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flow over the board and, second, to use the airflow in a thermal model of the board and the electronic components deployed upon it. The thermal engineering of air-cooled electronic packaging is being revolutionized by the increasing availability of CAD ~computer-aided design! tools designed to provide integration and compatibility among geometric modeling, mechanical design and layout, electrical design and layout, and pre and post-design simu-lation. From the point of view of thermal simulation, the predic-tion of device-junction temperature can be accomplished, in prin-ciple, by discrete decoupled models of the electronic component, the board, and the board’s environment. For example, a single chip component can be modeled routinely now using the finite element method ~FEM!. At the other end of the system hierarchy, there is an increasing need, or perhaps just temptation, to use commercial CFD ~computational fluid dynamics! tools to predict the airflow distribution in cabinets and enclosures so as to provide estimates of the card-level air flow. In the middle ground, the numerical computation of convective flow and heat transfer over the populated printed wiring board ~PWB! is, at present, compu-tationally prohibitive for routine design because of the complexity of the air flow over arrays of three-dimensional components on boards and the inadequacy of existing turbulence closure models for such flows. At this state in the evolution of CFD tools, ad-hoc decoupled methods for combining the component and board FEM conduction models with the coarse board convective flow pre-dicted by the CFD system simulation model are still required for design simulation. the two through the convective flow and through the board. The convective interaction arises from the fact that heat transferred to the air from the upstream component is advected downstream as internal energy. The energy diffuses in all directions, even as it is being advected, producing a three-dimensional temperature distri-bution, referred to as the ‘‘thermal wake,’ ’ behind the heated com-ponen