Thermal evaluation of ΘJA for varying board conductivity

Electronic packages are traditionally thermally characterized by the resistance parameter Θ_JA. In many cases, Θ _JA is measured on low thermal conductivity printed circuit boards. The simplicity of the signal planes and the usual lack of power or ground planes on the low conductivity thermal test board gives a misleading indication of thermal performance. A method of determining how Θ_JA changes when the thermal characteristics of the printed circuit board change is required to understand the actual thermal performance and operating temperatures of electronic packages. An analytical model and a finite element model were developed to examine the sensitivity of Θ_JA to changing board parameters such as board thermal conductivity and board temperature. A 352 pin, plastic two-layer ball grid array package was tested and modeled on a one-layer, two-layer and four-layer board in still air. The model was reasonably accurate in predicting Θ_JA in still and moving air for a variety of board thermal conductivities