1 Steady operating conditions exist. 2 Heat transfer can be
approximated as being one-dimensional, although it is recognized that heat
conduction in some parts of the plate will be two-dimensional since the plate
area is much larger than the base area of the transistor. But the large thermal
conductivity of copper will minimize this effect. 3 All the heat generated
at the junction is dissipated through the back surface of the plate since the
transistors are covered by a thick Plexiglas layer. 4 Thermal conductivities are
constant