I am using a thermal pad instead of a thermal compound between the mosfet and the heatsink.
Thermal pad (p/n 53-78-1ACG) has a thermal conductivity of 0.9 W/(m-C) and 0.15mm thickness.
Normally I am accustomed to see thermal resistance in C/W but I cannot found the way to get the thermal resistance in this units regarding to the thermal pad.
For a block of material separating two plates, the thermal conductance goes up linearly as the area of the interface goes up, and down linearly as the thickness goes up.
Bulk thermal conductance of a material (the inverse of resistance) is therefore normally quoted in the units you state \$W/(m.C)\$.
This can just be inverted to give the bulk thermal resistance of the material, and given that you know the pad thickness, we can requote as a thermal resistance of \$ 0.00017 (C.m^2/W)\$ or \$1.7 (C.cm^2/W)\$, where the area element corresponds to the size of the the pad.
If you have \$1cm^2\$ of thermal interface this tell you it will have a thermal resistance of \$1.7C/W\$, an area of \$2cm^2\$ a thermal resistance of \$0.85C/W\$, etc.
