Laser-Based Powder Bed Fusion of Copper Powder on Aluminum Nitride Ceramics for Power Electronic Applications

Aluminium oxide (Al2O3), Aluminium Nitride (AlN) and Silicon Nitride (Si3N4) are ceramics commonly used in power electronics modules, and are the current candidates for high temperature applications. A first study has previously shown different charge displacement behaviors at high temperature (up to 400°C). Surface potential measurements revealed a fast decay for AlN and Al2O3 with the increase of the temperature while on Si3N4 charges remain on surface at 400°C. However, these charges may move through the surface or be injected in the volume of the ceramics. - Surface potential results are correlated with broadband impedance spectroscopy and to current-voltage measurements of the ceramics. AlN and Al2O3 have a resistive-like behavior at high temperature, characterized by a space charge limited conduction current mechanism at high fields. For Si3N4, despite its resistivity decreases with temperature, it shows a dielectric behavior and an ohmic conduction mechanism over the studied temperature and field ranges.

To enable the rapidly emerging and imminent energy economy, high-voltage (HV) and high-current robust power electronic modules (PEMs) are needed at low cost. PEMs typically consist of a number of semiconductor power switches and driver chips; intelligent power modules often contain sensing and protection circuitry. In the entire supply chain of power electronics systems-from materials to end-user applications, including the original equipment manufacturers (OEMs)-the PEM is the key building block, as it forms the heart of a power electronic system. The performance, cost, and durability of the entire power electronic system critically hinge on those of the PEM. In addition, major business opportunities in power electronics are often enabled by the advances in power semiconductor devices. One such opportunity currently available to the power electronics community has been created by the advances in wide-bandgap (WBG) power switching devices, which were first introduced by Shenai et al. in the 1980s [1].

Our previous studies showed that geometrical techniques including (1) metal layer offset, (2) stacked substrate design and (3) protruding substrate, either individually or combined, cannot solve high electric field issues in high voltage high-density wide bandgap (WBG) power modules. Then, for the first time, we showed that a combination of the aforementioned geometrical methods and the application of a nonlinear field-dependent conductivity (FDC) layer could address the issue. Simulations were done under a 50 Hz sinusoidal AC voltage per IEC 61287-1. However, in practice, the insulation materials of the envisaged WBG power modules will be under square wave voltage pulses with a frequency of up to a few tens of kHz and temperatures up to a few hundred degrees. The relative permittivity and electrical conductivity of aluminum nitride (AlN) ceramic, silicone gel, and nonlinear FDC materials that were assumed to be constant in our previous studies, may be frequency- and temperature-dependent, and their dependency should be considered in the model. This is the case for other papers dealing with electric field calculation within power electronics modules, where the permittivity and AC electrical conductivity of the encapsulant and ceramic substrate materials are assumed at room temperature and for a 50 or 60 Hz AC sinusoidal voltage. Thus, the big question that remains unanswered is whether or not electric field simulations are valid for high temperature and high-frequency conditions. In this paper, this technical gap is addressed where a frequency- and temperature-dependent finite element method (FEM) model of the insulation system envisaged for a 6.5 kV high-density WBG power module will be developed in COMSOL Multiphysics, where a protruding substrate combined with the application of a nonlinear FDC layer is considered to address the high field issue. By using this model, the influence of frequency and temperature on the effectiveness of the proposed electric field reduction method is studied.

At present, power electronics advancements on renewable energy sources are mainly lying on converters and inverters (i.e.) moderators. The power electronic modules play a vital role in handling generation, transmission, distribution and consumer end appliances. Due to the lack of familiarity on the power electronic modules the researchers experience the complexity in choosing appropriate power electronic modules for standalone renewable energy sources like Photovoltaic panels, wind turbines etc. The improper selection of power modules leads to deteriorating the efficiency of the system and detriment of the equipments too. There is a discussion on the usefulness of hybridization of renewable energy resources by taking into account of optimum utilization and compensation of the investment done on the sources. So, choosing a suitable power module for the hybrid system further increases the complexity in choosing the appropriate power electronic modules. This proposed paper will emphasis on different power electronic converters that have been presented in the literature which are suitable for stand-alone and an array of renewable energy sources. In addition, it presents about different kinds of power converter topologies along with the technical requirements, control structures and boundaries. This paper also gives an insight on the evaluation of the existing converter topologies by considering the various parameters. Through which a suitable characterization of promising converter topology can be opted for either stand alone or hybrid renewable energy resources.

The need for effective electrical insulation coupled with good thermal conductivity in power electronics has led to an exploration of suitable solutions for components like Insulated-Gate Bipolar Transistors (IGBTs). Considering its material properties, AlN emerges as a promising candidate for this application due to its high thermal conductivity, good electrical insulation and ample dielectric strength. However, aluminium nitride (AlN) has a low deposition efficiency when applied by atmospheric plasma spraying (APS). In contrast to AlN, alumina has a very good deposition efficiency during thermal spraying. Feedstock development was conducted to enhance the coating deposition for AlN. Therefore, a parameter study was carried out with AlN feedstock material to form a protective alumina shell around the AlN particles. Subsequently, the heat-treated powder was applied on an aluminium substrate by APS. X-ray diffraction (XRD) analysis displayed that, the heat-treated feedstock material contained AlN and α-Al2O3 phases. It was observed from scanning electron microscopy (SEM) analysis that the AlN particles formed an oxide shell which led to an enhanced deposition efficiency with a high amount of AlN in the coating. The coatings were also investigated by XRD and SEM to prove the presence of AlN and alumina. For the first time, oxide shelled AlN was successfully applied by thermal spraying with sufficient coating deposition and enhanced AlN-content in the coating.

Reliability investigation of large area solder joints in power electronics modules and its simulative representation

Application of Kriging and radial basis function in power electronic module wire bond structure reliability under various amplitude loading

Understanding of chemical corrosion resistance of aluminum nitride (AlN) ceramics is strongly desired for their applications as structural parts in semi-conductor process. However, there is not enough information on the chemical corrosion. Two kinds of commercial AlN ceramics with high thermal conductivity of 170 and 200 W/mK were examined in acids and basic aqueous solutions such as KOH, NaOH, HNO{sub 3} and H{sub 2}SO{sub 4} with different concentration. Weight changes, phases present and microstructures on the surfaces before and after testing were evaluated. As a result, basic aqueous solutions exactly corroded AlN ceramics in comparison with acid aqueous solutions. Although secondary phases, such as yttrium aluminate, were mainly dissolved in the case of acid aqueous solutions, AlN itself was corroded into basic aqueous solutions. Furthermore, it was observed that weight loss of AlN ceramics with higher thermal conductivity was larger than that of lower thermal conductivity. It seems that oxygen content dissolved into AlN grain has a influence on corrosion of AlN ceramics. (orig.)

Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1100–1250 °C under 20 Torr total pressure with a V/III ratio of 180–300 in either N2 or H2 diluent. High-resolution x-ray diffraction measurements revealed that AlN epitaxial layers deposited on MP substrates were strained due to nucleation and coalescence of AlN grains on the mechanically damaged surfaces. AlN deposited on CMP substrates was epitaxial and strain-free. Thermodynamic models for nitridation and AlN deposition were also proposed and evaluated.

A newly developed rapid densify technique, spark plasma sintering (SPS), is used to prepare full-densified aluminum nitride (AlN) ceramics which are poorly sinterable. Some sintering aids are also used to promote the AlN ceramics' densification and improve its thermal conductivity. In this work, effects of sintering aid Li2O on densifica- tion, microstructure and thermal property of SPS sintered AlN ceramics were investigated. Results suggest that the ini- tial sintering temperature of AlN samples reduce from 1550℃ to lower than 1200℃ with 1.0wt% Li2O and 1.5wt% Sm2O3 (or Y2O3) adding as sintering aids . With Li2O addition, AlN compacts can be fully densified at 1650℃. The microstructure of AlN compacts indicates that Li2O is beneficial to generate aluminate liquid phase with better wet- tability and promote the densification of AlN ceramics, but it is unable to obtain higher relative density of AlN com- pacts because the escapement of gas phase though out liquid phase is very difficult in a rapid sintering process. Meanwhile, Li2O addition affects the growth of AlN grains, and the better wettability of aluminate with AlN grain in- duces the homogeneous distribution of grain boundary phase. The deterioration of thermal conductivity of AlN ceram- ics is caused by the fact that the scattering of phonon is enhanced by small grain size and the secondary phase spreading adequately along the AlN grain boundaries. The thermal conductivity of AlN samples with 1.0wt% Li2O and 1.5wt%Sm2O3 as sintering aids is lower than that of sample only with Sm2O3 as sintering aids.

A paste containing molybdenum (Mo) and titanium nitride (TiN) powders was printed on aluminum nitride (AlN) substrates and postfired. The adhesion strength of metallized substrates with Ni/Au plate was about 25 kgf/2.5 mm and was unchanged after the thermal cycle test. TiN-Mo does not adhere to the grain boundary phase in AlN substrate, or to the surface oxide layer, but to the AlN grain itself. This method, therefore, seems to be applicable to any kind of AlN substrate, which may have different grain boundary oxide phases and thermal conductivities. This TiN-Mo metallized AlN substrate was tried as a replacement for a beryllium oxide (BeO) heat sink, which has been used for RF power transistors. There was no trouble in assembling the AlN heat sinks into transistors. Thermal resistance and electrical properties for transistors with AlN heat sinks were almost equal to those for transistors with BeO heat sinks. The TiN-Mo metallized AlN substrates were found to be suitable for replacing BeO substrates as the heat sinks for semiconductor devices. >

In recent years, the number of power electronic assemblies has increased significantly both in products and machines for the end user and in products and machines for industry. Above all, however, power electronics is becoming more and more important in drive technology and the generation of renewable energies. Power electronic modules, such as inverters and rectifiers or DC converters, are required to provide more and more power in the advancing field of electromobility, while the installation space is constantly being reduced. At the same time, however, these assemblies must have the longest possible and most reliable lifetime. Previous power electronic modules have a maximum junction temperature of 150 °C due to the materials used in assembly and connection technology. New types of wide band gap semiconductors, however, already offer the possibility of enabling junction temperatures of 200 °C and above. The aim of the investigations in this paper is to qualify a sintering process that is suitable for efficient series production for an all-in-one joining process in assembly and interconnection technology. By replacing the solder layer with a silver sinter paste, operation at junction temperatures significantly above 150 °C is possible. However, if the maximum junction temperature is maintained at the current level of 150 °C, higher reliability and longer lifetime of the power electronic modules are expected. In active power cycling tests, the reliability of the demonstrators produced in the series sintering process is compared with the service life of soldered reference samples.

This report describes technical opportunities to serve as parts of a technological roadmap for Shoals Technologies Group in power electronics for PV applications. There are many different power converter circuits that can be used for solar inverter applications. The present applications do not take advantage of the potential for using common modules. We envision that the development of a power electronics module could enable higher reliability by being durable and flexible. Modules would have fault current limiting features and detection circuits such that they can limit the current through the module from external faults and can identify and isolate internal faults such that the remaining modules can continue to operate with only minimal disturbance to the utility or customer. Development of a reliable, efficient, low-cost, power electronics module will be a key enabling technology for harnessing more power from solar panels and enable plug and play operation. Power electronics for computer power supplies, communication equipment, and transportation have all targeted reliability and modularity as key requirements and have begun concerted efforts to replace monolithic components with collections of common smart modules. This is happening on several levels including (1) device level with intelligent control, (2) functional module level, and (3)more » system module. This same effort is needed in power electronics for solar applications. Development of modular units will result in standard power electronic converters that will have a lower installed and operating cost for the overall system. These units will lead to increased adaptability and flexibility of solar inverters. Incorporating autonomous fault current limiting and reconfiguration capabilities into the modules and having redundant modules will lead to a durable converter that can withstand the rigors of solar power generation for more than 30 years. Our vision for the technology roadmap is that there is no need for detailed design of new power converters for each new application or installation. One set of modules and controllers can be pre-developed and the only design question would be how many modules need to be in series or parallel for the specific power requirement. Then, a designer can put the modules together and add the intelligent reconfigurable controller. The controller determines how many modules are connected, but it might also ask for user input for the specific application during setup. The modules include protection against faults and can reset it, if necessary. In case of a power device failure, the controller reconfigures itself to continue limited operation until repair which might be as simple as taking the faulty module out and inserting a new module. The result is cost savings in design, maintenance, repair, and a grid that is more reliable and available. This concept would be a perfect fit for the recently announced funding opportunity announcement (DE-FOA-0000653) on Plug and Play Photovoltaics.« less

Cooling for power electronics in leisure vessels has so far been based in the use of forced air, even though such vessels are surrounded by water which can ultimately be used to dump the heat developed by power electronics converters and electrical machines. Some of the reasons to avoid the use of sea water for cooling are the need to deal with biological-fouling, salt water, corrosion, and condensation. We propose to apply a new concept where pre-filtered seawater is used to cool down power electronic modules in order to significantly reduce the size of the cooling system. A new type of cold plate with direct flow of seawater has been designed considering the mentioned challenges. The cold plate will cool down a 5kW (10kW peak) power electronics module. Such module consists of a 3 kW 3-phase AC-DC rectifier, a 2 kW (10kW peak) full-bridge DC-DC converter and a 5 kW (10 kW peak) DC-AC single phase inverter.

In order to overcome the purification difficulty of aluminum nitride (AlN) ceramics, the sintering of AlN ceramics with ammonium fluoride (NH4F) as an additive had been studied. The results demonstrate that the addition of NH4F evidently affects the phase compositions, the microstructure of grains and the contents of oxygen and nitrogen in the AlN sintered samples. NH4F not only removes oxygen out of AlN grains but also reduces the total oxygen content in AlN ceramics. It is found that relatively high purity of AlN can be acquired when the molar ratio of NH4F/O (oxygen element in raw AlN powder) increases to 0.8. With adequate amount of NH4F, the Al–O–N phases are removed. SEM and TEM results show the hexagonal structures of AlN grains with clean triple-grain junctions. The oxygen content decreases to 0.55 wt% and nitrogen content increases to 33.7 wt%. Thermodynamic analysis illustrates the oxygen removing effects of NH4F by the reaction of NH3 and Al2O3, which inhibits the formation of Al–O–N. NH4F should be at least 2/3 of the oxygen content.