Gerhard Mitic

Characterization of female breasts in vivo by time-resolved and spectroscopic measurements in the near infrared spectroscopy

Time-resolved and spectroscopic in vivo measurements were performed to determine the optical properties of the female breast in transmission. The time-resolved measurements were carried out at different positions on the female breast with a Ti:sapphire laser (800 nm) using a synchroscan streak camera. A diffusion model was used to calculate the absorption coefficient mA and the reduced scattering coefficient. In addition, spectroscopic in vivo measurements of more than 100 patients were performed in a wavelength range between 650 and 1000 nm. A variety of pathological alterations could be characterized by measuring patients of different ages, different breast sizes, and at varying locations on the breast. The results indicate that besides the pure detection of the amount of blood in the neovascular network, the volume concentrations of water and fat seem to be of particular importance for discrimination. In order to quantify this observation, an analytical model was developed that takes the volume percentages of fat and water, the concentration and oxygenation of hemoglobin, and the relevant optical parameters into account. Experiments were carried out with volunteers and patients in a clinical environment: Typical observations are presented and analyzed statistically.

Localisation of electrical-insulation- and partial-discharge failures of IGBT modules

The partial discharge (PD) and insulation resistance is very important in view of the increasing operating voltages of IGBT modules. Partial discharge spectroscopy showed that the PDs from metallisation edges and interfaces in silicone gel were the main sources of PD at high voltages. It also allows these types of PD to be clearly distinguished. As the PDs from interfaces in silicone gel increase strongly at high voltages, it is especially important for the silicone gel to adhere well to the ceramic.

Reliability of AlN substrates and their solder joints in IGBT power modules

Abstract The reliability of IGBT modules was investigated with respect to the metallized ceramic (substrate) and the solder layer between the substrate and copper baseplate. Thermal cycles were performed between −55°C and +150°C on substrates based on different technologies and from various manufacturers. An incipient delamination of the metallization could be predicted from the mechanical resonance frequency. The warping of the substrates after cycling due to crack propagation and the adhesion of the metallization were determined. Thermal and active-power cycles were performed on 1200 A / 3.3 kV IGBT power modules to investigate the reliability of the solder joint between substrate and baseplate.

IGBT module technology with high partial discharge resistance

The high operating voltages of 6.5 kV IGBT modules place additional demands on the insulation and partial-discharge resistance. The most important component affected here is the metallized aluminum nitride ceramic which is embedded in a silicone gel. A high electric field strength can cause a local electric discharge in the silicone gel known as partial discharge, leading ultimately to electric insulation failure and reducing the reliability of the IGBT module. For a 6.5 kV IGBT module, the insulation test must be performed up to a voltage of 10.5 kV rms. Technological steps have been carried out to reduce the maximum electric field strength along the edge of the copper metallization. The edge of the ceramic was coated with a high-impedance layer of doped amorphous silicon. The electric current along the edge of the ceramic homogenizes the electric field strength. The partial discharge was determined up to 11 kV and a considerable reduction was observed compared to standard modules. Without an a-Si:H coating, the partial discharge already increases strongly at low voltages of 3-4 kV. At high voltages, the interface between the silicone gel and the substrate is a major source of partial discharge. The a-Si:H coating reduces electric field peaks and the partial discharge does not exceed 10 pC up to a voltage of 10 kV.

MONTE CARLO SIMULATIONS AND LASER DOPPLER FLOW MEASUREMENTS WITH HIGH PENETRATION DEPTH IN BIOLOGICAL TISSUELIKE HEAD PHANTOMS

Laser Doppler flow measurements on biological tissuelike phantoms have shown that penetration depths of 30 mm could be obtained, thus exceeding the penetration depth of commercial instruments for the measurement of skin perfusion by more than an order of magnitude. Monte Carlo simulations were performed and compared with measurement results obtained on a headlike tissue model to quantify the influence of perfusion of the scalp on the cortex perfusion results. We found Doppler frequency spectra to be independent of the mean scattering angle and could be fitted with a sum of Gaussian functions, using a simple analytical model.

Reliability testing of high-power multi-chip IGBT modules

Abstract Power-cycling tests are among the most important tools used for evaluating the reliability of power modules. They are in most cases carried out at the rated module current and during a relatively short cycle time, i.e. under worst-case operating conditions. Test conditions must be defined which also permit information to be obtained about failure mechanisms in the various parts of the module. This paper describes the measurement of the temperature distribution, the test conditions, the rates of temperature change in modules with 36 semiconductor components as well as the results of power-cycling tests in which the thermomechanical stress principally affects the substrate-baseplate interface.

The thermal impedance of new power semiconductor modules using AlN substrates

Increasing operating voltages of IGBT modules result in higher dissipation heat as well as additional requirements on the insulation and partial discharge. The most important part affected here is the substrate. AlN ceramic with a thermal conductivity of typically /spl lambda/=180 W/mK is clearly superior to the conventional Al/sub 2/O/sub 3/ ceramic at /spl lambda/=27 W/mK. The thermal and thermomechanical properties of AlN substrates have been investigated in view of IGBT modules. Measurements of the thermal resistance R/sub th/ of the AlN substrates assembled with IGBTs gave values less than that of Al/sub 2/O/sub 3/ substrates by a factor greater than three. In complete IGBT power modules a factor of two is achieved. Temperature cycling test of AlN substrates and modules show a reliability similar to those of Al/sub 2/O/sub 3/ i.e. the reliability requirements are fulfilled.

AlSiC composite materials in IGBT power modules

The reliability of IGBT modules is limited by thermal fatigue of soft solder layers due to different coefficients of thermal expansion. A thermally matching conducting material can be produced from Al matrix composites containing high volume fractions of SiC particulates. Variously processed prototypes of AlSiC baseplates were investigated with a view to their suitability for IGBT power modules. The volume fraction of SiC particulates as well as their thermal conductivity, heat capacity, thermal expansion and mechanical properties were determined. The thermal conductivity of AlSiC increases with the SiC content to reach more than 200 W/mK, which exceeds that of the matrix alloy of about 180 W/mK. The remarkable plastic elongation of the material during initial heating indicates that it originally contained internal stresses which relax during heating by the plastification of the matrix. The mechanical and physical properties of the tested AlSiC materials are appropriate for applications in high-power IGBT modules. Annealing treatment is recommended for AlSiC to reduce the internal stresses.

Thermal Reliability and Environmental Testing of Advanced Metal Diamond Composites

Advanced diamond composites with matrix metal silver, copper and aluminum alloys were developed for heat sinks and base plates in high performance electronic packages. The reliability of these composites under extreme thermal and environmental conditions was investigated in the thermal cycling and pressure cooker tests. Thermal cycling tests on metal diamond composites were performed in a two chamber cycling oven between −55°C and +150°C for 1000 thermal cycles with 20 minutes storage time in each chamber. The effect of humidity and pressure was assessed from pressure cooker test at 121°C at 2 bar for 168 h. Thermal properties such as thermal diffusivity and specific heat were measured before and after the thermal cycling to observe any changes in thermal properties of the composites due to any possible interfacial degradation. The thermal properties of the composites did not decrease and the interfaces did not degrade after the thermal cycling tests in case of Ag-diamond composites. Al-diamond composites show a slight decrease of 5–8% in thermal diffusivity after thermal cycling. The thermal diffusivity of the Cu/Cr based diamond composite is unaltered before and after testing. The thermal diffusivity of Cu/Si diamond composites decreased by 43% at room temperature measured value. Al-diamond composites did not show any degradation during the pressure cooker tests although aluminum carbide is hygroscopic and deteriorates under moisture. Ag and Al-diamond composites survived the pressure cooker test. Cu/Si and Cu/Cr based diamond composites showed a slight change in coloration and a possible corrosion after the pressure cooker test.Copyright

Inaudible cooling: A novel approach to thermal management for power electronics based on acoustic streaming

A new approach for forced convection cooling based on “Reynolds streaming” is presented, which allows for additional heat dissipation in situations where conventional active cooling devices (e.g. fans) are not suitable. The system operates inaudible, has no moving parts and is therefore durable and immune to dust. Its operation is dependent on numerous influences that have been identified and analyzed through experiment and simulation. It employs a new kind of acoustic driver delivering high intensity fields within air, while maintaining a small build volume. This transducer was designed and optimized using FEM-Simulation and verified with real-world prototypes at each major simulation milestone. All of the taken measures improved the performance by a factor of nearly 2 while reducing the system size by a factor of over 6 at the same time when compared to the first working system. This last generation device delivers comparable cooling performance.