Emissivity is a material surface property which determines how much thermal energy is absorbed by or radiated from a solid surface, following the Stefan-Boltzmann Law.

The emissivity for most surfaces can be set to a default value in the General Settings > Wall Conditions menu (usually use a value of around 0.8), but then individual surfaces or groups of surfaces can be set to different values using the Radiative Surface type feature. Many plastics, ceramics, PCB’s and many other coated electronics components have an emissivity value of around 0.8 (0.7→0.9), but metals typically have a lower emissivity, so you may wish to override the default value for some heat sinks etc. However a good anodized heat sink surface may also approach a value of 0.8 or even higher.

The FloEFD Electronics Module provides a database of pre-defined radiative surface properties for real surfaces, so you should look at these. Note also that FloEFD allows you to specify the Emissivity Coefficient and Solar Absorptance values separately, but for electronics cooling applications these two numbers are usually the same value.

You can find surface emissivity properties online using Google and some links such as:

http://www.ib.cnea.gov.ar/~experim2/Cosas/omega/emisivity.htm

It’s important to note that emissivity is a surface property rather than a bulk material property, so for example a cube of any material could have 6 different emissivity values on each of its faces. However if in doubt I tend to use a value of around 0.5.