The main purpose of Thermal Grease is to aid thermal conduction between two mechanical mated surfaces, such as an IC and its heat sink. Thermal grease fills the gap between the mating surfaces, eliminating air gaps which are a big thermal insulator, and improving thermal conductivity in order to improve the heat transfer between two surfaces. The thinner your thermal grease layer, the better thermal conductivity performance you’ll get from your grease.
If a batch of thermal grease is expired, its fluidity will deteriorate and its ability to fill the space and eliminate air gaps between two surfaces will become poorer, so its thermal conductivity will suffer. Expired thermal grease will not always, or even often, perform up to its specified ratings. Therefore, it is not recommended to use expired thermal grease in your application, even it is just recently expired. Stick to the manufacturer’s ratings and expiration dates in order to get the best performance from your grease.
For more information about Thermal Interface Materials and their use, check out our Thermal Interface Materials EEWiki page!