Heatsink Compound. Apply small thin layer on each transistor or module you install to help the installation process.
ThermalcoteĀ is a superior silicone-based thermal joint compound uniquely developed to increase heat transfer to all heat sinks. Thermalcote’s thermal conductivity is more than 15 times that of air and more than 4 times the conductivity of typical silicone-based greases. It is non-toxic and extremely stable; it neither cakes nor runs between -40°C to 204°C.
The 251 and 251G are both Rohs compliant. If you do not have the Rohs complant requirement, you could purchase Therm251