Innovating 3D Printed Liquid Cooled Heatsinks

Aachen, Germany-based company IQ Evolution manufactures liquid cooled heatsinks which are made of stainless steel, ultra-light-weight, and can be even integrated into the PCB. Further they are 3D printed using a laser process to selectively melt powder to form the heatsinks. The water flow and disturbances within the heatsink have been further optimized, increasing the heatsinks heat flux density to values between 150-200W/cm² (2.000.000W/m²!!!).

 

Heatsinks from IQ Evolution can increase reliability by enabling extended life of power semiconductors, reduce size, weight, and volume of the system, and thus reduce system costs of the end application. In cooperation with the Institute for Power Electronics and Electrical Drives (ISEA), Aachen a 20kW DC/DC converter has been built with a world record power density of 53kW/kg or 98kW/l. For reference, NASA has asked in the past for 26kW/kg in their state-of-the-art converter.

 

Figure 1: IQ-Twin

IQ Evolution has been founded in 2006 by Dr. Thomas Ebert as a spin-off from RWTH Aachen. After acquiring the first printer machine, the first nickel-based heatsinks have been manufactured and sold into the high-power laser diode market. Throughout the years various patents have been granted worldwide and the latest has been received for a high-power diode laser cooler with a matched CTE (Coefficient of Thermal Expansion, a matched CTE is beneficial for improved reliability and thermal resistance). With various key players as reference customers it is safe to say that the technology has been successfully established in the diode laser market, which typically requires high performance and very reliable products.

 

Expanding Into the Power Electronics Market 

Recently, the company decided to expand its product portfolio and move into power electronic applications such as electric vehicles, electric aviation motor drives, energy storage, renewables and others. One key challenge has been that Nickel as a material is export restricted, thus complicating the quick adoption into various applications. Through an artificial intelligence-based optimization of the internal structure, the thermal performance of the heatsinks was significantly increased making it possible to use stainless steel. But first a few basics to explain the process in more detail.

 

The Versatility of 3D Printing

The 3D printing (additive manufacturing) manufacturing process of the metal heat sinks does not require any moulds or tools and allows customized cooling solutions. Highly effective cooling structures including inlet areas are manufactured in the micrometer range. The IQ Evolution process enables the production of (micro-) structures that are suitable for economical use in industry, for example, with exceptionally thin cooling plates for integration in multilayer printed circuit boards. Almost all materials can be processed or used as alloys, even adding a layer of insulation material is possible. Complex shapes, which were neither theoretically nor economically possible with conventional processes, can be produced economically with SLM (Selective Laser Melting). As a rule, almost all melt-able materials can be used (such as stainless steel, copper alloys, aluminum, titanium, chromium-cobalt-molybdenum, bronze, nickel, etc.). Various material combinations within a component are also possible. Interesting examples of applications in need of liquid cooling are industrial applications such as high-power laser diodes, power supplies for lasers, server, telecom and aviation or automotive applications such as onboard chargers, DC/DC converters, and traction inverters. But not only semiconductors can be cooled. Cooling passives elements such as coils and transformers is necessary as well and will be described in more detail in future articles.

In general, the process is suitable for rapid prototyping as well as fast ramp up series production. High volume quantities can be easily achieved as only a few steps are manual and scaling production is done through parallelization of process chains.

To go into production, the following steps are required. First, a technical discussion with the customer will ensure the best and efficient system cooling with an ideally as small cooler as possible. After the customer provides a CAD file a 3D model of the heat sinks is generated which Evolution will then feed into the machine. Metal powder is loaded into the machine and a laser is used to selectively melt powder particles where desired. As the process does not require any further tools, manufacturing identical parts, as well as even variants, are possible. One must remember that e.g. adding a hole means additional working steps in conventional technologies but in 3D printing, it means less machining time which equals cost. Besides, it is a paradigm shift to place all heated parts into one efficient but small and thin cooler instead of spreading the heated parts all over the PCB. A video of the manufacturing process is available at www.IQ-Evolution.com.

 

AI Optimization Increases Performance 

The great advantage of this process is that structures that are optimized in CAD or a corresponding simulation can be loaded directly into the machine and executed. This is particularly interesting when the optimization (up to 20%) is carried out using artificial intelligence. As a result, even very unusual shapes are possible. Complex structures are very suitable for being optimized by means of AI.

With an initial basic starting structure, the AI algorithm simulates the process under special observation of the pressure drop and heat flux. The results from this simulation are used to intelligently modify the internal cooling structures, and the process starts again. The AI algorithm keeps the beneficial changes and rejects negative ones. At the end, after several thousand modifications, the result is a near-optimal cooling and supporting structure. The AI work has been done by Diabatix, Belgium. This possible increase in performance has prompted IQ evolution to consider a change in the material used. Whereas pure nickel had previously been used, mainly because of its high thermal conductivity, it is now possible to think about other materials that trade off slightly worse thermal conductivity for lower material cost and ease of export restrictions.

 

Evaluating a 3-D Printed Heatsink

In this case stainless steel (Material No. 1.4404) was chosen because it is corrosion free which increases the lifetime of the heatsink. Despite its 10x lower thermal conductivity, the first tests with substitute heat sources showed no difference in cooling performance compared to the previously manufactured heat sinks of this series. On the contrary, the optimized heat sinks made from stainless steel showed a better performance and higher than expected dissipation capability. THERMAL MANAGEMENT

 

Figure 2: IQ-Four

 

With AI optimization the technology is capable of safe and reliable cooling with heat flux densities of over 200W/cm² (pure water, no TIM). Even higher densities are possible but need to be further evaluated and measured appropriately. Partnering with the Institute for Power Electronics and Electrical Drives at RWTH Aachen University, the cooling effect on components, and the potential savings in weight, volume and system cost in real use applications has been evaluated.

Examples of the new generation of stainless-steel heat sinks are the IQ-Twin (see Figure1) or IQ-Four (see Figure 2) which are designed to simultaneously cool two, respectively four TO-247 or similar packages. The measurements for this were carried out by ISEA. SiC MOSFETs (C3M0016120K) in a TO-247 package were used, which were energized with 50 A for the test. A Hi-Flow®-300P insulation foil is used against the metal heat sink to electrically isolate it. The cooler was operated at a fluid pressure of 1.2 bar, resulting in a flow rate of cooling medium (normal water) of one liter per minute. The ISEA experts measured the electrical power delivered to the MOSFETs, the actual electrical power absorbed by the MOSFETs and the amount of heat dissipated by the heat sink.

750 W dissipated power could be transferred from the stainless-steel heat sink into the water. This results in a dissipated power loss of 187 W per MOSFET. With a footprint (cooling area) of the TO-247 package of slightly more than one cm² this results in a heat flux density of more than 175 W/cm².

The AI-optimized stainless-steel heat sink has thus passed its test of courage and found its way into the standard product range of the IQ evolution. In general, stainless steel is now the standard material for these micro-heatsinks, which are manufactured using 3D printing. Should a power electronics application require even higher cooling performance, Nickel or Aluminum alloys could be used again in the heat sink base material at any time, so that customers can benefit from an up to 20% higher thermal conductivity.

But laboratory tests are only one side of the coin. What about the other side, the real application? ISEA subjected the metal heat sink to practical tests: Impressed by the enormous cooling capacity and the confidence now placed in the 3D-printed stainless-steel coolers, they found an application in which the new type of cooler should prove itself, a DC/DC converter.

The field test specified that four 1000V SiC MOSFETs in a TO-247 4L package with Kelvin Source connection should be effectively cooled by the printed heat sink, in this case an IQ Double Four (See Figure 3). In contrast to the laboratory setup, the cooler was now unfolded so that the four MOSFETs are no longer two pairs opposing each other but are all placed next to each other.

 

Figure 3: IQ-Double-Four. One side being used to cool the MOSFETs, the other to cool inductors and other components.

 

The separate supply of cooling water to the pairs of two is also retained in this arrangement; it is located inside the heat sink and is therefore not visible from the outside. The modified arrangement of the MOSFET packages has a specific purpose: The underside of the heat sink remains free and is thus able to take over further cooling tasks.

The heat sink is mounted with the underside close to the coils and cast together with the coils in the housing. In this way, not only the MOSFETs, but also the coils and the inner workings of the converter are cooled. In addition to the electronics required for the converter, the voltages are also monitored via sensors. A galvanically isolated microcontroller is also integrated for control and communication.

The use of the cooling capacity of the IQ heatsinks and their implementation in the converter design resulted in an extremely compact component, which would not have been possible without the use of the optimized heat sinks. The result is an ultra-compact bidirectional DC/DC converter (see Figure 4) that operates in the voltage range between 400 and 800 V at a frequency of 450 kHz. The DC/DC converter can transmit 20.6 kW. With dimensions of 90 mm x 65 mm x 35 mm, this means a power density of 98 kW per liter or 53kW/kg; according to Prof. De Donker from ISEA this value is a world record in power density.

 

Figure 4: 20kW DC/DC Converter. Picture courtesy of ISEA, RWTH Aachen

 

Outlook and Summary

In this article we demonstrated what impressive results and heat flux densities can be achieved with the IQ Evolution technology. However, we are just in the beginning of optimizing power electronic cooling. It is the target of IQ evolution to unleash the full potential of semiconductors by optimizing the packages with development partners. Heat sinks with an electrically insulated surface are already being developed, as is the possibility of applying conductive structures directly to the heat sinks. If one thinks about baseplate-less modules, sintered to an optimized, electrically isolated heatsink a quantum leap can be achieved in terms of system cost, weight, and volume reduction as well as efficiency. We are already well progressed in this development and the first measurements look very promising. Furthermore, we are working with partner on developing ideal solutions to cool passive elements such as inductors and transformers.

 

This article originally appeared in the Bodo’s Power Systems magazine.

 

About the Author

Dr. Thomas Ebert received his Doctor of Engineering in the Field of Machine at RWTH AACHEN University. He is the Managing Partner and CEO of IQ Evolution.

Christopher Rocneanu received his Electrical and Electronics Engineering degree at the University of California, then a Master's degree at the University of Kiel. He is skilled in Semiconductor Industry, wide bandgap, power electronics, and marketing and he is a strong sales professional with a Master's degree focused in Industrial Electronics Technology from Christian-Albrechts-Universität zu Kiel. He is the found and CEO of Foxy Power GmbH.