Six identical power transistors with aluminum casing
are attached on one side of a 1.2-cm-thick 20-cm
30-cm
copper plate (k  386 W/m · °C) by screws that exert an average
pressure of 10 MPa. The base area of each transistor is
9 cm2, and each transistor is placed at the center of a 10-cm
10-cm section of the plate. The interface roughness is estimated
to be about 1.4 m. All transistors are covered by a thick
Plexiglas layer, which is a poor conductor of heat, and thus all
the heat generated at the junction of the transistor must be dissipated
to the ambient at 15°C through the back surface of the
copper plate. The combined convection/radiation heat transfer
coefficient at the back surface can be taken to be 30 W/m2 · °C.
If the case temperature of the transistor is not to exceed 85°C,
determine the maximum power each transistor can dissipate
safely, and the temperature jump at the case-plate interface