Final answer:
Without specific thermo-physical data and information on the cooling method, the maximum temperature of a circuit board that dissipates 5 W of power cannot be accurately calculated. The uniform temperature for an isothermal surface condition is also indeterminate with the given details.
Step-by-step explanation:
To determine the maximum temperature of the circuit board, we would typically use Fourier's Law of heat conduction and Newton's Law of Cooling, but we are given a power dissipation value (5 W) rather than material properties and temperature differences. To estimate the heat loss to the surrounding air at 27 ℃, a more detailed thermo-physical analysis is needed, taking into consideration convection and radiation losses, which are not specified in the question.
Due to the lack of sufficient data, this calculation cannot be carried out. As for the condition of an isothermal surface, this implies that the entire board is maintained at a uniform temperature. This temperature should theoretically still be above the ambient air temperature due to the power dissipated by the components, but a precise value cannot be provided without more information on the thermal characteristics of the board and the nature of the cooling (natural convection, forced convection, etc.).