By Dr. Armin Veitl
Light-emitting diodes (LEDs) suffer from heat problems that understandably can limit their success as a light source. Much attention is given to the heat sink, and less 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 as well as a simplified system. Using ceramics as a heat sink, circuit carrier, and part of the product design needs some fresh thinking and the willingness to overcome traditional patterns.
A simulation process based on Computational Fluid Dynamics (CFD) supports thermal optimisation and technical product design. This article will explain the theoretical approach, the proof of concept, and how to ultimately achieve those improvements with ceramic heat sinks.
LEDs are known to be efficient and are favourites 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 2.500°C, LEDs are much colder. Thus, many ultimately realise 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 OPTIMISATION BLOCKS
Looking at Figure 1, Group 1 is the LED itself and mainly remains untouchable. Its centre 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 heat sink. Due to mass production, this concept is commercially unrealistic. We consider the LED as a standardised “catalogue” product that can’t be modified. It’s a black box.
Group 2 comprises the heat sink, 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. However, 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 PCB that’s glued on the metal heat sink. 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 heat sinks 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.
CERAMICS: TWO JOBS IN ONE MATERIAL
It’s common to optimise only the heat sink. 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 heat sink 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, corrosion-resistant, and RoHS-compliant. 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 highpower LED and the ceramic heat sink 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.
The CeramCool ceramic heat sink is an effective combination of circuit board and heat sink 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 can provide bonding surfaces by using metallisation pads. If required, customer-specific conductor track structures can be provided, even in 3D.
For power electronic applications, direct copper bonding is possible. The heat sink 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 behaviour of various designs, a method based on CFD was devised. Also developed was an optimised ceramic heat sink for 4W cooling. Manufacturing requirements where taken into account.
The optimised geometry allows operation of a 4W LED at a maximum temperature below 60°C, which was validated against physical tests. The design is square in shape (38mm x 38mm x 24mm) and comprises longer, thinner fins with a larger spacing. The identical geometry in aluminium with a PCBmounted LED showed significant higher temperatures. Depending on the thermal conductivity of the PCB (from λ = 4 W/mK to λ = 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 a LED on its optimum temperature assuring high life time and high lumen per Watt or you accept higher temperatures reducing life time and efficiency. A temperature spread from 50°C to 110°C is common. If more lumina are needed, the 4W heat sink can be equipped with 5W or 6W LEDs. Splitting the power into several 1W LEDs helps improve heat spreading: 65°C with 5W and 70°C with 6W (Fig. 4).
With the chip permanently and reliably bonded to the electrically insulating CeramCool, the heat sink 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 heat sink.
COOLING WATER AT 2MM
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 heat sinks—the shortest (thermal) distance between heat source and heat drain.
With ceramic, it’s feasible that water can be cooled only 2mm away from the LED heat slug. No other concept is able 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 CUSTOMISED SOLUTIONS
Since most of the applications that use CeramCool are customerspecific solutions, it’s essential that the performance can be proved before first expensive prototypes are made. Intensive studies were made 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.
Often retrofits with metal heat sinks don’t comply, since it requires larger distances (like 6mm in air) or double layers of insulation, which stop the heat sink 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 heat sink, even if the driver fails completely, the heat sink doesn’t conduct mains electricity and the product is safe.
The new 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 4W LED only reaches a maximum temperature below 60°C, increasing both the lifetime and the light output.
In all CeramCool heat sinks, the substrate becomes the heat sink. Here, it acts as the lamp, or even the luminaire. The simplified design delivers extremely high reliability. In addition, 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 EMC, and high mechanical and chemical stability.
SUBMOUNTS FOR IMPROVING EXISTING LED SYSTEMS
Ceramics can greatly improve both new and existing LED systems. Using a ceramic CeramCool Submount, the printedcircuit board between LEDs and metallic heat sink 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 in the outdoors.
High-power applications, especially those targeted for outdoor usage, are well-suited for CeramCool. In fact, a family of round heat sinks that will meet the demands of different power levels is under development. The concept combines costefficient production with high flexibility of usage. What will ultimately result is a “semicustomised” product family.
METALLISATION AND COMPONENT PART CARRIER
To fully exploit the optimisation potential, we have to look at metallisation 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 metallisation makes the heat sink’s entire surface usable as a circuit carrier, which can be firmly packed with LEDs and drivers on customised 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 centre for electronic services. They do prototyping as well as series production. The heat sinks are mechanically set in a production framework and a solder cream is automatically applied, which is pressed through a template.
In the next step, LEDs and other components are mounted onto the heat sinks, 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.