A brief introduction to several thermal materials

Speaking of the cooling system, many people think of the fan and heat sink. In fact, they ignore a media that does not understand, but plays an important role in the media. So we talk about thermal materials. I hope you discuss, to express their own power dissipation of various types of devices and views.

Why do you need thermal media?

Some people may think that the bottom of the CPU surface or heat sink is very smooth, they do not need between the thermal medium. This view is wrong! As the machining can not make an idealized flat surface, so there is a lot of gully or gap between the CPU and the radiator, which are air. We know that the thermal resistance of the air is high, so you must use other substances to reduce the thermal resistance, or the performance of the radiator will be greatly reduced, or even unable to play a role. So the heat conduction medium came into being, and its role is to fill the processor and the radiator between the large and small gaps, increase the heat source and the heat sink contact area. Therefore, the heat conduction is only a role in the thermal medium, increasing the effective contact area of ??the CPU and the radiator is its most important role.

What are the thermal media?

1,Thermal grease

Thermal grease is currently the most widely used as a thermal medium, it is a silicone oil as raw materials, and add thickeners and other fillers, after heating decompression, grinding and other processes formed after an ester, the material There is a certain degree of viscosity, no obvious grainy. Thermal grease operating temperature is generally in -50 ℃ ~ 220 ℃, it has a good thermal conductivity, high temperature, anti-aging and Waterproof properties.

In the heat dissipation process, after heating to a certain state, the thermal grease will show a semi-liquid state, and fully fill the gap between the CPU and the heat sink, making the connection between the two more closely, and then strengthen the heat conduction. Usually, the thermal grease is insoluble in water, is not easy to be oxidized, but also has a certain degree of lubrication and electrical insulation.

2,Thermal silica gel

And thermal grease, thermal silica gel is also added by the silicone oil chemical raw materials, and after chemical processing. But unlike the thermal grease, there is some kind of sticky material in the chemical raw material it adds, so the thermal silica gel of the finished product has a certain adhesive force.

The biggest feature of thermal silica gel is hard after solidification, the thermal conductivity slightly lower than the thermal grease. At present, there are two kinds of thermal silica on the market: one after solidification for the white solid, the other after solidification for the black with a glossy solid. General manufacturers are accustomed to using the first silicone as a heat sink and heat between the object of the adhesive, its advantage is very strong sticky, but it has just become its shortcomings. When we need to repair, often in the effort to separate the adhesive device and the radiator separation, you will find the contact surface of the large number of solid white silica gel, these silica is very difficult to clean.

In contrast, the second silicone advantage is more obvious: one of its cooling efficiency is higher than the first, and second it solidified after the formation of black solid brittle, the residue is easy to remove. In any case, thermal conductivity of thermal conductivity of silica gel is not strong, and easy to device and heat sink "stick dead", so unless the special circumstances recommended users.

3,Thermal silicone film

Soft silicone thermal insulation pad has a good thermal conductivity and high-grade pressure insulation, thermal conductivity 2.6W / mK, anti-voltage breakdown value of more than 4000 volts, is to replace the thermal grease of alternative products, the material itself has a certain Of the flexibility, a good fit between the power device and the heat of aluminum or the machine between the shell, so as to achieve the best thermal and cooling purposes, in line with the current requirements of the electronics industry for thermal conductivity material is to replace the thermal grease grease two The best product of the cooling system. Such products can be arbitrarily cut, to help meet the automated production and product maintenance.

Silicone thermal insulation pad of the process thickness ranging from 0.5mm ~ 14mm, every 0.5mm a plus, that is, 0.5mm 1mm 1.5mm 2mm ~ 5mm specifically for the use of the gap to pass the heat of the design program to fill the gap to complete the heat and heat Parts of the heat transfer, but also play a role such as shock insulation seal, to meet the miniaturization of social equipment, ultra-thin design requirements, is a great process and the use of new materials.

Flame retardant performance in line with U.L 94V-0 requirements, and in line with the EU SGS environmental certification

4,Synthetic graphite sheet

This thermal medium is relatively rare, generally applied to some of the smaller heat on the object. It uses graphite composite materials, after a certain chemical treatment, excellent thermal conductivity, suitable for electronic chips, CPU and other products of the cooling system. In the early Intel boxed P4 processor, the material attached to the bottom of the radiator is a graphite thermal pad called M751, the advantage of this thermal medium is no sticky, not in the demolition of the radiator The CPU from the base of the "uprooted."

Several of the above-mentioned common thermal medium, aluminum foil thermal pad, phase change thermal pads (plus protective film) are also thermal media, but these products are rare in the market.

Thermal conductivity materials also have performance parameters

As the thermal grease is a chemical substance, so it also reflects the performance characteristics of their own related parameters. We just understand the meaning of these parameters, you can determine the performance of a thermal conductivity material.

1,Thermal conductivity

Thermal conductivity of the thermal grease and heat sink is basically the same, its unit is W / mK, that is, the cross-sectional area of ??1 square meters of the cylinder along the axial 1 meter distance of the temperature difference of 1 Kelvin (1K = 1 ℃) Heat conduction power. The larger the value, the faster the heat transfer rate of the material, the better the thermal conductivity. At present, the thermal conductivity of the mainstream thermal grease is greater than 1.134W / mK.

2,Working temperature

The operating temperature is an important parameter to ensure that the thermally conductive material is in solid or liquid state. The temperature is too high and the thermal conductivity material is due to the liquid; the temperature is too low and it will become solid due to the increase in viscosity. Heat dissipation. Thermal grease operating temperature is generally in -50 ℃ ~ 220 ℃. For the thermal grease operating temperature, we do not have to worry, after all, through conventional means it is difficult to CPU temperature beyond this range.

3,Thermal resistance coefficient

The thermal resistance coefficient represents the effect of the object on heat conduction. The concept of thermal resistance is very similar to the resistance, the unit is also similar (℃ / W), that is, the object continued heat transfer power of 1W, the thermal path at both ends of the temperature difference. Thermal resistance is clearly the lower the better, because the same ambient temperature and thermal conductivity, the lower the thermal resistance, the lower the temperature of the heated object. The size of the thermal resistance and thermal grease used in the material has a great relationship.

4,Dielectric constant

For some CPU without metal cover protection, the dielectric constant is a very important parameter, which is related to the existence of a short circuit inside the computer. Common thermal grease used in the insulation are better materials, but some special grease (such as silver-containing silicone grease, etc.) may have a certain conductivity. Many CPUs are now fitted with metal caps for heat conduction and protection of the core, so there is no need to worry about the short circuit caused by the thermal grease spill. At present, the dielectric heat of the thermal grease used in the mainstream heat sink is greater than 5.1.

5,Viscosity

Viscosity is to guide the thermal grease viscosity. In general, the thermal grease viscosity of 68 or so.

6,Insulation

Advantages and Disadvantages of Thermal Silicone Film, Thermal Grease and Thermal Adhesive

Thermal conductivity of silicone film relative to the thermal grease and thermal double- sided adhesive has the following advantages:

1.Thermal conductivity range and stability

2.The structure of the difference between the work of the bridge to reduce the radiator and heat dissipation of the structural requirements of the workpiece

3.EMC, insulation performance

4.Shock the effect of sound absorption

5.Installation, testing, reusable convenience

6.Thermal conductivity range and stability

Thermal conductivity of silicone film in the thermal conductivity of the larger selectivity, from 0.8w / k.m 3.0w / k.m above, and stable performance, long-term use of reliable.

Thermal conductivity double-sided adhesive is currently the highest thermal conductivity of not more than 1.0w / k-m, the thermal effect is not ideal;

Thermal conductivity of silicone grease at room temperature curing process, in the high temperature conditions prone to dry surface, unstable performance, easy to volatilize and flow, thermal conductivity will gradually decline, is not conducive to long-term reliable system operation.

To bridge the structural process difference, reduce the radiator and heat dissipation of the structural parts of the work requirements

Thermal conductivity of silicone film thickness, hardness can be adjusted according to the design of the different, so in the thermal channel can be dispersed in the heat dissipation structure, chip size difference, reduce the structural design of the thermal device contact surface of the differential requirements, Flatness, roughness of the work difference, if the processing accuracy will improve the product cost to a large extent, so the thermal conductivity of silicone film can greatly increase the heating body and thermal device contact area, reducing the radiator production costs.

In addition to the traditional PC industry, now the new cooling solution is to remove the traditional radiator, the structural parts and radiators into a cooling structure. In the layout of the PCB layout will be scattered on the back of the chip layout, or in the front layout, the need for heat around the chip to open the hot hole, the heat through the copper foil and other lead to the back of the PCB, and then through the thermal silica chip to create a thermal conductivity guide to the PCB Bottom or side of the cooling structure metal bracket, metal shell), the overall cooling structure to optimize, but also reduce the cost of the entire cooling program.

EMC, insulation performance

Thermal conductivity of silicone film because of its material properties with thermal insulation properties, the EMC has a very good protection, the reasons for the silicone material is not easy to pierce and under pressure in the tear or damage, EMC reliability is better.

Thermal conductivity of double-sided rubber because of its material properties of its own restrictions, it is relatively low EMC protection performance, often fail to meet customer needs, when used in the more limited, generally only in the chip itself done insulation or chip surface made EMC protection Only when you can use.

Thermal grease because of the material properties of its own EMC protection performance is relatively low, many times fail to meet customer needs, in the use of more limited, generally only the chip itself done insulation or chip surface to do the EMC protection can be used.

Shock absorption sound effects

The silica gel carrier of the thermally conductive silicone film determines that there will be a good elasticity and compression ratio, so that it has a good cushioning effect, and then adjusting the density and the hardness can produce a good absorption effect on the low frequency electromagnetic noise.

The use of heat-conductive double-sided adhesive to determine the use of it does not have shock absorption sound-absorbing effect.

Thermal contact with the thermal grease to determine the use of it does not have shock absorption sound absorption effect.

Installation, testing, reusable convenience

Thermal silicone film for the stable solid, the plastic strength optional, easy to disassemble; flexible recovery, can be reused.

Thermal double-sided adhesive once used, not easy to disassemble, there is the risk of damage to the chip and the surrounding device, easy to disassemble thoroughly. In the scratch thoroughly, it will scratch the surface of the chip and pick up when the dust, oil and other interference factors, is not conducive to thermal conductivity and reliable protection.

Thermal grease can not be disassembled, must be carefully wiped swab, it is not easy to wipe thoroughly, especially in the replacement of thermal conductivity test, the reliability of the test data will have an impact, thus affecting the engineer's judgment.

Advantages and Disadvantages of Heat Preservation of Silicone Sheets

To the thermal silicone film adhesive generally have two forms, double-sided adhesive and single-sided adhesive, Then we introduce the thermal silicone film on the benefits and disadvantages:

One, to the thermal silicone film double-sided adhesive:

Double-sided adhesive is mainly because the product is not fixed or inconvenient fixed, double-sided adhesive can be used to fix the radiator, the IC and heat sink, do not need another design fixed structure.

Benefits: can be used to fix the radiator, no need to design a fixed structure.

Impact: thermal effect will be worse, the derivative coefficient will be much lower.

Second, to the thermal silicone film single-sided adhesive:

Single-sided adhesive is mainly easy to install the thermal silica gel fit, such as a backlight using thermal silica size of 452 * 5.5 such a long size, not easy to install, so the use of single-sided adhesive, the thermal silicone rubber paste On the PCB board, the other side is attached to the shell.

Benefit: You can stick the surface of the heater side, when the radiator or shell during the assembly process is relatively sliding when the thermal silicone film does not produce a position offset.

Impact: thermal conductivity will become low, but the effect than the double-sided effect is better.

The above that, to the thermal silicone film, whether single-sided adhesive or double-sided adhesive will lead to lower thermal conductivity.