The surfaces of the PCB and the heat sink are not completely flat, and the two must be in full contact to have a better heat dissipation effect. So thermal paste is used.
Thermal conductive grease, also called thermal paste, is an interface material used between the heating element and the radiator. It is often used in products such as CPU, LED light PCB, etc. It will maintain a semi-liquid state during the service life. The effect of heat conduction, after the end of the service life, will dry out and become a powder, which will hinder the heat conduction. Its components mainly include silicone oil and filler aluminum oxide.
Parameters of Thermal Paste
Thermal Conductivity (also called thermal conductivity), and Thermal Resistance.
Generally speaking, the higher the thermal conductivity, the lower the thermal resistance. The two are in an inverse relationship. Under normal circumstances, the higher the thermal conductivity, the better the thermal conductivity, and of course the more expensive it will be.
However, various brands are now mixed in the market. In order to attract people’s attention, many manufacturers will raise the thermal conductivity, such as the thermal conductivity of 10W/m.k, or even higher. Unless a large amount of metal powder is added, most of them are fake; silicone grease with metal powder added will have poor insulation effect; if insulation is required, it is recommended not to buy.
It is not ruled out that some manufacturers have adopted different test standards, but the internationally universal test standard for thermal conductivity is: ASTMD5470. If it is not for this standard, no matter how high the target is, it is meaningless. You can directly ask their test standard. However, even with the same standard, the measured thermal conductivity can be very high, but you must pay attention, because in addition to thermal conductivity and thermal resistance, there are two very important parameters that amateurs don’t know and will be ignored. It is the oil penetration rate and volatilization rate of thermal grease.
Oil penetration Rate and Volatilization Rate
These two parameters are an important basis for determining the quality and performance of silicone grease. Many brands can indeed achieve high thermal conductivity, but they are completely achieved by adding a large amount of silicone oil. Adding a large amount of silicone oil will bring a very serious consequence, that is, high oil permeability and high volatility.
The oil penetration rate and volatilization rate, as the name implies, are the amount of silicone grease penetrated and volatilized within a certain period of time under certain operating conditions and operating temperatures. If the oil penetration rate is too high, the silicone oil will penetrate into the electronic components and damage the product; if the volatilization rate is high, the silicone grease will be particularly easy to dry out, which will eventually affect the thermal conductivity.
Therefore, please pay attention to the two parameters of oil penetration rate and volatilization rate while paying attention to the thermal conductivity and thermal resistance. This is the guarantee of the quality, performance and life of the silicone grease.
In short, it refers to the change of the object under the action of external force, which is reversible. To give a simple example, the vanishing cream we used when we were young was exquisite solid before use. The process of applying it and the changes after applying it are a kind of thixotropy.
The good thixotropy of silicone grease is not only reflected in its simple and easy application, it can be pushed and smoothed well, but also in that it can reduce the extrusion phenomenon after long-term use. As we all know, electronic products will produce temperature changes during use. Temperature changes will cause objects to expand and contract with heat. Silicone grease will also expand and contract with heat at high and low temperatures. In addition, silicone grease contains liquids such as silicone oil. , Bubbles will be generated during thermal expansion and contraction. As time passes, the bubbles become larger and larger, and the silicone grease will be completely squeezed out, completely losing its role as a heat conductor. Therefore, good thixotropy is also a guarantee for the quality of silicone grease.
For high-quality products, the environmental protection level is higher, it will not corrode metal components, and it will not harm your health.