Convection & Radiation
Convection cooling can occur on either side, or both sides, of the board. Convective boundaries simulate either free or forced convection of the board and components to a forced convection boundary temperature.
If FORCED, it is assumed that the air temperature is a boundary temperature that does not change (i.e., replacement air is always being forced by the board). You must specify this boundary temperature. This thermal path can either cool or warm the board, depending on the air temperature relative to the board temperature.
If FREE, the air adjacent to the board is treated as a single non-boundary mass-less node that has a thermal conductance to each board and component node. Because it is modeled as a single node, it is implicitly assumed that the circulation of air within the cavity results in a uniform temperature.
Conduction and convection are usually the dominate forms of heat rejection. However don’t forget radiation to the chassis as long as it has a high emissivity (i.e. Paint or anodize). The specified radiative boundaries simulate radiation interchange with an adjacent board or chassis walls boundary temperatures.