Integrating Compact Thermal Models in CFD Simulations of Electronic Packages

Compact thermal models (CTMs) are simplified multi-nodal thermal resistor network representations of the detailed material and geometric structure of the electronic package. CTMs predict the thermal response of the package, in various environments, to within an accuracy of 2%. The junction temperature of the package is typically obtained by solving the linear algebraic network equations of the CTM, with the heat transfer to the ambience modeled by a convection coefficient obtained from handbooks, assuming identical ambient conditions imposed on all nodal surfaces. This approach may give misleading results as the ambience at each nodal surface can differ depending on the cooling flow patterns at that surface. In this work, a methodology is presented where the network equations of the CTM are integrated into the governing fluid flow and energy equations solved by computational fluid dynamics (CFD). The $CTM+CFD$ approach predicts a significantly (20–30%) higher junction temperature as compared to the conventional CTM network solver method, even when the convection coefficient used in the latter case is obtained more accurately from CFD, rather than from handbook correlations. It is also found that CFD computations assuming uniform flux at the package surfaces (and ignoring the internal resistance of the package) vastly under-predict the junction temperature. The new approach offers a promising alternative for electronic package thermal design and is highly advantageous where the internal geometric and material configurations are not known due to proprietary concerns.