Copper-Si₃N₄ composites as circuit carriers for power electronics

Structured metal-ceramic substrates are an important component for power electronic assemblies as circuit carriers. Existing solutions, such as direct bonded copper-aluminum oxide (DBC) or active metal brazing (AMB), no longer meet the requirements of electromobility and the potential of new silicon carbide (SiC)-based semiconductor components. SiC increases the demands on the substrates for mounting the semiconductors and cooling the circuits. Composites with silicon nitride ceramics (Si₃N₄) with their outstanding mechanical properties promise a solution. This makes metal-ceramic substrates possible that exhibit significantly improved stability against active and passive thermal cycles in power electronic assemblies.

In the CuSiN joint project, Fraunhofer IKTS and its partners developed reliable and high-performance copper-silicon nitride composites (Cu-Si₃N₄) using active brazing. Ceramic Si₃N₄ substrates with thicknesses < 320 μm, produced by multiwire sawing of sintered Si₃N₄ blocks, already exhibit thermal conductivities > 90-100 W m^-1K^-1 and strengths > 700 MPa. The basis for this is formed by particularly low-oxygen Si₃N₄ powders in combination with aluminum-free additives. Another new feature is the possibility of sintering compact Si₃N₄ compacts with dimensions of up to 5.5 x 7.5 inches to form homogeneous and high-quality structures.

New active solder pastes suitable for automated screen printing are required for the adhesive bonding of copper tapes to Si₃N₄ substrates using active soldering. In addition to good bond strengths (currently up to 25 N per mm copper width), joining zones with as few pores as possible must be realized to ensure high reliability of the bonds. This is achieved by using innovative pastes with homogeneously distributed, minimal proportions of active phases for solder layers of less than 25 μm. It is also possible to reliably remove organic binder components from printed brazing layers in flat Cu-Si₃N₄ arrangements in an ultra-high vacuum below 380 °C.

Titanium is often used as the active component. Depending on the joining temperature, the matrix phase of the active solder itself is a mixture of copper and silver; at lower joining temperatures, indium or tin is added to the solders. During the joining process, titanium forms a thin compound layer in contact with Si₃N₄. In addition, titanium silicides with brittle-hard properties can form at the interface and lead to a weakening of the bond strength. For the first time, investigations into the mechanical properties of actively soldered copper-Si₃N₄ composites were carried out at the IKTS at a microscopic level using micrometer cantilever equipment in an electron microscope. The results showed that a moderate formation of titanium silicides does not have a detrimental effect on the composite properties. A targeted adjustment of the microstructure in the property-determining joining zone between active solder and Si₃N₄ ceramic is achieved by an optimized paste formulation and adapted solder profiles, taking into account the individual behavior of the Si₃N₄ ceramics in the joining process.

The characterization of actively soldered Cu-Si₃N₄ composites with regard to partial discharge behaviour is generally a new topic for metal-ceramic composites. Partial discharge measurement is a non-destructive measurement method for detecting defects in dielectrics. With specially adapted measurement setups and analysis methods, partial discharge processes could be recorded with high temporal resolution and an understanding of these effects could be created by correlating them with material and composite properties.

The knowledge gained within the framework of CuSiN enables the development of Si₃N₄ ceramics, active solder pastes and active soldering processes as well as the materials science and electrical characterization of copper-ceramic substrates.