This can take a few hours or a few days, depending on load level and type of compound. It isn't until the compound has an aging period that maximum thermal efficiency is achieved. When freshly applied, thermalpaste hasn't yet cured and is still somewhat liquid-y. There's a “curing process” with thermal compound – a time period required for the paste to reach its peak efficiency. Larger surfaces may matter more, but we haven't yet confirmed this (LGA 2011 would be a good test). aluminum coldplates for thermal dissipation from a CPU, finding that – for smaller sockets (115X) – the difference is negligible. We've previously tested the efficacy of copper vs. 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. 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. Without some sort of interface filling the gaps, air will rest between the coldplate and IHS and generate heat pockets. 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).
Copper is rated at ~401W/mK at 25C, with aluminum coming in at 205W/mK. 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. 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.Ītmospheric air has a thermal conductivity of about 0.024W/mK (Watts per meter Kelvin) at 25C, so that's no good.
In an ideal world, a cooler's copper or aluminum coldplate makes full contact with the IHS, with zero TIM between the metal. 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. This is the most top-level definition of thermalpaste. 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). How Thermalpaste Works & Applying Thermal Compound