Guide to concepts, types and application of thermal pastes

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What is the thermal compound or thermal paste for?

It serves to improve the heat conductivity between two materials that are physically isolated from each other. It is used precisely to cover the imperfections between both materials as long as there is as little air as possible between both materials since this is a rather inefficient heat transmission medium.

 

The imperfections of the materials to be communicated, usually metals due to their great heat conduction capacity (among others), are filled with another material, which can be composed of various materials on which their conductive capacity will depend. The better this material is, the greater its ease in displacing the air and covering those micro fissures, the better the conductivity between the two adjacent materials.

Other important factors for good heat conductivity are having adequate pressure between the bonded materials, maintaining adequate temperatures so that the conductive material is in its reasonable range of use and the very nature of the materials that will bond the thermal compound to each other.

Its objective is not to improve the thermal conductivity capacity of our aluminum or copper heatsink but to reduce the points of low conductivity between them. The conductive material must have adequate density to expand by displacing air with pressure and temperature rise, but not overflowing out of contact areas. It is a complicated balance but it is more than solved with modern thermal pastes, at least with some of them.

Thermal pastes and thermal pads

PC users live with two types of heat conductors. On the one hand, the thermal pads , which are those more or less thick sheets that we see in contact with the RAM of our graphics card, with the new systems of heat dissipation of M.2 disks of the motherboards or between the heatsinks of The MosFETs of our motherboard. This type of thermal pad is thicker and consists of different materials, usually of different types of silicones.

We usually have the impression that these types of compounds, thick in sight, are bad conductors of heat, but, as in everything, there are quality levels in them. There are poor quality, medium quality and some of them are excellent and usually very expensive (up to 14W / mK , better than many thermal pastes). They are destined, normally, to elements with gross changes of temperature but that really do not have large consumptions. They absorb the heat of the emitting part and are good distributing it through the dissipating material in a homogeneous way.

The thermal compounds, or thermal paste, that we use with our processor or our GPU is another type of conductive material and there are also of many kinds and qualities. Its formulation may be a thermal advantage but some are also, by their very nature, aggressive with some of the materials with which they have contact. Some, with the right temperature or pressure, become a weld that persistently joins both materials to communicate and are sometimes adverse results.

The measure of heat conductivity

The conductivity of heat in a material can be measured in multiple ways and there are different units of measurement, but the “International System” measures the conductivity of heat in watts per meter-kelvin (w / mK) .

Basically we measure the difference of watts of energy in a homogeneous material with respect to the distance in which there is a difference of a degree Kelvin. That is, how much energy it is able to transfer within itself. The higher the value, the greater the energy absorption capacity and the faster removes heat from one point to another.

The thermal conductivity of aluminum is around 200W / mK and that of copper rubs 400W / mK (it depends a lot on the quality of the alloy used, the purer the more transmission capacity). They are two of the materials, more or less economical and accessible, that best transfer heat and also the most commonly used among commercial products that we handle in the PC world or in electronics in general. It is often combined with other materials that have a high heat absorption capacity, which need a lot of energy for a phase change, such as water.

A quality thermal paste normally has a conductive capacity of between 8-11W / mK , as you see very far from the conductive capacity of the two common metals that we have mentioned before. Thermal paste is not able to cool your processor, but it is able to improve those points, those irregularities.

It improves because the heat transfer capacity of the air you breathe, at sea level, is around 0.02W / mK. As you can see the difference is measured in a factor of thousands and that is where a good thermal paste takes out those few degrees of improvement that we see when we saturate our processor. Thermal pastes, also thermal pads, spoil over time and use and have to be applied in the correct way.

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