Simplifying LED cooling design with innovative ceramic approach

　　In addition to simplifying system implementation, changes in concepts and materials can also significantly improve thermal management capabilities and reliability. Using ceramics as heat sinks, circuit carriers, and part of product design requires not only new ideas, but also a willingness to overcome traditional models.

　　Computational fluid dynamics (CFD)-based simulation processes support thermal optimization and product process design. This article will explain the theoretical approach, proof of concept, and how these improvements were ultimately achieved with the help of ceramic heat sinks.

　　What is heat?

　　As we all know, LEDs are energy-efficient light sources and are loved by designers because of their small size. But they can only be truly called "small" when thermal management is not involved. Although compared with the operating temperature of incandescent light sources up to 2500℃, the temperature of LED light sources is much lower. Therefore, many designers finally realized that heat dissipation is an issue that cannot be ignored. Although LEDs still heat up, their temperature is relatively low, so this is not a big problem. However, based on semiconductor devices, the operating temperature of LEDs should be below 100℃.

　　According to the law of conservation of energy, heat (energy) must be transferred to nearby areas. LEDs can only operate between an ambient temperature of 25°C and a maximum of 100°C, with a temperature difference of only 75°C. Therefore, a large heat dissipation surface and very effective thermal management are required.

　　Two optimization blocks

　　As shown in Figure 1, Group 1 is the LED itself, which is still largely untouchable. In the center is the LED die and a heatsinking copper strip that connects the die to the base of the LED. From a thermal perspective, the ideal solution would be to bond the LED die directly to the heatsink. This concept is not commercially feasible due to mass production. We view the LED as a standardized "catalog" product that cannot be modified. It is a black box.

Figure 1 When defining the optimization block, three groups are constructed into a thermal management system.

　　Group 2 includes heat sinks, whose function is to transfer heat from a source of heat to a source of heat dissipation. Usually, the surrounding air is either free flowing or forced convection. The less attractive the heat sink material, the more it needs to be hidden. However, the deeper it is hidden, the less effective it is at cooling. Of course, materials that are both aesthetically pleasing and performant can also be chosen. These materials can be left 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 connection, electrical isolation, and thermal conduction. This may seem contradictory, as most materials that conduct heat well also conduct electricity. Conversely, almost every electrically insulating material also insulates thermally.

　　The best compromise is to solder the LED to a printed circuit board (PCB) glued to a metal heat sink. The original function of the PCB as a circuit board can be retained. Although PCBs have various thermal conductivities, they all act as a barrier to heat conduction.

　　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, little attention is paid to Group 3 and its significant impact on overall thermal performance. Adding together all thermal resistances except the LED (Group 1) itself gives the total thermal resistance (RTT) (Figure 2). The RTT allows for a true thermal comparison.

Figure 2 RTT specifies the total thermal resistance from the LED heat sink to the ambient environment. 　　Ceramics: One material performs two functions

　　It is common to optimize only the heat sink. There are hundreds of heat sink designs, and they are basically constructed of aluminum. But to further improve performance, it is necessary to increase or even eliminate Group 3. The electrical isolation function must be obtained from the heat sink itself by another material. We think that this material should be ceramic. Ceramic materials such as Rubalit (aluminum oxide) or Alunit (aluminum nitride) combine two key properties: electrical insulation and thermal conductivity.

　　Rubalit has a lower thermal conductivity than aluminum, while Alunit has a slightly higher thermal conductivity. On the other hand, Rubalit is not as expensive as Alunit (Figure 3). Their thermal expansion coefficients meet the requirements of semiconductors. In addition, they are hard, corrosion-resistant and meet the EU's Restriction of Hazardous Substances Directive (RoHS). Ceramics are completely inert and they are the most durable part of the entire system.

image 3

　　This simplified structure (no glue, insulating layers, etc.) directly and permanently bonds a high-power LED to a ceramic heat sink, creating ideal operating conditions for the entire assembly. This results in excellent long-term stability, safe thermal management and high reliability. We have patented this approach under the name Ceram Cool.

　　Theoretical basis

　　Ceram Cool ceramic heat sink is an effective integration of circuit board and heat sink, which can reliably dissipate heat for heat-sensitive components and circuits. It supports direct and permanent connection between devices. In addition, ceramic itself is non-conductive, which can provide a bonding surface by using metal pads. If necessary, even customer-specific three-dimensional conductor track structures can be provided.

　　For power electronics applications, direct copper bonding can be used. The heat sink becomes a module substrate on which LEDs and other components can be densely placed. It can quickly dissipate the generated heat without creating any thermal barriers.

　　Proof of concept

　　The idea of ​​using ceramics was first cross-validated using several simulation models. A CFD-based method was developed to predict the thermal performance of various designs. In addition, an optimized 4WLED ceramic heat sink was developed. Manufacturing requirements were taken into account during the development.

　　The optimized geometry allows the 4WLED to operate at a maximum temperature of 60°C, which has been physically tested. The design is a square layout (38×38×24mm) and contains elongated fins that occupy a larger space. An aluminum substrate with the same geometric layout (with LEDs mounted on a PCB) can withstand much higher temperatures. Depending on the thermal conductivity of the PCB (from 4W/mK to 1.5W/mK), the temperature can rise by 6 to 28K.

　　Lowering the temperature of the hotspot by 6K means significantly less stress on the LED. The total thermal resistance of a Rubalit component of the same shape is at least 13% better than that of aluminum. Using Alunit improves Ceram Cool performance by at least 31%. If a heat drop of 28K is taken into account, the tangible benefits of both ceramic materials, Rubalit and Alunit, are much more significant.

　　Flexibility of approach

　　This is a flexible approach that can be used for different purposes. You can choose to operate the LED at its optimal temperature to ensure long LED life and higher lumens per watt; or you can choose to accept the reduced life and efficiency brought by higher operating temperature. 50℃ to 110℃ is a common temperature range. If higher lumens are required, a 5 or 6W LED can be equipped with a 4W heat sink. Spreading the power with several 1W LEDs can help improve heat dissipation: 65℃ for 5W; 70℃ for 6W (Figure 4).

Figure 4

　　As the chip is permanently and reliably bonded to the Ceram Cool electrically insulating material, the ceramic heat sink absorbs more heat and gets hotter. It removes the heat sink burden from the LED, keeping the critical components cool. The reduced die temperature also allows for a smaller surface, so the heat sink can be made smaller.

　　2mm pitch water cooling

　　At very high power densities, air cooling reaches its limits. Liquid cooling is an alternative. Ceram Cool is an example of a water-cooling solution that benefits from the inert characteristics of ceramics. This approach is consistent with the goal of air-cooled heat sinks: to minimize the distance between the heat source and the heat dissipation path.

　　With ceramic heat sinks, water cooling can be implemented at a distance of only 2 mm from the LED heat sink. There is no other way to achieve this while retaining the durable properties of ceramic. Multilateral circuits can be printed directly on ceramic without creating a thermal barrier.

　　Simulation models for customized solutions

　　Since most applications using Ceram Cool are customer-specific solutions, it is essential to verify its performance before building the first expensive prototype. To this end, intensive research has been carried out to establish simulation models. These simulation models have been validated by various tests and have proven their reliable correlation with the respective test results. Based on this knowledge, new methods or new variations can be easily evaluated.

　　Lighting Retrofitting and Insulation

　　The main issue with retrofit lamps involves insulation. Any retrofit lamp must be of Class II construction, as you cannot guarantee the provision of an electrical ground. This means that any exposed metal parts must be isolated from the mains supply line by double or reinforced insulation.

　　Metal heat sinks often cannot be modified to do this because they require larger spacing (such as 6mm air spacing) or double insulation layers, which will affect the operation of the heat sink. The electronic driver integrated in the GU10 LED is subject to extremely demanding space constraints, making the product very complex. With a ceramic heat sink, even if the driver fails completely, the heat sink still insulates the main power supply, so the product is still safe.

　　The Ceram Cool GU10 LED spotlight works with any LED. The socket and reflector are made of the same high-performance ceramic material. As a result, it has a simple Class II construction with safe insulation. The maximum temperature that a high-voltage 4W LED can reach does not exceed 60°C, which both extends the product life and increases the light output.

　　The base plate of all Ceram Cool heat sinks functions as a heat sink. In this case, it acts as a luminaire or even a light source. This simplified design offers extremely high reliability. Furthermore, the mounting bracket and reflector of a GU10 LED spotlight are usually made of different materials. This solution uses much less material and makes full use of the properties of ceramics such as electrical insulation, good electromagnetic compatibility (EMC) and high mechanical and chemical stability.

　　Submounts for retrofitting existing LED systems

　　Ceramics can significantly improve the performance of new and existing LED systems. With the ceramic Ceram Cool submount, the PCB between the LED and the metal heat sink can be easily replaced, greatly reducing the total thermal resistance of the system. This provides important advantages such as good thermal conductivity while providing excellent electrical insulation and high temperature stability. Whether it is a submount or a complete circuit board, ceramic is absolutely corrosion-resistant, which eliminates galvanic corrosion, especially in the open air.

　　Ceram Cool is ideally suited for high-power applications, especially those outdoors. In fact, a range of round heat sinks is being developed to meet the needs of different power classes. This approach combines the requirements of cost-effective production with a high degree of flexibility in use. The end result will be a "semi-customized" product range.

　　Metallization and component carriers

　　To fully exploit the optimization potential, we must also consider the metallization possibilities. Ceramics can be coated directly using proven thick film technology and its high adhesion [WNi (gold), silver, silver palladium, gold, DCB, AMB, etc.] or with the help of thin film processes and their smooth surfaces (which allow for precise light angles). Better soldering quality is achieved using chemical nickel or gold plating (immersion or cathodic deposition method).

　　The possibility of metallization makes it possible to use the entire surface of the heat sink as a circuit carrier, which can be securely packaged with LEDs and drivers on a customized circuit while providing reliable electrical insulation. The process can be simplified by directly bonding the chip to a specially designed metal surface.

　　Assembly and quality inspection

　　BMK, a well-known German electronic service center, can perform this assembly. The company can realize prototypes and also carry out mass production. In production, a mechanical structure is used to realize the heat sink, and the solder paste automatically provided by the component itself is squeezed onto the component through a template.

　　The next step in the process is to mount the LEDs and other components on the heat sink before the subsequent reflow process. After cooling, a permanent bond is obtained. The solder joints and component positions are visually checked, followed by a 100% functional test. At this point, the product is finished and ready to be delivered to the customer.