The heat dissipation problem of LEDs will be the main factor limiting its future success in the market. Much of the research in the industry has focused on heat sinks, but there has been less research on the barrier between LEDs and heat sink surfaces.
However, as long as we make some changes in design ideas and material usage, we can not only significantly improve thermal management performance and reliability, but also get a more simplified system. The use of ceramics as a heat sink, circuit carrier and part of product design requires us to have some new modes of thinking and willingness to overcome traditional design patterns.
Computational fluid dynamics (CFD) based simulation processes support thermal optimization and product technology design. This article will explain the theoretical basis, proof of concept, and how to ultimately achieve these improvements with ceramic heat sinks.
As we all know, LEDs have high luminous efficiency, and they are also favored by designers because of their small size. But they are really "small" only when heat management is not considered. Although the operating temperature of an incandescent light source is as high as 2500 ° C, the LED light source temperature is much lower. Therefore, many designers finally realized that heat dissipation is a big problem. Although LEDs also generate heat, they are relatively low, so heat dissipation is not a problem for the LED itself. However, semiconductor devices that drive LEDs operate at operating temperatures below 100 °C.
According to the law of conservation of energy, thermal energy must be transferred to the surrounding area. The LED can only use a small temperature gap between the 100 ° C hot spot and the 25 ° C ambient temperature, so only 75 Kelvin is available. The result is a large surface and powerful thermal management.
The two optimization blocks are shown in Figure 1. Group 1 is an LED that is basically untouchable. Its center is a die and a heat-dissipating copper metal block for connecting the die to the bottom of the LED. From the perspective of heat dissipation, the ideal solution is to bond the die directly to the heat sink. But from the perspective of mass production, this idea is commercially unrealistic. We see LEDs as a standardized, unmodified "catalog" product. It is a black box.
Figure 1: Two optimization modules
Group 2 contains a heat sink that transfers the energy of the heat source to the air. Normally, the surrounding air is free or forced convection. The less aesthetically pleasing the heat sinking material, the more it needs to be hidden. But the more you hide, the lower the efficiency of cooling. In contrast, aesthetically pleasing and high value materials can be used. These heat sinking materials are directly exposed to the air and become part of the visible product design.
Between Groups 1 and Groups 2 is Groups 3, which provides mechanical connections, electrical insulation and heat transfer. This seems contradictory because most materials have good thermal and electrical conductivity at the same time. Vice versa, almost every electrical insulating material is also a thermal barrier material.
The best compromise is to solder the LED to the PCB and glue it to the metal heat sink. This allows the PCB to be maintained as an initial function of the board. Although PCBs have many different thermal conductivities, they are still an obstacle to heat transfer. (Text / European Altair Design Director Armin Veitl)
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