Benjamin Leyrer

Copper thick-film substrates for power electronic applications

Substrates for high power electronic systems are dominated by DCB-technology. Recently, new copper thick-film pastes have been proposed for use as high power substrates. They are compatible with Al2O3- and pre-oxised AlN-substrates. This paper investigates production processes to build up highly reliable power modules and explores basis electrical and thermal properties of thick-film copper substrates. Fired copper film thicknesses of 300 μm have been produced by subsequent print-dry-fire cycles. Smooth surfaces and copper films with a density of about 70 % of bulk copper have been produced. A power module comprising of 650 V IGBTs, diodes and an intelligent hall sensor with copper traces and spaces of 200 μm is presented. Wire bonding processes on copper thick-films with 500 μm aluminium wire and 400 μm copper wire are discussed. Test units with a 1200 V IGBT were built up. The IGBT was attached at 250 °C and a pressure of 15 MPa using a novel silver sinter paste. This paste can be directly used on copper. The current-carrying capacity of the thick-film test samples was found to be reduced by 10% in comparison to the DCB test device. No significant difference was found in the performance of both technologies in active power pulse tests lasting a few seconds. The number of cycles for test devices with sintered chips, bonded with 400 μm copper wire bonds exceeded 450,000 cycles in a cycles from 25 °C up to 150 °C.

Highly integrated power modules based on copper thick-film-on-DCB for high frequency operation of SiC semiconductors — Design and manufacture

This paper encompasses the design and the manufacture of a full-SiC module based on copper thick-film. Both DC-link capacitors as well as gate drives are implemented onto the substrate in order to minimise parasitic inductances. Thus, the module is especially suitable for high-frequency operation such as inductive energy transfer and inverter systems for renewable energies and electrical vehicles. In order to maintain high mechanical strength of the modules substrate, a Direct Copper Bond (DCB) provides the basis for multiple thick-film layers. The used thick-film dielectric insulates the gate-drive islands and also works as solder-stop material. The heat-spreading capabilities of DCB substrates are investigated by simulations.

Thermal improvements for high power UV LED clusters

We present a high power density UV LED module for a wavelength of 395 nm with an optical power density of 27.3 W/cm2. The module consists of 98 densely packed LED chips soldered onto an Al2O3 ceramic board. Thermal and optical measurements were conducted. The module was cooled by a forced air heat sink for the characterization experiments. A room temperature liquid gallium alloy was used as thermal interface material between substrate and heat sink with a TiN barrier layer to prevent corrosion. Further a technology to replace Al2O3 ceramics by aluminum as substrate is presented. An initial characterization shows that aluminum with a dielectric layer for electrical insulation is a competitive substrate material for efficient heat removal.

Index matched fluidic packaging of high power UV LED clusters on aluminum substrates for improved optical output power

We present an improved cooling for a high power density UV LED module for a wavelength of 395 nm. The module consists of 98 LED chips soldered on a thick film printed alumina substrate on an area of 2.11 cm2. We investigated cooling by a commercial water cooler as well as by a surface micro cooler developed by our own. Further we describe a technology to replace alumina by aluminum as substrate material. A module consisting of 25 UV LEDs was optically characterized without and with liquid encapsulation. Finally we conducted numerical studies to develop an easily producible, sufficiently powerful, and robust water cooler. Based on the results we present a water cooler design with cooling channels embedded in the aluminum substrate of an LED module, removing the interface between LED substrate and heat sink.

High power density LED modules with silver sintering die attach on aluminum nitride substrates

Current research studies deal with the investigation of the thermal and optical properties of four LED modules on different substrate materials. The LED modules consist of arrays of 98 blue emitting LEDs with an emission wavelength of 457 nm within an area of 2.11 cm2. The modules are based on aluminum oxide or aluminum nitride substrates and the LED chips are attached by using a soldering or a pressureless silver sintering process. The modules are mounted on a high performance microstructured heat exchanger. By using the water driven cooler a maximum optical power density of 106.2 W/cm2 at a forward current of 2100 mA and 837.5 W electrical input power is achieved. A saturation of the optical power density over the input current due to thermal degradation is not observed.

Very high power density LED modules on aluminum substrates with embedded water cooling

We present optical measurements of an LED module consisting of 98 UV LEDs with an emission wavelength of 395 nm soldered onto a ceramic substrate within an area of 211 mm2. The module is mounted to a high performance mi-crostructured water cooler. This cooler enables a maximum optical power density of 45.9 W/cm2 at a forward current of 1350 mA and 447.9 W electrical input power. Further we describe the development of an LED module based on an aluminum substrate with thick film printed insulator and conductor layers and embedded, meander shaped water cooling channels. Numerical and experimental studies with different channel cross-sections are shown. Finally experimental results for this kind of UV LED module with 98 LED chips are presented and compared to the ceramic based module.

Evaluation of Ag-sinter pastes for the die attachment in power electronic modules using design of experiments

Silver sintering is a potential die attach technology to replace the solder technology for power electronic systems. A design of experiments (DoE) is performed in order to investigate the influences of sinter parameters as sinter pressure, temperature, duration, drying temperature and the interactions of these parameters on the shear strength of the Ag-sinter connection. Four sinter pastes have been evaluated. All significant parameters and parameter interactions are identified in order to derive a quadratic model function used to fit the measured shear values. This fitted function predicts the shear strength as a function of the significant parameters and, moreover, the function is used to determine the optimal sinter parameter space. The shear tests reveal that for three tested sinter pastes an excellent average shear result exceeding 90 MPa is achieved. One sinter paste outperforms these results by an average shear strength of 121 MPa. The DoE shows that all sinter pastes are significantly dependent on the sinter pressure and sinter temperature. However one sinter paste is influenced by the drying temperature and a second sinter paste is independent of the drying temperature as well as the sinter duration.

Low-temperature silver sintering processes on high performance ENIG, EPIG, ENEPIG and ISIG surfaces for power electronic systems and huge battery systems

Low-temperature low-pressure silver sintering is a die attachment process for highly reliable power modules. The quality of the sintered interconnection strongly depends on the properties of substrate metal, the die metallization and the sinter paste. This paper investigates the properties of chips sintered at 10 MPa and 250 °C on recently proposed gold layers, which are electrochemically deposited on nickel, palladium and silver layers in a mixed displacement and autocatalytic reaction. This specific deposition process leads to ENIG, EPIG ENEPIG and ISIG finishes comprising a gold layer of high purity, which was proven utilizing X-ray Photoelectron Spectroscopy (XPS) and FIB-SEM sections. Shear tests demonstrated the high quality of the sintered interconnection. Shear values at room temperature exceeded 80 N/mm2. After storing the substrate for two hours at 200 °C or for one hour at 350 °C prior to the sintering process shear values over 80 N/mm2 were measured. Gold layers deposited by this new process are very suitable for silver sintering processes and tolerant to various sinter pastes from different manufactures. Shear values derived from a paste of a different vendor exceeded 180 N/mm2, resulting in copper torn out of the DCB.

Blue and white light emitting high power density LED modules

We present optical measurements of an optimized LED module consisting of 98 blue light emitting LED chips silver-sintered onto an aluminum nitride ceramic substrate within an area of 211 mm2. The module is cooled by a high performance microstructured water cooler. Using this cooler a maximum optical power density of 111.6 W/cm2 at a forward current of 3003 mA and 1255.6 W electrical input power could be achieved. Placing sintered glass discs doped with yellow phosphor in different concentrations in front of the module, white light with correlated color temperatures between 3600 K and 4200 K was produced.