# Calculating Case-to-Sink thermal resistance from bulk thermal conductivity of thermal pad material

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.

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.