Introduction to LED heat dissipation methods and heat dissipation material design

1. Radiation Radiation refers to the body's heat dissipation by emitting infrared rays. When the skin temperature is higher than the ambient temperature, the body's heat is lost by radiation. The amount of radiation heat dissipation is related to factors such as skin temperature, ambient temperature and the effective radiation area of ​​the body. Under normal circumstances, radiation heat dissipation accounts for 40% of the total heat dissipation. Of course, if the ambient temperature is higher than the skin temperature, the body will absorb radiation heat. Steel workers working in front of the furnace and farmers working in the fields under the sun in the hot summer will also encounter this situation.

2. Conduction and convection Conduction is the way the body dissipates heat by transferring molecular kinetic energy. When the human body is in direct contact with objects that are cooler than the skin (such as clothes, beds, chairs, etc.), heat is transferred from the body to these objects. In clinical practice, the use of ice caps, ice packs, and other methods to cool down high fever patients is based on this principle.

Convection is the flow of air, which uses air as the medium to dissipate heat.

Heat exchange between radiator and environment

When the heat reaches the top of the radiator, it is necessary to dissipate the heat to the surrounding environment as quickly as possible. For air-cooled radiators, this means heat exchange with the surrounding air. At this time, heat is transferred between two different media, and the formula followed is Q=α XAX ΔT, where ΔT is the temperature difference between the two media, that is, the temperature difference between the radiator and the surrounding air; and α is the thermal conductivity of the fluid, which is a fixed value after the heat sink material and air composition are determined; the most important A is the contact area between the heat sink and the air. Under the premise that other conditions remain unchanged, such as the volume of the radiator, there will generally be some restrictions. The space in the chassis is limited, and too much will increase the difficulty of installation. By changing the shape of the radiator, increasing its contact area with the air and increasing the heat exchange area, it is an effective means to improve the heat dissipation efficiency. To achieve this, the surface area is generally increased by using a fin design supplemented by surface roughening or threads.

When the heat is transferred to the air, the temperature of the air in contact with the heat sink will rise rapidly. At this time, the hot air should take away the heat as much as possible through convection and other heat exchange methods with the surrounding cold air. For air-cooled radiators, the most important means is to increase the speed of air flow and use fans to achieve forced convection. This is mainly related to the design of the fan and the wind speed. The efficiency of the radiator fan (such as flow rate, wind pressure) mainly depends on the fan blade diameter, axial length, fan speed and blade shape. The fan flow rate is mostly measured in CFM (English, cubic feet per minute), and one CFM is approximately 0.028mm3/minute.

Pure aluminum radiator

Pure aluminum heat sinks are the most common heat sinks in the early days. They have simple manufacturing processes and low costs. So far, pure aluminum heat sinks still occupy a considerable part of the market. In order to increase the heat dissipation area of ​​its fins, the most commonly used processing method for pure aluminum heat sinks is aluminum extrusion technology. LED light controller The main indicators for evaluating a pure aluminum heat sink are the thickness of the heat sink base and the Pin-Fin ratio. Pin refers to the height of the fins of the heat sink, and Fin refers to the distance between two adjacent fins. The Pin-Fin ratio is the height of the Pin (excluding the thickness of the base) divided by the Fin. The larger the Pin-Fin ratio, the larger the effective heat dissipation area of ​​the heat sink, which means the more advanced the aluminum extrusion technology.

Pure copper radiator

The thermal conductivity of copper is 1.69 times that of aluminum, so under the premise of other conditions being the same, pure copper radiators can take heat away from the heat source faster. However, the texture of copper is a problem. Many "pure copper radiators" are not actually 100% copper. In the list of copper, copper content of more than 99% is called acid-free copper, and the next level of copper is red copper with a copper content of less than 85%. At present, the copper content of most pure copper radiators on the market is between the two. Some inferior pure copper radiators contain less than 85% copper. Although the cost is very low, their thermal conductivity is greatly reduced, affecting the heat dissipation. In addition, copper also has obvious disadvantages. The high cost, difficult processing, and the large mass of the radiator have hindered the application of all-copper heat sinks. The hardness of red copper is not as good as that of aluminum alloy AL6063, and the performance of some mechanical processing (such as grooving, etc.) is not as good as that of aluminum; the melting point of copper is much higher than that of aluminum, which is not conducive to extrusion (Extrusion) and other problems.

Although the most commonly used heat sink materials are copper and aluminum alloy, aluminum alloy is easy to process and has low cost, so it is the most widely used material. The higher thermal conductivity of copper makes its instantaneous heat absorption ability better than that of aluminum alloy, but its heat dissipation speed is slower than that of aluminum alloy. Therefore, whether it is pure copper, pure aluminum, or aluminum alloy radiator, there is a fatal flaw: since only one material is used, although the basic heat dissipation capacity can meet the needs of mild heat dissipation, it cannot well balance the requirements of heat conduction capacity and heat capacity, so it is somewhat incapable in occasions with higher heat dissipation requirements.

Copper-aluminum bonding technology

After considering the shortcomings of copper and aluminum, some high-end heat sinks on the market often use a copper-aluminum combination manufacturing process. These heat sinks usually use a copper metal base, while the heat sink fins use aluminum alloy. Of course, in addition to the copper base, there are also heat sinks that use copper columns and other methods, which are also based on the same principle. With a higher thermal conductivity, the copper bottom surface can quickly absorb the heat released by the CPU; the aluminum fins can be made into the shape that is most conducive to heat dissipation with the help of complex process methods, and provide a larger heat storage space and release it quickly. This has found a balance point in all aspects.