I've a power module that generate large amount of heat we are using heat sink to cool it, I want to validate the heat sink design using cosmos, but the problem is that I wan tot define thermal paste layer, actually I've never done that before, could anyone tell me how to so that.
I guess you mean a thermal resistance between two components.
To do so, you first need to define a contact between two faces of your assembly. Then in the contact user interface, you should activate an option "thermal resistance". You can input a total or distributed resistance in °K/W or °K.m²/W.
Of course you need conductivity (or resistivity) of your layer material...:)