For decades soldering has been the technology of choice in die bonding. However, due to worldwide health protection regulations, the most common solder alloys, which contain lead, have been banned. Furthermore, standard solders cannot fulfil the reliability requirements of future power electronic devices. New interconnection technologies have to be developed. One of them is pressure sintering (p=30..50 MPa) of silver flakes below 300 °C. It forms a strong, highly electrically and thermally conductive bond. In order to lower the level of pressure, silver nanoparticles can be used. Shear tests have shown that even 5 s of sintering, a temperature of 225 °C, or a pressure as low as 2 MPa is sufficient to generate bonds comparable to solder and high pressure sinter joints if the remaining parameters (p, t and T, respectively) are set correctly. However, strength is only a necessary criterion as aging comes into play. Therefore, reliability tests using thermal cycling and pow er cycling were run. These returned superior reliability of the sintered samples. 160 million of the power cycles between +45 and +175 °C run in this work can be extrapolated using a Coffin-Manson model. Solder joints failed at about 40,000 cycles. ©2010 IEEE
High electric field strengths at the edge of the metallization of insulated gate bipolar transistor (IGBT) power modules are, besides defects in the substrate or the potting gel, the main reason for partial discharge. These critical electric field strengths occur at the energized contact where it is bordered by the insulating ceramic and the cover (mostly silicone gel). The reduction of high electric field strengths for increasing the threshold voltage for partial discharge has been studied in several publications based on experiments as well as on simulations. Simulations allow the localization of the critical spots and the quantification of the maximum electric field strength. However, a systematic study of the singularities of the electric field strength at the sharp edges is lacking. Such singularities are investigated in this article. The calculation of an absolute electric field strength value is only possible for finite edge radii. For sharp edges, however, the maximum electric field strength returned by simulation depends on the grid size: Through the finite grid size a virtual edge radius is induced that suppresses the singularity at the edge. To get around this problem, a mesh-independent evaluation procedure is introduced. With this procedure it is possible to quantitatively evaluate the electric field strength in the vicinity of the sharp edge. As an example, the influence of the offset between the top and bottom metallization layer on the maximum electric field strength is studied. Moreover, the influence of the thickness of the involved layers and of the shape of the electrodes is discussed. Also, the impact of the material properties of the involved dielectrics is examined. In addition to electrostatic simulations we have carried out electric transient simulations, which show that the ratio of the conductivities of the involved dielectric materials plays a major role for determining the maximum electric field strength
Wide bandgap materials have become very attractive for power electronics due to their physical properties that allow junction temperatures up to a theoretical limit of 600°C. In contrast, the maximum operation temperature of conventional silicon semiconductors is limited to approximately 200°C. The high-temperature operation of wide bandgap switches allows an increasing power density of power converters due to the reduced complexity of thermal management systems, leading to highly miniaturized power converters for example for automotive and aircraft applications. However, the reliability of wide bandgap devices at high temperatures is limited by the maximum operation temperature of conventional interconnection materials. The aim of this study is to investigate die attach technologies that are suitable to fulfill high temperature and high power requirements. Therefore, this work focuses on solder joints made of gold-germanium (AuGe12), zincaluminum (ZnAl5), and lead tin (PbSn5) alloys, as well as die bonding by low temperature sintering of silver nano particles. For this reason, the evolution of the interfacial microstructure of test devices, assembled with different high temperature die attachment technologies, were monitored using cross sectioning techniques and scanning electron microscope (SEM) images. The evolution of the shear strength with time during high temperature storage was investigated. A comparison between shear test results and the evolution of the microstructure is given. The results show that sintered test devices feature a much higher shear force after high temperature storage due to the proceeding sintering of the particles, while the mechanical stability of all solders decreases with storage time. ©2010 IEEE
High voltages and the edges of the metallization on ceramic substrates (AMB, DBA, DBC, HTCC, LTCC) lead to high electric field strengths. In the vicinity of the metal edges these high electric field strengths induce partial discharges in the ceramic insulation and in the potting and thereby represent one key degradation mechanism of power modules. In this work the correlation of the simulated electric field strength with phase resolved partial discharge (PRPD) measurements has been investigated. For the simulation of the electric field strength a new method was used to bypass numerical artifacts. The simulated values show that it is possible to reduce the electric field strength by an adaption of the metallization structure. There the distance of the upper and the lower metallization to the rim of the ceramic was changed relative to each other. Due to this variation a reduction of the electric field strength of about 30% can be reached by choosing the optimum distance compared to state of the art design. In PRPD measurements for insulating ceramic substrates (AlN/Al 2 O 3 by DCB) we examined whether the electric field strength reduction leads to higher partial discharge inception voltages (PDIV). The measurements were executed on layouts with different dimensions of the upper and lower metallization relative to each other as well as for 3 different thicknesses of the ceramic insulation layer. An increase from 20% to 35% of the PDIV was measured for layouts designed according to the findings from the simulation with respect to electric field strength reduction. Finally, the calculated electric field strength and the measured PDIV were correlated.
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