Electronic Design

Cool Ceramics Help Simplify LED Heat Dissipation

Ceramic heatsinks bring a fresh approach to thermal management in LED design.

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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THE THEORY
The CeramCool ceramic heatsink is an effective combination of circuit board and heatsink for the reliable cooling of thermally sensitive components and circuits. It enables the direct and permanent connection of components. Also, ceramic is electrically insulating per se, and it can provide bonding surfaces by using metallization pads. If required, customer-specific conductor track structures can be provided, even in 3D.

For power electronics applications, direct copper bonding is possible. The heatsink becomes a module substrate that can be densely populated with LEDs and other components. It quickly dissipates the generated heat without creating any barriers.

VALIDATION AND PROOF OF CONCEPT
The idea to use ceramics was first cross-checked in several simulation models. To predict thermal behavior of various designs, a method based on CFD was devised. Also developed was an optimized ceramic heatsink for 4-W cooling. Manufacturing requirements were taken into account.

The optimized geometry allows operation of a 4-W LED at a maximum temperature below 60°C, which was validated against physical tests. The design is square in shape (38 by 38 by 24 mm) and comprises longer, thinner fins with a larger spacing. The identical geometry in aluminium with a PCB-mounted LED showed significantly higher temperatures. Depending on the thermal conductivity of the PCB (from ? = 4 W/mK to ? = 1.5 W/mK), the temperature rose to between 6 and 28K.

Already, a 6K reduction at the hotspot implies significantly less stress for the LED. The total thermal resistance of the Rubalit assembly is at least 13% better than aluminium with an identical shape. Using Alunit, the minimum improvement of CeramCool reaches 31%. These solid results are outperformed largely for both ceramics if the heat drop of 28K is considered.

FLEXIBILITY OF CONCEPT
The concept is flexible and can be used for different targets. It’s your choice whether you run an LED on its optimum temperature ensuring a long lifetime and high lumen per Watt, or you accept higher temperatures, reducing lifetime and efficiency. A temperature spread from 50°C to 110°C is common. If more lumina are needed, the 4-W heatsink can be equipped with 5- or 6-W LEDs. Splitting the power into several 1-W LEDs helps improve heat spreading: 65°C with 5 W and 70°C with 6 W (Fig. 4).

With the chip permanently and reliably bonded to the electrically insulating CeramCool, the heatsink takes on more heat and becomes hotter. It removes the burden from the LED, cooling the critical components. The reduced die temperature allows a downsized surface and, thus, a smaller heatsink.

COOLING WATER A 2 MM
Air cooling reaches its limits at very high power densities. At this stage, liquid cooling is the option. One such example is CeramCool water cooling, which benefits from the inertness of ceramics. The concept follows the same goal as for air-cooled heatsinks—the shortest (thermal) distance between heat source and heat drain.

With ceramic, it’s feasible that water can be cooled only 2 mm away from the LED heat slug. No other concept can do this in combination with the durable nature of ceramics. Multilateral electrical circuits can be printed directly on the ceramic without creating thermal barriers.

SIMULATION MODELS FOR CUSTOMIZED SOLUTIONS
Since most of the applications that use CeramCool are customer-specific solutions, it’s essential that the performance can be proved before the first expensive prototypes are made. Intensive studies were conducted to build up simulation models. These simulation models have been verified against various tests and showed reliable correlations to test results. Based on this knowledge, new concepts or variations are easily evaluated.

RETROFIT LAMPS AND ISOLATION
The problem with retrofit lamps mainly concerns isolation. Any retrofit lamp has to be Class II construction because you can’t guarantee an electrical earth. This means that any exposed metal part has to be isolated from the mains wiring by double or reinforced insulation.

Retrofits with metal heatsinks often don’t comply, since they require larger distances (like 6 mm in air) or double layers of insulation, which stop the heatsink from working well. The integrated electronic driver in a GU10 LED is so restricted for space that the product becomes very complex. With a ceramic heatsink, even if the driver fails completely, the heatsink doesn’t conduct mains electricity and the product is safe.

The CeramCool GU10 LED spot works with any LED. The socket as well as the reflector are made from a single material—a high-performance ceramic. Consequently, it has simple Class II construction with safe insulation. A high-voltage 4-W LED only reaches a maximum temperature below 60°C, increasing both the lifetime and the light output.

In all CeramCool heatsinks, the substrate becomes the heatsink. Here, it acts as the lamp or even the luminaire. The simplified design delivers extremely high reliability. Furthermore, the mount and reflector of GU10 LED spots are usually made of different materials. This solution uses far fewer materials, and ceramics are exploited for their electrical insulation, good electromagnetic compatibility (EMC), and high mechanical and chemical stability.

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SUBMOUNTS FOR IMPROVING EXISTING LED SYSTEMS
Ceramics can greatly improve both new and existing LED systems. Using a ceramic CeramCool Submount, the PCB between LEDs and the metallic heatsink can be replaced with ease, considerably reducing the total thermal resistance of the system. This offers important advantages, such as very good thermal conductivity while delivering perfect electrical insulation and high temperature stability. Whether it’s a submount or complete circuit board, ceramic is absolutely corrosion-resistant, which eliminates galvanic corrosion, especially outdoors.

High-power applications, especially those targeted for outdoor usage, are well-suited for CeramCool. In fact, a family of round heatsinks that will meet the demands of different power levels is under development. The concept combines cost-efficient production with high flexibility of usage. What will ultimately result is a “semi-customized” product family.

METALLIZATION AND COMPONENT PART CARRIER
To fully exploit the optimization potential, we have to look at metallization possibilities as well. Ceramic can be coated directly with proven thick-layer technologies and their high adhesion force (WNi(Au), Ag, AgPd, Au, DCB, AMB...) or thin-film processes with their smooth surfaces (allowing precise light angles). A finish for better soldering can be obtained using electroless nickel or gold (immersion or cathodic deposition).

The possibility of metallization makes the heatsink’s entire surface usable as a circuit carrier, which can be firmly packed with LEDs and drivers on customized circuit layouts while providing reliable electrical insulation. The process can be simplified by bonding the chip directly onto the specially designed metallic surface.

ASSEMBLING AND QUALITY CHECK
One potential assembler is BMK, a well-known German center for electronic services. The company performs prototyping as well as series production. The heatsinks are mechanically set in a production framework, and a solder cream that’s automatically applied is pressed through a template.

In the next step of the process, LEDs and other components are mounted onto the heatsinks, followed by subsequent processing in reflow ovens. A durable bond is established after cooling. The soldering points and position of the components are visually inspected, followed by a 100% functional test. At this point, the product is complete and ready to be sent to the customer.

TAGS: Components
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