Generally, the heat dissipation design of aluminum substrate LED is based on fluid dynamics software simulation and basic design. Resistance of fluid flow: due to the viscosity of fluid and the influence of solid boundary, the fluid is subjected to resistance in the flow process. This resistance is called flow resistance, which can be divided into two types: resistance along the aluminum substrate and local resistance.
Along-the-way resistance: the friction resistance of fluid along the whole process in the area where the boundary along the way is constant.
Local resistance: in areas with sharp changes in boundary, such as sudden expansion or contraction of cross section, bend and other local positions, it is the flow resistance caused by sharp changes in fluid state.
Aluminum substrate usually uses radiator for natural heat dissipation of LED. The design of radiator can be divided into three steps: 1. Design the outline of aluminum substrate according to relevant constraints. 2. According to the relevant design criteria of radiators, the tooth thickness, tooth shape, tooth spacing and substrate thickness of radiators are optimized. 3. Check and calculate.
Design method of aluminum substrate LED natural cooling radiator
Considering that the temperature boundary layer is thick during natural cooling, if the tooth spacing is too small, the thermal boundary layer between the two teeth will easily cross, which will affect the convection on the tooth surface. Therefore, in general, it is recommended that the tooth spacing of the radiator with natural cooling be larger than 12mm. If the tooth height of the radiator is lower than 10mm, the tooth spacing of the radiator can be determined according to the tooth spacing ≥ times the tooth height. The heat transfer capacity of the natural cooling radiator surface is weak, and adding corrugations on the surface of the radiating teeth will not have much influence on the natural convection effect, so it is suggested that the surface of the radiating teeth should not be added with corrugations. Generally, the surface of natural convection radiator is blackened to increase the radiation coefficient of the radiating surface and strengthen the radiation heat transfer. As it takes a long time for natural convection to reach thermal equilibrium, the thickness of the base plate and teeth of natural convection radiator should be enough to resist the impact of instantaneous heat load, and it is recommended to be more than 5mm.
LED circuit board design In order to better solve the heat dissipation problem, LED and some high-power IC need aluminum-based circuit boards. Aluminum PCB is composed of circuit layer (copper foil layer), thermal insulation layer and metal base layer. The circuit layer requires a large current-carrying capacity, so a thicker copper foil should be used, with a thickness of 35μ m “280μ m generally; Thermal insulation layer is the core technology of PCB aluminum substrate. It is generally composed of special polymers filled with special ceramics, with low thermal resistance, excellent viscoelastic properties, thermal aging resistance and mechanical and thermal stress resistance. This technology is used in the thermal insulation layer of high-performance PCB aluminum substrate such as IMS-H01, IMS-H02 and LED-0601, which makes it have excellent thermal conductivity and high-strength electrical insulation. The metal base is the supporting member of the aluminum substrate, which requires high thermal conductivity. Generally, it is aluminum plate or copper plate (copper plate can provide better thermal conductivity), which is suitable for conventional machining such as drilling, punching and cutting. Process requirements include: gold plating, tin spraying, osp oxidation resistance, gold deposition, lead-free ROHS process, etc.
Substrate: Aluminum substrate Product features: thin insulation layer, low thermal resistance; Non-magnetic; Good heat dissipation; High mechanical strength Product standard thickness:,,,,,, Copper foil thickness: Features: high heat dissipation, electromagnetic shielding, high mechanical strength and excellent processability. Purpose: LED special power hybrid IC(HIC).
The aluminum substrate is used to carry the heat conduction of LED and devices, and the heat dissipation mainly depends on the area. For concentrated heat conduction, a plate with high thermal conductivity can be selected, such as Beggs plate in the United States; Slow heat conduction or heat dissipation can be made of domestic general materials. There is a big difference in prices. It takes about 4,000 square meters for Beggs plate to produce finished products. Generally, domestic materials cost more than 1,000 square meters. Generally, the voltage of LED is not very high, so the insulation layer with a thickness of 1mil can withstand more than 2000V.
Why do aluminum substrates need thermal design?
Impact on high-temperature electronic products: insulation degradation; Components are damaged; Thermal aging of materials; Low melting point weld cracks and solder joints fall off. Influence on temperature components: Generally speaking, the resistance value of resistance decreases with the increase of temperature; High temperature will reduce the service life of capacitor; High temperature will reduce the performance of insulation materials of transformers and chokes. Generally, the allowable temperature of transformers and chokes is lower than 95 C; Excessive temperature will also cause the change of alloy structure of solder joints-—IMC thickening, solder joints becoming brittle and mechanical strength decreasing; With the increase of junction temperature, the current amplification factor of the transistor will increase rapidly, which will lead to the increase of collector current, further increase of junction temperature, and finally lead to the failure of the component.
The purpose of thermal design
Control the temperature of all electronic components inside the product, so that it will not exceed the maximum temperature specified in the standards and specifications under the working environment. The calculation of the maximum allowable temperature should be based on the stress analysis of components, and be related to the reliability requirements of products and the failure rate assigned to each component.
