About the problem of heat dissipation of touch screen and display screen, the cognition of heat conduction products briefly introduces several heat conduction products
When it comes to cooling systems, many people think of fans and fins.In fact, they ignored a medium of heat conduction which is not remarkable but plays an important role.In this regard, let's talk about thermal conductive materials. I hope you can discuss it and express your opinions on heat dissipation of various power devices

Why do we need heat conducting medium?
Some people may think that the surface of CPU or the bottom of heat sink are very smooth, and there is no need for heat conduction medium between them.This view is wrong! Because it is impossible to make an ideal flat surface by machining, there are many gullies or gaps between CPU and radiator, all of which are air.We know that the thermal resistance of air is very high, so we must use other materials to reduce the thermal resistance, otherwise the performance of the radiator will be greatly reduced, or even can not play a role.So the heat conduction medium came into being, its role is to fill the gap between the processor and the radiator, and increase the contact area between the heat source and the radiatorTherefore, heat conduction is only a function of heat conduction medium, and increasing the effective contact area between CPU and radiator is its most important role.

What are the heat conducting media?

1、Thermal conductive silicone grease
Thermal conductive silicone grease is one of the most widely used thermal conductive media at present. It is an ester like substance formed by heating, decompression, grinding and other processes with silicone oil as raw material, thickener and other fillers. The substance has a certain viscosity and no obvious particle sense.The working temperature of thermal conductive silicone grease is generally - 50 ℃～ 220 ℃，

It has good thermal conductivity, high temperature resistance, aging resistance and waterproof properties.

In the process of device heat dissipation, after heating to a certain state, the thermal conductive silicone grease presents a semi liquid state, which fully fills the gap between the CPU and the heat sink, making the joint between the two more closely, thus strengthening the heat conduction.In general, thermal conductive silicone grease is insoluble in water, not easy to be oxidized, and has certain lubricity and electrical insulation.

2、Thermal conductive silica gel
Like thermal conductive silicone grease, thermal conductive silicone is also made by adding certain chemical raw materials to silicone oil and chemical processing.But different from the thermal conductive silicone grease, there is a kind of viscous substance in the chemical raw materials, so the thermal conductive silicone of the finished product has a certain adhesive force.   The most important feature of thermal conductive silica gel is that it is hard after solidification, and its thermal conductivity is slightly lower than that of thermal conductive silicone grease.At present, there are two kinds of thermal conductive silica gel on the market: one is a white solid after solidification, and the other is a black glossy solid after solidification.General manufacturers are accustomed to using the first kind of silica gel as the adhesive between the radiator and the heating object. Its advantage is very strong viscosity, but this has just become its disadvantage.When we need to repair, we often find a large amount of solid white silica gel on the contact surface of the bonded device and the radiator after we try our best to separate them, which is very difficult to remove.

In contrast, the second silica gel has obvious advantages: first, its heat dissipation efficiency is higher than that of the first one; second, the black solid formed after solidification is brittle, and the residue is easy to remove.In any case, the thermal conductivity of thermal conductive silica gel is not strong, and it is easy to "stick" the device and radiator. Therefore, it is recommended to users only under special circumstances. 
3、Thermal conductive silica gel sheet.
Soft silicone thermal insulation pad has good thermal conductivity and high-grade voltage insulation, thermal conductivity is 2.6w/mk, voltage breakdown resistance is more than 4000V, it is a substitute for thermal conductive silicone grease，The material itself has a certain flexibility, which fits well between the power device and the heat sink aluminum or the machine shell, so as to achieve the best heat conduction and heat dissipation purpose. It meets the requirements of the current electronic industry for heat conduction materials, and is the best product of the binary heat dissipation system to replace the heat conduction silicone grease heat conduction paste
.This kind of products can be cut at will, which is conducive to meet the requirements of automatic production and product maintenance.
                        
The process thickness of silica gel thermal insulation pad ranges from 0.5mm to 14mm. It is added every 0.5mm, that is, 0.5mm, 1mm, 1.5mm, 2mm ~ 5mm. It is specially produced for the design scheme of using gap to transfer heat. It can fill the gap, complete the heat transfer between the heating part and the heat dissipation part, and also play the role of shock absorption, insulation and sealing, which can meet the design requirements of miniaturization and ultra-thin of social equipment, It is a new material with high technology and usability

Flame retardant and fireproof properties meet the requirements of U.L. 94V-0, and meet the EU SGS environmental certification.

4、Synthetic graphite sheet
This kind of heat conduction medium is relatively rare, and is generally applied to some objects with small calorific value. It is made of graphite composite material. After a certain chemical treatment, it has excellent heat conduction effect. It is suitable for the heat dissipation system of electronic chips, CPU and other products.In the early Intel boxed P4 processor, the material attached to the bottom of the radiator is a graphite heat-conducting gasket named m751. The advantage of this heat-conducting medium is that it has no stickiness and will not pull the CPU from the base when removing the radiator.In addition to the above common heat conduction media, aluminum foil heat conduction gasket, phase change heat conduction gasket (with protective film) are also heat conduction media, but these products are rare in the market.    
Thermal conductive materials also have performance parameters
As thermal conductive silicone grease is a kind of chemical substance, it also has relevant performance parameters reflecting its own working characteristics.As long as we understand the meaning of these parameters, we can judge the performance of a thermal conductive material.
1、Thermal conductivity
The heat conduction coefficient of the thermal conductive silicone grease is basically the same as that of the radiator, and its unit is w / MK, that is, the heat conduction power when the temperature difference between a cylinder with a cross-sectional area of 1 square meter and a distance of 1 meter along the axial direction is 1 kelvin (1K = 1 ℃).The larger the value is, the faster the heat transfer rate is and the better the thermal conductivity is.At present, the thermal conductivity of mainstream thermal conductive silicone grease is more than 1.134w/mk.

2、Working temperature
Working temperature is an important parameter to ensure that the thermal conductive material is in solid or liquid state. If the temperature is too high, the thermal conductive material is liquid；If the temperature is too low, it will become solid due to the increase of viscosity, which is not conducive to heat dissipation.The working temperature of thermal conductive silicone grease is generally - 50 ℃ ~ 220 ℃.For the working temperature of thermal conductive silicone grease, we don't have to worry about it. After all, it is difficult to exceed the temperature of CPU by conventional means.

3、Thermal resistance coefficient
The thermal resistance coefficient represents the effect of an object on heat conduction.The concept of thermal resistance is very similar to that of resistance, and the unit is also similar (℃ / W), that is, the temperature difference between the two ends of the heat conduction path when the continuous heat transfer power of the object is 1W.Obviously, the lower the thermal resistance, the better, because under the same ambient temperature and heat conduction power, the lower the thermal resistance, the lower the temperature of the heated object.The thermal resistance is closely related to the material used in the thermal conductive silicone grease.

4、Dielectric constant
Dielectric constant is a very important parameter for some CPU without metal cover protection, which is related to whether there is short circuit in the computer.Common thermal conductive silicone grease is made of materials with good insulation, but some special silicone grease (such as silver containing silicone grease) may have certain conductivity.Now many CPUs are equipped with metal covers for heat conduction and core protection, so there is no need to worry about the short circuit problem caused by the overflow of heat conduction silicone grease.At present, the dielectric constant of thermal conductive silicone grease used in mainstream radiator is more than 5.1.

5、Viscosity
Viscosity is the viscosity of hot silicone grease.Generally speaking, the viscosity of thermal conductive silicone grease is about 68.

6、Insulation
Comparison of advantages and disadvantages of thermal conductive silicone film, thermal conductive silicone grease and thermal conductive double-sided adhesive
Thermal conductive silicone film has the following advantages over thermal conductive silicone grease and thermal conductive double-sided adhesive
   *Range and stability of thermal conductivity
   *The structure of the gap between the process, reduce the radiator and cooling structure of the process difference requirements
   *EMC，Performance of insulation
   *The effect of shock absorption
   *Installation, testing, reusability, convenience,
   **Range and stability of thermal conductivity  
The results show that the thermal conductivity of the silica gel sheet is more selective, from 0.8w/k.m to more than 3.0w/k.m
   At present, the maximum thermal conductivity of thermal conductive double-sided adhesive is less than 1.0w/k-m, and the thermal conductivity is not ideal;
   Thermal conductive silicone grease is a room temperature curing process, which is easy to produce surface cracking at high temperature, unstable performance, easy to volatilize and flow, and the thermal conductivity will gradually decline, which is not conducive to long-term reliable system operation
   **Bridge the structural process difference, reduce the requirements of radiator and heat dissipation structure
  The thickness and hardness of the thermal conductive silica gel sheet can be adjusted according to the different design. Therefore, in the thermal conductive channel, the size difference of the heat dissipation structure and the chip can be bridged, and the requirement of the contact surface difference of the radiator in the structural design can be reduced, especially for the flatness and roughness. If the processing accuracy is improved, the product cost will be greatly increased, Therefore, the thermal conductive silica gel sheet can fully increase the contact area between the heat generator and the heat sink, and reduce the production cost of the radiator.
   In addition to the traditional PC industry, now the new cooling solution is to remove the traditional radiator and unify the structure and radiator into a cooling structure.In the PCB layout, the heat sink chip is placed on the back, or in the front layout, the heat sink holes are opened around the chips to be cooled, and the heat is led to the back of the PCB through the copper foil，Then, the heat conduction channel is filled with the heat conduction silicon film to lead to the heat dissipation structure (metal bracket, metal shell) under or on the side of the PCB, so as to optimize the overall heat dissipation structure and reduce the cost of the whole heat dissipation scheme.

The above is my personal opinion, welcome to discuss, thank you!