How Thermalpaste Works & Applying Thermal Compound
Thermalpaste (also known as: thermal compound, TIM, thermal glue) is used to fill microscopic imperfections in the surface of a CPU cooler's coldplate and the CPU's IHS (integrated heat-spreader). This is the most top-level definition of thermalpaste.
Were you to use a high-accuracy laser to measure the smoothness of either surface, it would be revealed that neither a coldplate nor an IHS are perfectly flat surfaces, and this means that perfect, direct contact cannot be made. In an ideal world, a cooler's copper or aluminum coldplate makes full contact with the IHS, with zero TIM between the metal. It's not an ideal world, though, so we're forced with two primary choices: Fill the little gaps with some kind of thermally conductive material or leave them alone, in which case air will fill the gaps.
Atmospheric air has a thermal conductivity of about 0.024W/mK (Watts per meter Kelvin) at 25C, so that's no good. For comparison, the average tube of thermalpaste will sit somewhere in the range of 4 – 8.5W/mK; a lot of the stock paste is ~4W/mK, though silver and diamond compounds can be had at higher conductivity ratings. Copper is rated at ~401W/mK at 25C, with aluminum coming in at 205W/mK. Even in the case of aluminum, it is clear that thermalpaste doesn't come anywhere close to the thermal efficiency of metal – but metal isn't going to deform to fit the surface, so we've got to use something more malleable (at least, without reheating and melting it).
|Material||Thermal Conductivity (W/mK) at 25C|
|Thermalpaste (AVG)||~5.3 - 8.5|
Without some sort of interface filling the gaps, air will rest between the coldplate and IHS and generate heat pockets. Filling the gaps with thermal interface will provide a material of higher conductivity, with the objective of serving as a pathway for heat to reach the coldplate from the IHS. This is TIM's only goal. Using too much thermal compound will actually diminish the thermal efficiency of the entire system, because it limits direct contact between the coldplate and IHS and creates a thick thermal wall of a lower conductivity than the copper.
We've previously tested the efficacy of copper vs. aluminum coldplates for thermal dissipation from a CPU, finding that – for smaller sockets (115X) – the difference is negligible. Larger surfaces may matter more, but we haven't yet confirmed this (LGA 2011 would be a good test).
Curing, Aging, & Cracking
There's a “curing process” with thermal compound – a time period required for the paste to reach its peak efficiency. When freshly applied, thermalpaste hasn't yet cured and is still somewhat liquid-y. It isn't until the compound has an aging period that maximum thermal efficiency is achieved. This can take a few hours or a few days, depending on load level and type of compound. Were you to thermally test your CPU immediately after application and then test it again a week later, the results should be marginally different. Not much, but enough to pick-up with accurate equipment and methodology.
Eventually, the thermal compound reaches and passes its peak efficiency, potentially spiraling downward into aging and cracking. Good compound won't do this within the average PC lifespan – diamond and silver compounds are a good example of high-endurance paste – but cheaper stuff (like silicone) will decompose with age. Under intense enough heat, the paste begins to crack and lose its ability to efficiently transfer heat from one surface to another.
Laptops are an excellent example of this process. A lot of our readers likely have experience replacing some sort of internal laptop component – a fan, the GPU's thermalpaste, reflowing the solder, or similar. Laptops undergo a lot of abuse, they're potentially exposed to external sources of heat (like the sun, if used outside), their ventilation ports are often smothered, the internals are predisposed to higher thermals resultant of a tight enclosure, the cooling abilities are relegated to smaller fans, and so forth. We've replaced laptop GPU compounds a number of times, normally because the stock compound has dried-up and lost its ability to adequately cool the silicon. During the replacement process, an astute technician will spot flakes of dried compound falling from the coldplate upon removing the heatsink / fan combo. This is aging.
Different Types of Compound
There are dozens of thermalpaste brands available on retailers. Price is normally set based upon thermal conductivity and the amount of compound in the tube (generally in the range of 3g, which is a few uses). A tube of 8.5W/mK carbon-based compound, which is resistant to aging, costs about $10 for 4g.
The type of compound is normally listed as a metal-based material (silver), diamond/carbon (often called “nano diamond”), or ceramic. Metal-based compounds, like silver compound, use tiny flakes of metal to aid in conducting the heat to the coldplate. Diamond compounds are usually a bit more hard coming from the tube, requiring some extra work to dispense, but are theoretically stronger over long durations of use.
For most system builders, the differences between compound types aren't necessarily going to make a noticeable impact. Overclockers should care, given the higher voltages and heat, but general purpose builders can grab any ~5.3W/mK tube and be fairly happy. There are some tubes of compound as low as 1.5W/mK that we've seen – which we'd strongly recommend avoiding – but that's the main item to watch out for. If building a long-life system where minimal maintenance will be involved, we'd recommend getting a carbon-based compound (like diamond) for its endurance.
Stock paste with coolers is normally fine, though as a personal note, I do have some things I avoid. The Cooler Master compound included with AMD heatsinks is one of them – it likes to stick (like glue) to the IHS, meaning that removing the CPU cooler often rips AMD CPUs from their sockets. This is a hazard to the pins (mounted on the CPU, not the socket) and can damage a CPU irreparably. I always use aftermarket compound when presented with AMD stock paste.
Other thermal interfaces exist in a system than just thermalpaste, and you've likely seen some of them. Thermal pads are the most prevalent. Thermal pads are used to mount VRM heatsinks to the chokes, capacitors, and MOSFETs, they're used to mount the copper/aluminum GPU coolers to VRM and VRAM modules, and pads are used heavily in laptop systems. A thermal pad is less thermally aggressive than paste, for the most part, but is cheaper and can better conform to the surface. If the manufacturer wants coverage on the sides of a choke, for instance, a thermal pad will provide some bleed-over from the pressure applied by the heatsink.
That about covers it for now. If you've got questions, leave them for us below or post on our one-on-one forums!
- Steve "Lelldorianx" Burke.