Fraunhofer Institute for Ceramic Technologies and Systems IKTS
Copper pastes for ceramic substrates
Power electronics systems play a pivotal role in renewable energy supply and electromobility. At the heart of these systems are modules that integrate power semiconductors and other components. However, since the DCB (Direct Copper Bond) substrate currently used in the majority of power modules limits the integration density of these modules, thick-film copper systems are increasingly being employed. At Fraunhofer IKTS, such copper paste systems are available for Al2O3 and AlN substrates. Using screen or stencil printing, copper layers with thicknesses ranging from 15 µm to 300 µm can be produced, which are suitable for soldering and bonding. These novel systems enable finer patterning due to high printing resolution (< 100 µm) as well as greater design flexibility than commercially available DCB substrates. Additionally, they demonstrate higher reliability under thermal cycling conditions.
Conductive and dielectric pastes for the fabrication of multilayer structures
In order to fabricate complex multilayer structures on copper-ceramic substrates, additional paste systems such as conductive and dielectric pastes are required. For the high-temperature (650 °C to 950 °C) and low-temperature (200 °C to 280 °C) ranges, Fraunhofer IKTS has developed various paste systems that are perfectly adapted to the copper-ceramic substrate and can be sintered under a nitrogen atmosphere. Among other things, a thick-film insulation paste is available that is suitable for processing electrical insulation layers on copper surfaces of copper thick films and copper DCB in the high-temperature range. Based on a barium-zinc-containing glass, firmly adhering glass insulation layers with low residual porosity are applied to the copper thick-film surfaces via screen printing. The insulating layers exhibit excellent electrical properties in terms of insulation resistance and breakdown voltage. Insulation layers on copper surfaces of ceramic wiring substrates thus enable chip-level sensor integration directly on the DCB substrate and significantly increase the integration density of power electronic modules. This results in more accurate and reliable chip monitoring while simultaneously saving space and weight.
Base metallization pastes for thermocouples and heaters
Another key area of research is the development of base metallization pastes. Base metals are characterized by low material costs and advantageous electrical properties, such as high electrical conductivity and very low temperature coefficients. This makes them ideal for use as sensors to measure state variables such as temperature, strain, and humidity in power modules. In current work, Fraunhofer IKTS is focusing on the development of CuNi pastes and their use for screen-printed Type T (Cu-CuNi) thermocouples on insulated copper surfaces of ceramic wiring substrates. It has already been demonstrated that the thermocouple is suitable for temperature measurement in the range from 25 °C to 300 °C. In addition to CuNi pastes, the focus is also on the development of NiCr pastes. The latter are also of interest as a suitable functional material for the manufacture of thermocouples or strain gauges.
All of these innovative IKTS developments contribute significantly to the functionalization of DCB substrates, thereby enabling the future production of compact, reliable, and cost-effective power modules.