Definition of a Boundary Condition?

I am working on a Heat transfer application of an antenna board with semiconductor chip as the heat source (on top of the board). Since the design of the antenna board consists of vias and different material layers, specifying boundary conditions is turning out to be difficult.

Can someone explain me the different factors which needs to considered for specifying boundary conditions? Since the heat source (Semiconductor chip) is at the top of the board, do I have to specify heat flux as the boundary or the temperature as the boundary for the heat source?

I don't know the temperature of the board or the chip, I know the Power dissipation which is 5W and the ambient temperature which is 25 deg C. Apart from this, since the simulation is done in 2D I know the length and height of the board and the chip plus all the thermal conductivity of the materials being used.