US Dollars
Compound/Grease
Thermal compound is a common material that is spread on the bottom surface of the heatsink. It doesn't have any mechanical properties, so it is usually used in conjunction with screws, clips, or other mechanical fasteners to keep the heatsink in place. Grease comes in different conductivities and will become very thin as more force is applied. It also tends to be messy, so watch where it is used.