Light-emitting diodes (LEDs) suffer from heat problems that understandably can limit their success as a light source. Much attention is given to the heatsink, and less is given to the layers and barriers between the LED and the heat-dissipating surface.
A change of concept and material allows for significant gains in thermal management and reliability in addition to a simplified system. Using ceramics as a heatsink, circuit carrier, and part of the product design requires some fresh thinking and the willingness to overcome traditional patterns.
A simulation process based on computational fluid dynamics (CFD) supports thermal optimization and technical product design. This article will explain the theoretical approach, the proof of concept, and how to ultimately achieve those improvements with ceramic heatsinks.
WHAT'S HOT LEDs are known to be efficient and are favorites among designers for being tiny. But they’re only really “tiny” as long as heat management isn’t involved. While incandescent light sources work with temperatures up to 2500°C, LEDs are much colder. Thus, many designers ultimately realize that heat is such an issue. Being relatively cold, LEDs still do produce heat, which isn’t yet a problem. However, they’re based on semiconductors that, roughly speaking, allow temperatures below 100°C.
According to the law of energy conservation, the thermal energy must be transferred to the surrounding area. The LED can only use a small temperature gap between 100°C of the hot spot and 25°C ambience temperature, offering just 75 Kelvin. Consequently, a larger surface and powerful thermal management are needed.
TWO OPTIMIZATION BLOCKS Looking at Figure 1, Group 1 is the LED itself and mainly remains untouchable. Its center is a die and a heat slug—a copper part that connects the die with the bottom of the LED. Thermally, the ideal solution is to bond the die directly to the heatsink. Due to mass production, this concept is commercially unrealistic. We consider the LED a standardized “catalog” product that can’t be modified. It’s a black box.
Group 2 comprises the heatsink, which transmits energy from a heat source to a heat drain. Usually, the surrounding air is either free or forced convection. The less aesthetic the material, the more it needs to be hidden. Yet the more you hide it, the less efficient the cooling. Alternatively, pleasing and worthy materials can be used. These can be directly exposed to the air and become part of the visible product design.
In between Groups 1 and 2 is Group 3, which provides mechanical connection, electrical isolation, and thermal transmittance. That seems contradictory, since most materials with good thermal conductivity conduct electricity, too. Vice versa, almost every electrical isolation material translates into a thermal barrier.
The best compromise is soldering the LED to a printed-circuit board (PCB) that’s glued on the metal heatsink. The original function of a PCB as a circuit board can be maintained. Although PCBs exist with various thermal conductivities, they remain an obstacle to thermal transfer.
THERMAL RESISTANCE FOR VALID SYSTEM COMPARISON The thermal resistance of LEDs (die to heat-slug pad) and heatsinks can be obtained from the manufacturer. However, there’s little focus on Group 3 and its significant influence on the total thermal performance. When adding all thermal resistances but the LED (Group 1), you get the total thermal resistance (RTT) (Fig. 2). The RTT allows a real comparison of heat.
CERMAICS: TWO JOBS IN ONE MATERIAL It’s common to optimize only the heatsink. Hundreds of designs are available, essentially consisting of aluminium. But for further improvement, it’s necessary to advance or even eliminate the third group. Electrical isolation has to come from the heatsink itself via other materials. Our conclusion is ceramic. Ceramics, e.g., Rubalit (Al2O3) or Alunit (AlN), combine two crucial characteristics— they are electrically isolating and thermally conductive.
Rubalit has a lower conductivity than aluminium, while Alunit’s is slightly higher. On the other hand, Rubalit is less expensive than Alunit (Fig. 3). Their thermal expansion coefficient is adapted to semiconductors. Also, they are rigid and corrosion-resistant, and they comply with the European Union’s Restrictions on Hazardous Substances (RoHS). Completely inert, they are the last part of a system to give out.
The simplified construction (without glues, insulation layers, etc.) combined with a direct and permanent bond between the high-power LED and the ceramic heatsink create ideal operating conditions for the entire assembly. Put simply, what isn’t there won’t wear out, and materials that expand in proportion to each other won’t separate. The result is excellent long-term stability, secure thermal management, and high reliability. A patent has been filed for this concept, named CeramCool.
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