Thermal pads (also known as gap fillers and gap pads) are a preformed rectangle of solid material commonly used between a heat sink and a heat generating compound to transfer heat in an electronic assembly. The advantages of thermal pads are that they are die-cut precisely to achieve a consistent part repetitively. The electronics manufacturer knows exactly what they are going to get. The solid thermal pads are easy to handle and install, leaving less room for assembly error.
Thermal Grease is the original thermal interface material with a proven history. It has high thermal conductivity and can be reworked easily. Thermal grease is also messy and difficult to apply. Application uniformity is often difficult to achieve during manufacturing, leaving more room for error and inconsistency.