LED Cooling Solutions

LED aluminum substrate design selection

LED circuit design In order to better solve the heat dissipation problem, LEDs and some high- power ICs need to use aluminum-based circuit boards.

Aluminum substrate PCB consists of circuit layer (copper foil layer), thermal insulation layer and metal base layer. The circuit layer is required to have a large current carrying capacity, so thicker copper foil should be used, with a thickness of 35μm~280μm in general; the thermal insulation layer is the core technology of PCB aluminum substrate, which is generally composed of special polymers filled with special ceramics, with low thermal resistance, excellent viscoelastic properties, resistance to thermal aging, and the ability to withstand mechanical and thermal stress. The thermal insulation layer of high-performance PCB aluminum substrates such as IMS-H01, IMS-H02 and LED-0601 uses this technology, which makes it have extremely excellent thermal conductivity and high-strength electrical insulation performance; the metal base layer is the supporting component of the aluminum substrate, which is required to have high thermal conductivity, generally aluminum plate, and copper plate can also be used (copper plate can provide better thermal conductivity), suitable for conventional mechanical processing such as drilling, punching and cutting. The process requirements are: gold plating, tin spraying, OSP anti-oxidation, immersion gold, 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: 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0mm Copper foil thickness: 1.8um35um70um105um140um Features: high heat dissipation, electromagnetic shielding, high mechanical strength, excellent processing performance. Application: LED dedicated power hybrid IC (HIC).

The aluminum substrate is used to carry the heat conduction of LED and devices. The heat dissipation mainly depends on the area. For concentrated heat conduction, you can choose a plate with high thermal conductivity, such as the American Bergis plate; for slow heat conduction or heat dissipation, ordinary domestic materials can be used. The price difference is large. It takes about 4,000 yuan per square meter to produce a finished product of Bergis plate, while ordinary domestic materials cost more than 1,000 yuan per square meter. The voltage used by LED is generally not very high, so you can choose a 1mil thick insulation layer with a withstand voltage greater than 2000V.

Thermal reference design methodology:

Why thermal design?

The impact of high temperature on electronic products: insulation performance degradation; component damage; thermal aging of materials; cracking of low-melting-point welds and solder joints falling off. The impact of

temperature on components: Generally speaking, the resistance value decreases with increasing temperature; high temperature will reduce the service life of capacitors; high temperature will reduce the performance of transformer and choke insulation materials. Generally, the allowable temperature of transformers and chokes should be lower than 95C; excessive temperature will also cause changes in the alloy structure of solder joints - IMC thickening, solder joints becoming brittle, and mechanical strength reduction; the increase in junction temperature will rapidly increase the current amplification factor of the transistor , resulting in an increase in collector current, which will further increase the junction temperature and eventually lead to component failure.

Purpose of thermal design

Control the temperature of all electronic components inside the product so that it does not exceed the maximum temperature specified by the standards and specifications under the working environment conditions. The calculation of the maximum allowable temperature should be based on the stress analysis of the components and be consistent with the reliability requirements of the product and the failure rate assigned to each component.

LED heat dissipation The design is generally based on fluid dynamics software simulation and basic design.

Resistance to fluid flow: Due to the viscosity of the fluid and the influence of the solid boundary, the fluid encounters resistance during the flow process. This resistance is called flow resistance, which can be divided into two types: along-the-line resistance and local resistance.

Along-the-path resistance: The friction resistance of the fluid along the entire process in an area where the boundary remains unchanged along the path.

Local resistance: In areas where boundaries change dramatically, such as sudden expansion or contraction of the cross section, elbows and other local locations, the flow resistance is caused by the rapid change in the fluid state of the fluid.

Usually, LEDs use heat sinks to dissipate heat naturally. The design of the heat sink is divided into three steps:

1: Design the contour according to the relevant constraints.

2: Optimize the radiator tooth thickness, tooth shape, tooth spacing, and substrate thickness according to the relevant design criteria of the radiator.

3: Perform verification calculations.

Radiator design method

Design Method of Natural Cooling Radiator

Taking into account the thicker temperature boundary layer during natural cooling, if the tooth spacing is too small, the thermal boundary layers of the two teeth are easy to cross, affecting the convection on the tooth surface. Therefore, under normal circumstances, it is recommended that the tooth spacing of the radiator for natural cooling be greater than 12mm. If the radiator tooth height is less than 10mm, the tooth spacing of the radiator can be determined as tooth spacing ≥ 1.2 times the tooth height.

The heat exchange capacity of the natural cooling radiator surface is weak. Adding corrugations on the surface of the heat dissipation teeth will not have much impact on the natural convection effect, so it is recommended not to add corrugated teeth on the surface of the heat dissipation teeth.

The surface of the natural convection radiator is generally treated with blackening to increase the emissivity of the heat dissipation surface and enhance radiation heat transfer.

Since natural convection takes a long time to reach thermal equilibrium, the base and tooth thickness of the natural convection radiator should be sufficient to resist the impact of instantaneous heat load. It is recommended to be greater than 5mm.