Tim comes in a few forms. There is Thermal Paste, Liquid Metal, and Thermal Pad.
Thermal paste is often a mixture of silicon, grease and/or powdered mineral. They can be isolating or conductive, liquid or viscous. They are universal, performant and cost effective.
Liquid metal are alloys often including gallium. They're conductive, runny, and reactive, but superbly performant. Require select coldplate material, isolation with conformal coating, protective die bracket, thorough cleaning and extreme caution in application. Use between 200w+/high thermal density processors in direct die and/or water cooling scenarios.
Thermal pad is a reusable, yet unproven replacement for pastes to reduce cost for frequent reapplication. Preliminary results indicate they're minorly behind a good paste performance wise with a few other caveats in practicaliy. Thermal Pastes
Application 1. Separate the heatsink and the processor 2. Clean both surfaces, ideally with isopropyl alcohol and coffee wipe. Allow it to dry completely. 3. Apply the paste in a cross fashion that provides enough paste for the entire processor surface. 4. (Optional) Spread with paper towel or the included spatula if there is one. 5. Mount the heatsink atop the processor 6. Tighten the heatsink in a star pattern that provides even and sufficient mounting pressure to the cooler manufacturer's instructions.
Caution - A good paste application job far outweighs the choice of paste on mainstream desktop. - Use enough paste to coat the entire surface of the heat spreader or processer. - Tighten the mounting screws by an equal small amount in a star pattern. Do not tighten one screw fully at once. - If using a conductive paste, take caution not to spill it over to exposed surface mounted components. Paste Choices
Cooler included pastes While the quality varies, the included pastes are usually usable at least. Apply enough paste and even, sufficient mounting pressure, and usually you can see at least near MX-4 performance. A good application is much more consequential than the paste choice.
Arctic MX-4 MX-4 is a well-documented good paste that is widely known, well proven, safe, performant, universal and suit for mainstream CPU and GPU. It is easy to apply, has a long shelf life, does not need frequent reapplication, cheap, and widely available. The paste dries out in the tube easily upon exposure to air, so cover the tube well after application and don't leave the paste without mounting for too long. Isolating, medium viscosity, and non-toxic upon contact and ingestion per Arctic safety information. It is great for replacing emptied or subpar stock pastes.
Higher end pastes: IC Diamond, CM Mastergel Maker, GC Gelid Extreme. First one is very viscos, so it is very nice for loose mounts. Otherwise, they're similar in performance.
Top end pastes: KPx, Kyronaut. First one is extremely viscos, and latter very runny. Both are very high performers designed for sub-zero cooling. Kyronaut is known to have drying issues at 80C and above.