Consider a surface-mount type transistor on a circuit board whose temperature is maintained at 35°c as shown in the figure below. air at 20°c flows over the upper surface of dimensions 4mm x 8mm with a convection coefficient of 50 w/m 2 . k. three wire leads, each of cross- section 1mm x 0.25mm and length 4mm, conduct heat from the base to the circuit board. the gap between the case and the circuit board is 0.2mm air transistor case wire lead circuit board gap
assuming the case is isothermal and neglecting radiation, estimate the case temperature when 150 mw is dissipated by the transistor and (i) stagnant air or (ii) a conductive paste fills the gap. the thermal conductivities of the wire leads, air, and conductive paste are 25,0.0263, and 0.12w/m - k, respectively.