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.
High-resolution microstructural investigations of interfaces between light metal alloy substrates and cold gas-sprayed coatings
Interfaces between light metal alloys, aluminum AA7022, and magnesium AZ91, and optimized cold gassprayed zinc-based coatings are characterized. The analyses include scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). Investigations by SEM show a seam with intensive mixing of the substrate and coating material, which is indicated by different values of gray due to element contrast. In energy-dispersive spectroscopy analyses, increased zinc concentrations compared with the substrate material are detected in <1 μm thick vortexes inside the seam. The TEM investigations prove that these areas consist of a homogeneous solid solution and submicron-sized or nanosized intermetallic phases with different concentrations of aluminum, zinc, and magnesium. Because diffusion processes cannot result in the observed microstructure. local melting followed by precipitation of intermetallic phases is concluded as the consequence of the intensive mechanical interaction at the substrate-coating interface during particle impact during the cold gas spraying of zinc on magnesium or aluminum substrates.
Bonding Mechanisms in Cold Spraying: The Contributions of Metallurgical and Mechanical Components
The mechanism of bonding in cold spraying is still a matter of some debate. In this work, copper has been cold sprayed onto aluminium alloy substrates, the surfaces of which had been prepared in a variety of ways. The coating-substrate bonding was assessed via a novel intermetallic growth method along with adhesive pull-off testing, and related to the substrate preparation method. The bond strength has been rationalized in terms of a modified composite strength model, with two operative bonding mechanisms, namely (i) metallurgical bonding and (ii) mechanical interlocking of substrate material into the coating. In most cases, mechanical interlocking is able to account for a large proportion of the total bond strength, with metallurgical bonding only contributing significantly when the substrate had been polished and annealed prior to spraying. In addition, grit-blasting has been shown to significantly reduce the bond strength compared to other substrate preparation methods.