Development and Testing of Cold gas Sprayed Circuit Boards for Power Electronics Applications
Within the framework of the presented project, new mounting concepts for power electronics were implemented by applying Al/Cu coat systems to insulators by means of cold gas spraying (CGS). The layers sprayed with cold gas were investigated and good material properties are observed. The applied metallizations show an extremely small porosity, no thermally induced phase conversions or process-related oxides and a high adhesive strength. Also the processing of assemblies with standard processes such as a soldering, adhesive bonding and heavy wire bonding is possible. The temperature shock stability of the new substrates was determined in temperature cycle experiments and it was compared with the temperature shock stability of the DCB substrates. The substrates were tested actively but also passively in bicameral temperature shock chamber and in power cycling experiments at power electronic function samples. The accomplished reliability investigations showed that the characteristic lifetime of the cold gas sprayed substrates is approximately 80% of the characteristic lifetime of the DCB substrates. These demonstrate that the substrates produced in cold-gas technology can be used for power electronic applications.
The effect of a simple annealing heat treatment on the mechanical properties of cold-sprayed aluminum
Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300°C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spray as a bulkforming process.
Microstructural characterisation of titanium coatings deposited using cold gas spraying on Al2O3 substrates
Abstract Microstructure formation in cold gas sprayed titanium coatings was investigated using optical microscopy, scanning and transmission electron microscopy and X-ray diffraction. Optical microscopy was employed to obtain an overview about the microstructure of the Ti coatings and to determine their porosity. Scanning electron microscopy in combination with the electron back-scatter diffraction was used to quantify the grain and sub-grain size in titanium and to get information about the mean misorientations of sub-grains within the Ti grains. X-ray diffraction revealed microstrain in titanium before and after the cold gas spraying process, from which the changes of dislocation density were calculated. Transmission electron microscopy yielded information about the morphology of Ti/Al 2 O 3 interfaces, about the size of Ti grains in different distances from the interface and about the mutual orientation of Ti and Al 2 O 3 crystallites in vicinity of the interface. Particularly the information about the mutual orientation of Ti and Al 2 O 3 crystallites supported the hypothesis that titanium can grow hetero-epitaxially on corundum. Furthermore, it was concluded from the experimental results that titanium particles are severely plastically deformed during the impact on the corundum substrate. This plastic deformation created microstructure defects in titanium that have a high stored energy. Together with a local increase of the temperature of the coating that is due to the impact of Ti on the substrate, the energy stored in the microstructure defects is considered to contribute to the local hetero-epitaxial growth of nanocrystalline Ti on Al 2 O 3 .