Why use anything?
Good thermal conductivity is important to the dissipation of unwanted heat from active power components and also important to the application of thermal platforms for testing electronics.
Perfectly flat, parallel and particle free surfaces might show better thermal conductivity directly pressed together, however some of the real-world realities generally prevent that best-case best performance. Here are a few reasons to use thermal grease:
- Metal surfaces that appear to the eye to be perfectly flat, free of voids and parallel are usually are not.
- Electronics likes cleanliness, not all products can be built in clean rooms. A small unseen particle between two surfaces can easily prevent them from mating properly
- Keeping even pressure on two surfaces is not always as easy as it might seem. Keeping an appropriate amount of pressure on the two surfaces is important to transfer heat. Grease fills voids caused by uneven clamping pressure.
Thermal grease to the rescue?
So - given that close enough to perfect flatness, cleanlieness and parallelness is hard to achieve, thermal grease has been used for decades to improve the thermal conductivity between two metal surfaces. Much has been said lately in the world of competitive computing & overclockers about heat transfer thermal grease. Generally the most important point is less is better. there are articles and stories reporting varying levels of improvement in heat transfer from different grades of thermal transfer grease. In general, what I have seen says that the quality of the grease can make some difference and the high end stuff is expensive but generally far from proprtionally better. One youtube video even sugested Nutella, the food product worked about as well as many thermal transfer product. Well I'd say don't try that but is interesting that it could work at all, even as a short term solution. I am pretty sure over time it would shrink and prove corrosive to metals to mention a few. Properly applied thermal compound should have little risk of making a mess over the rest of the electronics or in general. This is also a good time to note that often electrical conductivity doesn't matter that much but depending on the application, some thermal compounds are specified to be electrically conductive and some are requires to be insulating. clearly you wouldnt want unwanted electrical conductivity on a circuit assembly.
Silpads to the rescue from messy grease.
Silpads and similar products are pretty much what the name says, silicone based pads used in lieu of grease and designed to be either electrically conductive or not based on the application. They compress siightly to fill in voids and are often reusable providing faster, more repeatable results. Silpads are a good solution for many heat transfer requirements. Due to their generally thicker nature, they are generally slightly less thermally conductive than a very thin layer of thermal compound. When it is important to maintain electrical isolation, they are often better than other alternatives.