The mixture and diffusion of the high temperature fluid and the low temperature finally transfer the heat from the solid surface to the fluid, while the radiation does not need the aid of any medium. The heating body directly releases heat into the surrounding space.

The radiator pass is a piece of heat with a lot of blades, good allograft, in practical application, the radiator through the device, chip surface close contact so that the device, the heat conduction to the radiator, its fully expanded surface makes the heat radiation greatly increased, at the same time, the circulation of air can also take away more heat.

Thermal resistance The "resistance" to heat release between a radiator and the surrounding air when using a radiator.

Called thermal resistance, the "heat flow" between the radiator and the air flows from the radiator to the air due to the existence of thermal resistance, there is a certain temperature difference between the radiator and the air, just like the current flowing through the resistance will produce voltage drop. Similarly, there is a certain thermal resistance between the radiator and the device and chip surface. When choosing a radiator, in addition to mechanical size considerations, the most important parameter is the heat resistance of the radiator. The smaller the thermal resistance, the stronger the heat dissipation capacity of the radiator, the device operating temperature is easy to reduce. In every practical application, there is a specific thermal impedance target value, which can only be met in order to maintain the critical temperature of the connection site under harsh external conditions.

In general, the critical temperature that the radiator must maintain is the enclosure temperature of the device.

Power consumption Electronics need energy to work. As the device is not ideal, a lot of electric energy becomes heat energy, which increases the temperature of the device. If the heat is not dissipated effectively, its temperature will continue to rise, eventually leading to thermal failure. The task of heat dissipation is to provide effective heat dissipation channels (i.e. heat paths). The most important heat flux is the heat consumed.

Using thermal analysis software to achieve the optimal design of the air cooling radiator seems to be relatively simple, but in order to achieve the best heat dissipation performance, the selection of power loss, air flow rate, radiator width, length and height, radiator thickness and spacing, air guide plate size and environmental conditions and other variables are considered comprehensively, its complexity is visible. In order to achieve the best practical results, the radiator should be designed according to the different working environment in the specific application. Computational fluid dynamics analysis software (CFD) is a good software tool for radiator performance design in a cabinet or a space with a certain volume. The existing CAD software was used to build the model, and the geometric data of all components in the space were obtained by CFD to calculate and determine the actual direction and velocity of the air flowing through the radiator. Each unit volume in the space is analyzed with CFD and the results are calculated according to the conservation equations of mass, energy and momentum. Each side of the radiator is analyzed to determine the ratio of free area, which characterizes the total volume of air allowed to flow.

Thermal analysis software can be very effective in analyzing the heat dissipation performance of the radiator, but if the design of the radiator is considered after the device layout is complete, the location of the radiator is likely to conflict with other components, because the remaining space often cannot meet the requirements of heat dissipation, or even worse, the air in the system is blocked by other components and cannot flow into the radiator. This will not give full play to its advantages. Therefore, it should be used at the early stage of design to bring maximum design flexibility to the practical person, so as to achieve the purpose of saving trouble, saving money and avoiding design mistakes.

In the process of parameter optimization design with CFD software, the following factors should be paid attention to the influence of heat dissipation performance: 1) With the increase of the number of heat sinks, the static pressure will decrease. If the number of heat sinks is too large, the air velocity decreases and the heat dissipation efficiency decreases.

In addition, if the air is not directed to the radiator, it will actually flow past the radiator, leaving an air dead zone around the radiator. The volume impedance is related to the number of heat sinks when the width of each radiator surface is given. Vertically extruded radiators cannot allow a consistent flow of air, as this type of radiator has zero volume impedance, whereas cross-cut radiators allow air to flow in all directions.

2) The total height of the radiator The total height of the radiator is the height of the heat sink plus the thickness of the bottom plate. When the height of the radiator is certain, if the thickness of the bottom plate increases, the height of the heat sink will be reduced. The function of the thickness of the bottom plate is to dissipate heat from the radiator. The thickness value is determined by the location of the heat source and the location of the heat sink in the radiator. In general, the heat source is much smaller than the heat sink, just a dot. Heat can be transmitted stably between the two contact surfaces, but as it is transmitted further down the radiator, it encounters a "transmission impedance" that affects the overall heat transfer efficiency.

3) The location of the heat source the thermal resistance between the radiator and the surrounding environment decreases with the increase of the size of the heat source until the heat source inches closer to the radiator mounting surface 60 so far. Centralized arrangement of heating elements can greatly reduce the cross-sectional area of the radiator. Because the propagation impedance is relatively small, the thickness of the floor can be reduced, so that the radiator can be optimized in the limited space.

Choosing the right radiator installation method to reduce the thermal resistance is the most effective way to reduce the temperature of the device. Therefore, when the radiator is fixed on the heating device, the most important thing is to reduce the thermal resistance between the device and the radiator, so as to maximize the heat transfer efficiency between the interface. Other requirements, such as dielectric properties, conductivity, adhesion strength and reinstallation possibilities, should also be considered.

1) Mechanical fastener method This is the traditional installation method of radiator, using rivets, clamping parts and threaded fasteners. Because the surface of the binding part is not absolutely smooth, there is always a certain gap between the contact surface, which is a thermal insulator, seriously affecting the heat transfer efficiency. Therefore, during installation, the contact surface should be as smooth as possible to increase the contact area, tighten all bolts to increase the contact pressure, and use enough connecting parts to ensure uniform contact.

2) Thermal lubricant method This method mainly refers to the use of non-curing conductive medium such as silicone lubricant between the surfaces of the bonding parts, this lubricant will fill the gap between the surfaces of the matching parts, compared with the mechanical fastener method, has better heat transfer. Thermal lubricants can be used with mechanical fasteners because they do not bind the surfaces of parts together. If electrical insulation is required, mica gaskets can be used. The disadvantages of non-curing compounds include dust absorption, contamination, and the difficulty of controlling usage.

3) Compressible gaskets and gaskets method Gaskets and gaskets are made of silicone, composite materials and other materials, which can withstand a large enough clamping force and avoid the loosening of mechanical fasteners during use. When using this method, a sufficient quantity of premachined bedding of different shapes and sizes is required.

4) Thermal conductivity adhesive method The use of thermal conductivity adhesives such as epoxy resin and acrylic resin filler can effectively provide adhesion and thermal conductivity interface between the bonded parts. The resins in these binders are organic, poor conductors of heat, and are filled with pure metals and metal oxide oxides to increase thermal conductivity. The adhesive eliminates stress caused by mechanical fasteners, eliminates the need to procure large quantities of pre-machined gaskets and pads, and avoids contamination problems with non-curing compound methods.

Of course, adhesives have some drawbacks, including packing mixing problems, curing time required, and limited storage time. One thing that needs to be noted is that the thickness of the adhesive should be as thin as possible.

When choosing the installation method of the radiator, the following parameters should be carefully considered: 1) Electrical parameters According to the actual application of the working environment to decide whether to use conductive adhesive or electrical insulating adhesive. When using electrical insulating adhesives, the important factors are dielectric constant, dissipation factor and insulation strength.

The adhesive must maintain good insulation properties in high temperature and humid environment; 2) Mechanical parameters The different chemical bases of the polyesters in adhesives will affect their physical properties, such as hardness, toughness, strength, operating temperature range and coefficient of thermal expansion. Epoxy resin can provide high hardness, high strength and high temperature resistance; Silicone, polyurethane, sulfur polymers and other synthetic rubbers provide toughness. The addition of filler to the adhesive can reduce the coefficient of thermal expansion, so that it matches the coefficient of thermal expansion of the base material. The strength and elongation characteristics of the adhesive are determined by the filler selected.

3) Bond strength adhesives can have either controlled bond strength for reuse or high bond strength for permanent installation. The bonding strength mainly refers to the shear strength, tensile strength, cutting strength, twisting strength or splitting strength. For repairable strength, it is important to choose the right polymers and fillers. The adhesion of different substances depends on the polyester used: epoxy resins are used to increase hardness due to their good adhesion to most substances; To increase elasticity, sulfur polymers and polyurethane are preferred. Polyurethane adheres well to plastics and glass, but not to metals. Silicone, on the other hand, has limited adhesion without primer.

4) Thermal conductivity The heat transfer efficiency of the interface between the heating component and the radiator depends on the air residual, the type of filler and the thickness of the adhesive. When the surface of the joint part is made of thick adhesive or gel pad, air residue will occur due to moisture. If the adhesive is used, there will also be air residue in the mixing process of the adhesive. Degassing methods such as centrifuges can be used to eliminate the residual air. If you want to avoid this problem during assembly, you can drop the adhesive in the center of the adhesive surface so that when the adhesive parts are closed, the adhesive is radially extruded, draining air out of the adhesive layer. Using metal and metal oxide as filler increases the thermal conductivity of the polymer. When the proportion of the filler increases, the thermal conductivity increases proportionately with the decrease of the distance between the metal particles in the polymer.

At the end of the above two aspects of the design and installation of the radiator selection of some experience and criteria are discussed and summarized, these experience and criteria in the author's specific practice has been used, and achieved good results. Practice has proved that as long as the above steps are taken in the process of selecting the heat dissipation scheme of the whole electronic system, coupled with detailed consideration, better results can be obtained.