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Low cost thermally sprayed and structured conductive layers for power electronic printed circuit boards

Final Report Summary - SPRAYTEC (Low cost thermally sprayed and structured conductive layers for power electronic printed circuit boards)

Nowadays, electronic modules on printed circuit boards (PCB) are glued to the heat sink. Although thermally conductive glues are used, the thermal resistance of the junction limits the achievable power density of the modules. Thus the heats sinks, which are essential parts of the module's housings, have to be dimensioned much bigger than needed solely by the dissipated power. Higher thermal conductivity at the PCB / heat-sink junction would therefore allow to significantly reducing the size of such modules, saving space, weight and material. State of the art in manufacturing of the module substrates for PCBs is done by gluing the different material, slices (aluminium and alumina) together. Gluing the different layers is the start of the process of manufacturing module substrates.

In SPRAYTEC, thermal spraying will be used to replace the gluing process in the manufacturing of module substrates by direct thermal spraying of the materials one after another. The adhesive characteristics of layers sprayed onto materials such as ceramics and metals will be improved. Also the manufacturing process time will be reduced by using of thermal spraying technology. Thermal sprayed coatings are used extensively for a wide range of industrial applications. The technique generally involves the spraying of molten powder or wire feedstock, the melting being achieved by oxy-fuel combustion or an electric arc (plasma). The molten particles are accelerated by the flame, followed by impacting onto a properly prepared substrate, usually metallic. To adapt this process to spray the substrates of PCB's, several barriers like the implementation of the right materials or the adjusting of the PCB manufacturing process have to be overcome through the help of the European consortium.

The overall objective of the SPRAYTEC project is to develop low cost thermally sprayed and structured conductive layers for power electronic printed circuit boards and to gain an understanding of thermal spraying principles as well as materials for printed circuit boards. The industrial benefit of thermal spray coatings is the achievement of cost-effective production of high thermally resistant PCBs. The technology of atmospheric plasma spraying (APS), PureCoat and high velocity oxy-fuel (HVOF) spraying allows tailoring of components or specific areas to counteract damaging effects, prolonging the life of new parts or providing cost effective repair of worn parts. Thermal spraying also allows use of cheaper substrate materials such aluminium and is a flexible technology for the spraying of different layer materials to provide a cost-effective product with high thermally resistance. The understanding of the thermal spraying principle, the use of applicable materials and suitable processes for thermal sprayed of PCBs has been build up by the SME partners. Especially, detailed knowledge and the influence of the different parameters like layer thickness, layer porosity, sealing of copper and ceramic layers have been gained and opened new markets for the SME. The main requirements from customers to the new PCBs are a high dielectric strength, high thermal conductivity and good thermal shock resistance.

In the development of the technology, thermal sprayed PCBs provide a cost efficient and fast solution, with low start up costs and fast lead times to prototypes. In addition, quick turnaround can be achieved and low cost changes are possible, hence there is high design flexibility. In the process of thermal sprayed layers the use of plastic materials is reduced to a minimum, thus the thermal sprayed PCBs are especially suited to high temperature applications. Only ceramic and metals are be processed. Costly and time consuming sticking and soldering processes are replaced by thermal spraying method. With this method, coatings with low porosity and high thermal shock resistance were realised. Furthermore, new test methods to verify the capabilities of thermal sprayed PCBs have been developed. The significantly improved thermal conductivity and the use of smaller heat sinks enhanced the usability for high temperature applications in the power electronic market. The SME partners built up detailed knowledge about thermal sprayed PCBs by understanding the requirements and possible enhancement of the customer needs in sectors such as automotive and power electronics including the knowledge of the industrial implementation of the SPRAYTEC technology. The greatest benefits will be for SME partners, which can use the new thermal sprayed power PCBs to improve their power electronic components in various areas as functionality, module size, module weight and different design.

The close collaboration of the partners enabled the development of a new power supply unit (PSU) and a new LED panel for automotive lighting. It turned out that the SPRAYTEC technology is more suitable to high temperature applications like the insulated metal substrate (IMS) technology than to assembling technologies like direct copper bonding (DBC). The joining techniques contrary to the thermal sprayed PCBs differ primarily in the lamination processes of the copper and dielectric layer by gluing and costly etching processes.

The project started with a detailed scientific characterisation of the spraying technology, the characterisation of the suitable materials for the different layers and the definition of the market requirements on the thermal sprayed PCBs.

In SPRAYTEC two spraying methods are applied, APS and PureCoat. APS is a method where the energy is based on the plasma produced by ionising an inert gas, typically mixture of argon and hydrogen or helium, between the anode and cathode of the spray gun. Due to the high energetic ionised plasma, the temperature of the plasma is very high, as high as 10 000 degrees Celsius. The speed of the plasma is approximately 400 m/s. Due to the high process temperature, APS which enables good melting of the ceramic particle is often used to produce a ceramic coating.

PureCoat is an arc spraying process that uses a wire feedstock melted by an arc. Because of this it is not possible to produce unmelted particles. This thermal metal spraying process has been developed which is reported to be able to give coatings of near HVOF quality at less than half the cost. The process, known as PureCoat, has been developed in the United Kingdom by Metallisation Limited, in collaboration with six other companies in Europe. The PURECOAT process uses a computer-designed spray head which solves clogging and oxidation problems encountered with conventional metal spraying in the open air. It produces coatings of a quality close to that of the feedstock metal.

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