Thermal paste (also called thermal compound or thermal grease) is a thermal interface material applied in a thin layer between a heat source, such as a CPU or GPU, and a heat sink or cold plate. Its job is to fill microscopic gaps and surface irregularities that would otherwise trap air, which is a very poor conductor of heat, and so reduce the thermal resistance of the joint.
In computer power & cooling, thermal paste is a small but critical link in the heat path. A good cooler with dried-out or badly applied paste can perform worse than a modest cooler with a correct, thin, complete layer.
Why thermal paste is needed
Two machined metal surfaces look flat but touch only at a small fraction of their apparent area. The rest of the interface is filled with air, whose thermal conductivity is only about 0.026 W/m·K. Even a mediocre paste conducts heat dozens of times better than air, while copper reaches about 400 W/m·K.
The paste does not need to conduct as well as metal, because it forms a very thin bond line, typically 20 to 50 micrometers after mounting pressure is applied. The thermal resistance of the layer depends on its conductivity, its thickness and how completely it wets both surfaces. That is why thinner and complete coverage matters more than adding extra material.
Thermal paste sits at one of two interfaces in a typical desktop CPU. Inside the package, the die is joined to the integrated heat spreader by solder or a factory compound; outside, the user-applied paste joins the heat spreader to the cooler. GPUs and most laptop processors have bare dies, so the paste touches the silicon directly and its quality has an even larger effect.
Composition and types of thermal interface materials
Most pastes consist of a carrier fluid, often silicone oil or synthetic oils, loaded with thermally conductive filler particles. The filler type, particle size distribution and fill ratio determine conductivity, viscosity and long-term stability.
| Material | Typical conductivity | Electrically conductive | Notes |
|---|---|---|---|
| Ceramic paste | About 1 to 8 W/m·K | No | Zinc oxide, alumina or boron nitride fillers; safe and common |
| Metal-filled paste | About 5 to 10 W/m·K | Slightly, in some formulas | Silver or aluminum particles; avoid contact with circuits |
| Carbon-based paste | About 5 to 14 W/m·K claimed | Usually no | Carbon or diamond fillers; good stability |
| Phase change material | Effective performance similar to good pastes | No | Solid at room temperature, softens when hot; resists pump-out |
| Liquid metal | About 30 to 70 W/m·K | Yes | Gallium alloys; corrodes aluminum, risk of short circuits |
| Thermal pad | About 1 to 15 W/m·K | No | Thicker gaps such as VRAM and VRMs |
Manufacturer conductivity figures are not measured consistently and are often optimistic. Independent tests on real coolers usually show that well-made pastes differ by only about 1 to 3 °C in CPU temperature, while liquid metal can lower temperatures more on hot, small dies.
Thermal paste compared with pads and liquid metal
Paste, pads and liquid metal solve different gap problems. The decisive parameters are gap size, required performance and risk tolerance.
| Situation | Recommended material | Reason |
|---|---|---|
| Desktop CPU with air or AIO cooler | Quality ceramic or carbon paste | Good performance, no electrical risk |
| Laptop CPU or GPU with high heat cycling | Phase change material or stable paste | Resists pump-out over time |
| Memory chips and voltage regulators | Thermal pad of correct thickness | Gaps of 0.5 to 3 mm too large for paste |
| Delidded CPU, enthusiast overclocking | Liquid metal with insulation | Largest temperature reduction, highest risk |
| Aluminum heat sink | Any paste except liquid metal | Gallium attacks aluminum |
Applying thermal paste correctly
Correct application aims for a void-free, thin layer under even mounting pressure. Common methods include a pea-sized dot in the center, a line, an X pattern or manual spreading. For large rectangular processors, an X or spread method covers corners better than a single dot.
- Remove old paste with isopropyl alcohol of 90 percent or higher and a lint-free cloth.
- Apply a modest amount suited to the die or heat spreader size.
- Mount the cooler evenly, tightening screws in a diagonal pattern.
- Check temperatures under load and compare with expected values.
- Reapply after removing the cooler, because a broken layer traps air.
Too much paste is usually squeezed out harmlessly with non-conductive formulas, but it can contaminate sockets. Too little paste is worse, because uncovered areas create hot spots that raise temperatures and can trigger thermal throttling.
Curing time is mostly a myth for modern compounds. A few formulas improve slightly over the first days of heat cycling, but most reach near-final performance immediately, so a temperature test right after assembly is meaningful.
Aging, pump-out and common mistakes
Thermal paste degrades. Repeated heating and cooling makes the surfaces flex, slowly pushing paste outward, an effect called pump-out. The carrier oil can also separate or evaporate, leaving a dry, cracked layer. Desktop systems often run fine for many years, while laptops and GPUs with bare dies tend to show rising temperatures after two to five years of heavy use.
| Symptom | Likely cause | Fix |
|---|---|---|
| Temperatures rise slowly over years | Pump-out or dry-out | Clean and reapply |
| One core much hotter than others | Incomplete coverage or uneven mounting | Remount with better spread |
| High temperatures after new build | Protective film left on cooler base | Remove film, reapply paste |
| Corroded heat sink surface | Liquid metal on aluminum | Replace cooler, use proper material |
Another frequent mistake is reusing a disturbed layer after lifting the cooler briefly, which traps air pockets. The paste should always be cleaned off and reapplied whenever the cooler and chip are separated.
In plain words
Thermal paste is a gray goo that helps heat move from a computer chip to its cooler. The two metal surfaces look smooth, but they are full of tiny bumps and valleys. Air gets trapped there, and air blocks heat like a blanket.
The paste fills those gaps so heat can flow out of the chip. A thin, even layer works best. Old paste dries out after some years and should be replaced.
A real-world example
A video editor notices that his four-year-old laptop has become loud, and exports that once took 10 minutes now take almost 15. Monitoring software shows the CPU reaching 100 °C within seconds and lowering its clock speed to protect itself.
A technician opens the laptop and finds the original paste dry and pushed to the edges of the die. After cleaning both surfaces and applying a phase change pad designed for laptops, the processor peaks around 88 °C under the same export, the fans run quieter and the export time returns to about 10 minutes.