCB) poses a serious problem when preparing a thermal model of the board. Thermal analysts have resorted to the use of an average or so called, “effective thermal conductivity”, K_eff, treating the board as a homogeneous medium in their PCB thermal models. This approach carries with it the possibility of significant error in the prediction of board temperatures. A typical PCB will have large variations in the density and pattern of the circuit traces, and a single value of K_eff cannot accurately represent all board locations. An alternative approach to this long standing problem is presented in this paper. In the new procedure the thermal conductance between pairs of nodes is computed using all of the details of the circuit traces in the internodal region. The trace information is obtained from bitmap files of each circuit layer, files which may be generated from the board CAD files. The conductances are utilized in a general purpose thermal analyzer for computation of system temperatures. Using the details of the local circuit traces in the computation of internodal conductances results in a more accurate thermal model.