Consider a surface-mount type transistor on a circuit board whose temperature is maintained at 35°c. air at 20°c flows over the upper surface of dimensions 4 mm × 8 mm with a convection coefficient of 50 w/m2 · k. three wire leads, each of cross section 1 mm × 0.25 mm and length 4 mm, conduct heat from the case to the circuit board. the gap between the case and the board is 0.2 mm. (a) 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 0.12 w/m · k, respectively.