It is understandable that a light-emitting diode (LED) is prevented from becoming an ideal light source due to the problem of heat generation. We pay a lot of attention to the heat sink, but we don't think much about the layers and barriers between the LED and the heat sink surface.
In addition to simplifying system implementation, changes in concepts and materials can significantly improve thermal management capabilities and reliability. The use of ceramics as a heat sink, circuit carrier, and part of product design requires not only new ideas, but also the willingness to overcome traditional models.
Computational fluid dynamics (CFD) based simulation processes support thermal optimization and product process design. This article will explain this theoretical approach, proof of concept, and how these improvements can be ultimately achieved with ceramic heat sinks.
What is heat?
As we all know, LED is an energy-efficient light source, and because of its small size, it is loved by designers. But they can only be called "small" only when they are not involved in thermal management. Although the operating temperature of an incandescent light source is as high as 2500 Â° C, the LED light source temperature is much lower. Therefore, many designers finally realized that heat dissipation is a problem that cannot be ignored. Although the LED still heats up, its temperature is relatively low, so this won't be a big problem. However, based on semiconductor device LEDs, the operating temperature should be below 100 Â°C.
According to the law of conservation of energy, heat (energy) must be transmitted to a nearby area. The LED can only operate between an ambient temperature of 25 Â° C and a maximum temperature of 100 Â° C with a temperature difference of only 75 Â° C. Therefore, a large heat dissipation surface and very efficient thermal management are required.
Two optimization blocks
As shown in Figure 1, Group 1 is the LED itself and is still largely untouchable. The center position is the LED die and a heat sink copper strip that connects the die to the bottom of the LED. From a thermal point of view, the ideal solution is to bond the LED die directly to the heat sink. Due to mass production, this concept is not commercially viable. We see LEDs as a standardized "catalog" product that cannot be changed. It is a black box.
Figure 1: When defining an optimization block, the three groups are built into a thermal management system.
Group 2 includes a heat sink whose function is to transfer heat from a heat source to a heat sink. Usually, the surrounding air is either free flowing or forced convection. The more unobtrusive the material of the radiator, the more it needs to be hidden. However, the deeper it hides, the lower its cooling efficiency. Of course, materials with suitable appearance and performance can also be selected. These materials can be directly exposed to the air and become a visible part of the product design.
Between Group 1 and Group 2 is Group 3, which provides mechanical connections, electrical isolation, and heat transfer. This may seem contradictory because most materials with good thermal conductivity are also electrically conductive. On the contrary, almost every electrical insulating material is also insulated.
The best compromise is to solder the LED to a printed circuit board (PCB) that is attached to a metal heat sink. The original functionality of the PCB as a board can be preserved. Although PCBs have various thermal conductivities, they all act as a barrier to heat transfer.
Effective system thermal resistance comparison
The thermal resistance between the LED (die to thermal pad) and the heat sink is available from the manufacturer. However, few people are concerned about Group 3 and its significant impact on overall thermal performance. Adding all the thermal resistances other than the LED (Group 1) itself, you can get the total thermal resistance (RTT) (Figure 2). A true thermal comparison can be made through RTT.
Figure 2: RTT indicates the total thermal resistance from the LED heat sink to the surrounding environment.
Ceramic: A material that performs two functions
It is common to optimize the heat sink only. There are currently hundreds of radiator designs, which are basically constructed of aluminum. But to further improve performance, it is necessary to upgrade or even cancel Group 3. Electrical isolation must be obtained from the heat sink itself through other materials. We think this material should be ceramic. Ceramic materials such as Rubalit or Alunit combine two key properties: electrical insulation and thermal conduction.
Rubalit's thermal conductivity is lower than aluminum, while Alunit is slightly higher than aluminum. On the other hand, Rubalit is not as expensive as Alunit (Figure 3). Their coefficient of thermal expansion meets the requirements of semiconductors. In addition, they are hard, corrosion resistant and meet the European Union's Restriction of Hazardous Substances (RoHS). Ceramics are completely inert and they are the most durable part of the system.
This simplified structure (no glue, insulation, etc.) directly and permanently binds a high power LED to a ceramic heat sink, creating the ideal working conditions for the entire assembly. This brings excellent long-term stability, safe thermal management and high reliability. We have applied for a patent called CeramCool for this method.
CeramCool ceramic heatsinks are an effective integration of circuit boards and heat sinks to reliably dissipate heat sensitive components and circuits. It supports direct and permanent connections between devices. In addition, the ceramic itself is non-conductive and it can provide a bonded surface by using a metal liner. A three-dimensional conductor track structure for the customer can even be provided if desired.
For power electronics applications, direct copper bonding can be used. The heatsink becomes a module substrate on which LEDs and other components can be placed intensively. It quickly dissipates the heat generated without creating any thermal barriers.
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