Direct Bonded Copper Substrates Research Articles

Improvement of Bonding Strength and Thermal Shock Reliability for Ag Sinter Joining Direct on Al Substrate

Ag sinter paste joining as a proven die bonding technology have been used in the electric vehicle. However, for the Ag sinter paste joining, metallization layers on substrate are usually necessary such as Ni-P/Au or Ni-P/Ag to get a good bonding quality. The direct bonding on DBC (direct bonded copper) or DBA (Direct Bonded Aluminum) substrates without metallization layers are very attractive. In this study, we investigated the direct bonding on an aluminum (Al) substrate by Ag sinter paste and propose an abrasive blasting processes to treat the Al surface to increase the initial bonding quality. The result show that the shear strength of SiC/Al joint structure by Ag sinter paste can be improved by treating the Al surface. The shear strength of joint structure without surface treatment increased from 26.9 MPa to 32.2 MPa for a strong blasting treatment on Al surface. In addition, the results of thermal shock test (-40 ℃~150℃) also show that the blasting treatment on Al surface can improve the shear strength and reliability of SiC/Al joint structure.

Automotive Silicon Carbide Power Module Cooling With a Novel Modular Manifold and Embedded Heat Sink

Abstract The next generation of integrated power electronics packages will implement wide-bandgap devices with ultrahigh device heat fluxes. Although jet impingement has received attention for power electronics thermal management, it is not used in commercial electric vehicles (EVs) because of the associated pressure drop and reliability concerns. In this paper, we present a modular thermal management system designed for automotive power electronics. The system achieves superior thermal performance to benchmarked EVs while adhering to reliability standards and with low pumping power. The system utilizes a low-cost and lightweight plastic manifold to generate jets over an optimized heat sink, which is embedded in the direct-bonded-copper (DBC) substrate. The embedded heat sink concept leverages additive manufacturing to add elliptical pin fins to the DBC substrate. The heat sink geometry is optimized for submerged jet impingement using a unit-cell model and an exhaustive search algorithm. The model predictions are validated using unit-cell experiments. A full-scale power module model is then used to compare the DBC-embedded heat sink against direct DBC cooling and baseplate-integrated heat sinks for single-sided (SS) and double-sided (DS) cooling concepts. Using the SS and DS DBC-embedded cooling concepts, the models predict a thermal resistance that represents a reduction of 75% and 85% compared to the 2015 BMW i3, respectively, for the same water-ethylene glycol inverter flowrate. We have shown that an inverter with a 100-kilo-Watt-per-liter power density is achievable with the proposed design.

Heat Dissipation Design of High-Power Wide-Bandgap Power Module with Insulated Metal Substrate

In general, power modules are manufactured as direct bonding copper (DBC) substrates to withstand high temperatures and high power. The DBC substrate has a sandwich structure in which copper layers are formed on upper and lower parts and ceramic is formed as an insulating layer therebetween. Ceramic has excellent insulation properties but is difficult to process and is vulnerable to mechanical stress due to a difference in coefficient of thermal expansion (CTE) between copper. In this regard, research on the development of power modules using various materials is being conducted. In this paper, an insulating metal substrate (IMS) was designed and analyzed using a dielectric film as an insulating layer. IMS substrates are easier to process than ceramics and have a higher CTE, so they are more resistant to mechanical stress than DBC substrates. In addition, it may be made very thin through easy processing, thereby overcoming low thermal conductivity compared to ceramic. In this paper, IMS substrates of various thicknesses were designed based on the easy processability of the dielectric film, and simulations were performed to compare heat dissipation characteristics with Al2O3-based DBC substrates.

Point-Contact Bonding of Integrated Three-Dimensional Manifold Microchannel Cooling Within Direct Bonded Copper Platform

Abstract A microchannel heat sink integrated with a three-dimensional manifold using direct bonded copper (DBC) is promising for high power density electronics due to the combination of low thermal resistance and reduced pressure drop. However, this requires much progress on the fabrication and high-quality point-contact bonding processes of the microchannel substrate and three-dimensional manifold DBCs. In this study, we have developed processing techniques for surface preparations and high-quality point-contact solder bonding between the two DBC substrates. We utilized chemical polishing followed by electroless plating to prevent excess solder from blocking the microchannels. We performed a parametric study to investigate the impact of bonding time and surface roughness on the tensile strength of the bonding interface. The bonding strength increased from 1.8 MPa to 2.3 MPa as the bonding time increased from 10 to 30 min while reducing the surface roughness from Rz = 0.21 to 0.05 μm, resulting in increasing the bonding strength from 0.16 MPa to 2.07 MPa. We successfully tested the microcooler up to the inlet pressure of 70 kPa and pressure drop of 30 kPa, which translates to the tensile strength at the bonding point contacts, which remains well below the 2.30 MPa. We achieved the junction-to-coolant thermal resistance of 0.2 cm2 K/W at chip heat flux of 590 W/cm2. Thus, our study provides an important proof-of-concept demonstration toward enabling high power density modules for power conversion applications.

Pyroelectric energy harvesting from power electronic substrates

Energy harvesting from available resources can lead to implementation of more controlling and actuating devices for industrial internet of things (IIoT). Energy harvesting from temperature fluctuations in power electronic devices by pyroelectric materials is a key solution to achieve self-powered sensor systems. In this study, performance of a pyroelectric energy harvester (PEH), utilized as a dielectric substrate in power electronic modules, is investigated. Accordingly, a comprehensive analytical model is developed in MATLAB software, and the theoretical results are validated with experimental data. The results of this study show that, the pyroelectric substrate can harvest an average power of 50 µW/cm2. Effect of amplitude and frequency of input heat, cooling density, and load resistance over PEH are studied. The results reveal that, frequency of the input heat rate higher than 2 Hz does not have a sensible impact on the power generation by the pyroelectric module. Convective cooling density with low heat transfer coefficients led to higher power generation. On the contrary, it can cause operating at higher temperature than the Curie temperature. The energy harvesting concept developed, for the first time, in this study shows significant potential of pyroelectric direct bonded copper substrate to provide self-powered sensors for IIoT in a vast range of power electronic applications.

Interfacial Diffusion Mechanism between ALD Al2O3 passivation/Cu in Direct Bonded Copper Substrate for Power Electronics

Direct bonded copper(DBC) substrate requires surface treatments for oxidation prevention and adhesion enhancement to die-attachment. Conventional surface treatment, electroless nickel immersion gold (ENIG) has a cost and environmental problems, and immersion silver is sensitive to temperature and humidity. To solve these problems, this study presents the invention of aluminum oxide (Al2O3) passivation for DBC Cu pads, which meets the requirements of oxidation prevention and adhesion enhancement to die-attachment. Ultrathin Al2O3 passivation layers (~20nm) by atomic layer deposition(ALD) were coated on DBC Cu pads for replacing conventional ENIG and immersion silver surface finishes. This paper presents bondability and interfacial diffusion mechanism between ALD Al2O3 passivation/Cu in DBC substrate.

Single-Phase Jet Impingement Cooling for a Power-Dense Silicon Carbide Power Module

The adoption of silicon carbide (SiC) devices in the electric vehicle (EV) industry is increasing due to their superior performance over silicon devices. SiC devices enable further miniaturization of EV inverters, increasing their power density, which results in thermal management challenges. In this paper, the limits of single-phase jet impingement cooling are explored for an automotive SiC power module. We propose embedding pin fins in the direct-bonded-copper (DBC) substrate of the power module package using laser powder bed fusion additive manufacturing. The thermalhydraulic performance of the DBC-embedded pin fins is compared against folded fins that are directly soldered to the DBC substrate. A heat conduction analysis was conducted on a SiC package to determine the target heat transfer coefficient (HTC) for the heat sink. A water-ethylene glycol (WEG) jet impingement on the proposed concepts was studied using unit-cell models to achieve the target HTC. The studied designs put emphasis on the reliability and manufacturability requirements of the automotive industry. The thermal performance of DBC-embedded pin fins outperformed the DBC-soldered folded fins. The performance of the DBC-embedded pin fins is benchmarked against WEG-based cooling systems of commercial EVs. With the proposed cooling solution, we have shown a pathway of reducing the specific thermal resistance by 75% compared to BMW i3 thermal management system without any penalty on pressure drop or parasitic power.

High-temperature resistant interconnection using Ni nanoparticles and Al microparticles paste sintered in an atmosphere

Next-generation power devices using wide bandgap semiconductors, such as SiC, are expected to operate at higher temperatures than conventional Si power devices, and their operating temperatures are expected to exceed 250 °C. We developed a novel high-temperature resistant interconnection technology for die-bonding of SiC power devices using Ni nanoparticles and Al microparticles composite paste. The bond strength of the Al-metallized Si chip to Ni-plated direct bonded copper substrate was evaluated using shear tests. The initial shear strength of samples from pressureless sintering at 350 °C for 15 min in the air exceeded 30 MPa. Furthermore, no significant degradation was observed in a high-temperature storage test at 250 °C for 1000 h.

Low Parasitic-Inductance Packaging of a 650 V/150 A Half-Bridge Module Using Enhancement-Mode Gallium-Nitride High Electron Mobility Transistors

Because of their fast-switching speed and small die size, gallium-nitride high electron mobility transistors are challenging to package for low parasitic inductance and high heat dissipation in power electronics applications. In this article, a packaging technique was developed for making half-bridge modules of a 650 V, 150 A enhancement-mode gallium-nitride power transistor. The package consists of the device chips interconnected between a printed circuit board and an aluminum-nitride direct-bonded copper substrate. The dice are bonded on the insulated ceramic substrate by silver-sintering to ensure high thermal performance and joint reliability. The source, drain, and gate pads are connected by silver-sintering gold-plated pins or silver rods, which are aligned by holes or grooves in the circuit board. For electrical insulation and mechanical robustness, the space surrounding the device is filled by injecting and curing an underfill polymer. Electrical and thermal simulations of the package show that the half-bridge module has a 1.122 nH power-loop inductance, and 0.099 °C/W junction-to-case thermal resistance. Packages of the half-bridge module were fabricated, and their static and dynamic performances were characterized.

Packaging of a 10-kV Double-Side Cooled Silicon Carbide Diode Module With Thin Substrates Coated by a Nonlinear Resistive Polymer-Nanoparticle Composite

Medium-voltage silicon carbide (SiC) power modules are a critical component in grid-bound power conversion systems, and the packaging of these modules dictates the performance and reliability of the systems. One of the key issues for packaging the modules is managing the tradeoff between heat dissipation and insulation. To improve thermal performance without sacrificing insulation, a 10-kV SiC full-wave diode rectifier was designed and fabricated by incorporating double-sided cooling and wirebond-less interconnection and by utilizing thin alumina direct-bond copper substrates. To ensure that the substrates met insulation requirement, triple points on the substrates were coated by a nonlinear resistive polymer-nanoparticle composite to reduce electric field concentration. The nonlinear resistive coating increased the partial discharge inception voltage of the substrate with 0.5-mm thick alumina to 17.3 kV, an 84% improvement over that of the substrate without the coating. Electrical and thermal simulations of the module showed a low power loop inductance of 3.51 nH and a low junction-to-case thermal resistance of 0.114°C/W. The feasibility of the packaging techniques was demonstrated from successful fabrication and functional testing of the packagedmodule.

Ultra-thermostable embedded liquid cooling in SiC 3D packaging power modules of electric vehicles

As the enhanced acceleration performance of electric vehicles demands improved mileage capabilities, 3D packaging is a promising solution for developing high-power-density SiC power modules. However, 3D packaging causes instability owing to the increased heat flux and heat dissipation inside the power module. Thus, this paper proposes an ultra-thermostable embedded liquid cooling strategy for the thermal management of SiC 3D packaging power modules in electric vehicles. We constructed an embedded micro pin–fin (E-MPF) with non-uniform density in the SiC substrate to mount the SiC Schottky barrier diodes (SBDs). The thermostability and temperature uniformity of the SiC 3D packaging power modules containing E-MPF SiC and direct bond copper (DBC) substrates were verified. The results revealed that the SiC SBDs mounted on the E-MPF SiC substrate could stably operate up to a total power dissipation of 320 W at a 100 mL/min coolant (water) flow rate, whereas those attached on the DBC substrate could be operated only under 144 W (operation terminated at a maximum junction temperature of 175 ℃). Furthermore, the junction-to-coolant thermal resistance of the E-MPF SiC substrate could be reduced by 78.89 % compared to that of the DBC substrate. Moreover, the prepared 3D stacking package effectively improved the temperature uniformity of the SiC power modules by 68.69 %. The presented ultra-thermostable E-MPF substrate SiC power module can steadily render a motor torque elevation that is one magnitude greater than that required for sustaining the boiling-free condition on the DBC substrate. This embedded liquid-cooling architecture affords a feasible solution for the thermal management of upcoming high-power electric vehicle SiC inverters.

Aging investigation of the latest standard dual power modules using improved interconnect technologies by power cycling test

The latest standard “New Dual” power module has been developed for both silicon and silicon-carbide devices to meet the increasing demands in high reliability and high temperature power electronic applications. Due to the new package is just starting to release on the market, the reliability performance has not been fully studied. This paper investigates the power cycling capabilities of a 1.7 kV/1.8 kA IGBT power module based on the new package. Both the electrical and thermal performances have been studied before and after the power cycling. Neither the chips nor bond wires have noticeable degradation after 1.2 million cycles with ΔTj = 100 K and Tjmax = 150 °C. Nevertheless, the end-of-life criterion of an increased conduction voltage (Vce) in the test environment has been reached at about 600 k cycles. The further scanning acoustic microscopy test unveils that the fatigue site shifts from the conventional near-to-die interconnect (e.g., bond wire lift-offs) to the direct bond copper (DBC) substrate and baseplate layers. Considering the new package has more than ten times cycle life than the traditional power module, it expects the thermo-mechanical fatigue is not the life-limiting mechanism with further improved interconnect technologies. Meanwhile, as the previous bottlenecks (e.g., bond wires) are addressed, some new fatigue mechanisms, e.g., delamination of the DBC, become visible in this new package.

Identification of High Resolution Transient Thermal Network Model for Power Module Packages

A transient thermal network model is utilized to design and evaluate the transient thermal characteristics of power modules. Static test method identifies the transient thermal network model from the time response of the junction temperature, which is obtained by using the temperature dependency of I-V characteristics for power devices. This paper experimentally evaluates the effect of the sampling frequency and the resolution of the AD conversion on the accuracy of the obtained transient thermal network model. The high sampling frequency and the high resolution in the obtained time response of the junction temperature enable to clearly identify the transient thermal network model of the power module with the direct bonding copper substrate.

Characterization of a Nonlinear Resistive Polymer-Nanoparticle Composite Coating for Electric Field Reduction in a Medium-Voltage Power Module

Medium-voltage (MV) silicon carbide power devices are emerging and have the potential to serve in various power grid applications. However, the high blocking voltage and reduced size of the power modules require innovative insulation solutions. In this letter, a nonlinear resistive polymer-nanoparticle composite coating was characterized and, for the first time, demonstrated for reducing the high electric field at the triple point on a patterned substrate of a typical MV power module. The electrical properties of the coating were measured. Its conductivity showed nonlinear dependency on the applied electric field. The coating, which is about 20 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m thick, was applied along the triple-point edges on a patterned direct-bonded copper substrate. The partial discharge inception voltages of the coated substrates were measured in a transformer oil or in a silicone gel. The average increases in the inception voltage compared to the substrates without the coating were over 85% in the silicone gel. The increase agreed with the results of field simulations, which showed a 53% reduction of the maximum field in the silicone gel for a coated substrate. The processing simplicity and effectiveness of the coating present a cost-effective solution for insulating high-power-density, MV power modules.

Study on the Performance of Insulating Substrate Based on Silver Sintering

The reliability and stability of the insulating substrate are crucial for the service lifetime of high-power electronic devices. In this study, silver sintered copper (SSC) substrate was fabricated with silver paste as the bonding material, and its reliability was evaluated through simulations and experiments compared with the direct bonding copper (DBC) substrate. Under temperature cycling (−40 °C–125 °C), the stress distribution and changes of the SSC substrate and the DBC substrate are simulated by the finite element method. Further experimental tests were carried out to compare the shear strength and interfacial bonding rate of the two substrates. The results indicate that compared with DBC substrate, the stress of the SSC substrate is largely reduced, and the maximum stress is not at the vertices of the substrate. Moreover, the degradation of shear strength and interfacial connection rate of SSC substrate are much lower than that of DBC substrate. The SSC substrate has better reliability than the DBC substrate.

Packaging Design of 15 kV SiC Power Devices With High-Voltage Encapsulation

Silicon carbide (SiC) is capable of improving the blocking voltage of power devices. It is essential to package SiC power devices for high-voltage applications. This article proposes a high-voltage packaging method for &#x003E;15-kV SiC power devices by designing an optimized stacked direct bond copper (DBC) substrate with a copper ring and a field-dependent conductivity (FDC) encapsulation using SiC as the filler. The proposed FDC encapsulation is proven as a promising encapsulation as commercial silicone. The partial discharge initial voltage (PDIV) of the demonstrated module using the proposed method can be improved to at least 18.31 kV, which is increased by 163.77&#x0025; compared with the typical 34-mm insulated gate bipolar transistor (IGBT) modules. The maximum voltage rating of the demonstrated module could be enhanced to at least 15 kV with a sufficient protection margin, i.e., 20&#x0025;. It gives guidance to the packaging of &#x003E;15-kV SiC power devices.

A Stress-Relieved Method Based on Bottom Pattern Design Considering Thermal and Mechanical Behavior of DBC Substrate

Traditionally, reliability-improved methods like etched dimples are set at the edge of the copper pattern to relieve the thermal-mechanical stress on direct bonded copper (DBC). When the copper ratio of the top copper pattern is less than the copper ratio of the bottom copper pattern, these methods are less effective at the inner edges of the top ceramic interface where the initial crack appears. Currently, no explanation is found for this phenomenon and other solutions should be taken to this problem. Hence, this paper analyzed the mechanism of the phenomenon and proposed a bottom pattern design to solve the problem. Based on a bi-material model and FE analysis, it can be found that the stress concentration on the top ceramic interface is caused by the opposite bending actions on the two ceramic interfaces. To eliminate the geometrical difference between two copper layers, Parallel dimple traces are used on the bottom instead of mirror-copied gap on the consideration of thermal resistance and mechanical strength. The parameters of dimple trace were chosen by simulations on stress-strain results under thermal cycling and thermal resistance calculations in a module packaging. FE results show that the bottom dimple trace with a diameter of 0.4mm and a gap of 2mm can adjust the thermal-induced strain behavior of the singularities at the top copper to be even and sacrifices only a 2% thermal resistance rise of the module packaging. Life prediction model based on strain shows that the bottom design can improve the life time of the substrate. The thermal cycling test result on sample DBC and thermal resistance measurement on module packaged by DBC with bottom pattern design verify the FE analysis. This method can be widely used on the packaging design of power module using stand-alone DBC substrate.

Effect of Ald Al2o3 Passivation Layer Thickness on the Surface Treatment of Direct-Bonded Copper Substrates: Wetting, Oxidation, and Mechanical Property Analyses

Effect of Ald Al2o3 Passivation Layer Thickness on the Surface Treatment of Direct-Bonded Copper Substrates: Wetting, Oxidation, and Mechanical Property Analyses

Assembly Reliability and Molding Material Comparison of Miniature Integrated High Power Module With Insulated Metal Substrate

Abstract Given the increasing demand for power density and lightweight specifications, the discrete transistor outline-type package is no longer sufficient for personal vehicle. The new generation of high-power drive needs excellent heat dissipation and miniaturized system simultaneously. However, a traditional architecture of power module, direct bonding copper substrate, has serious warpage deformation and limitation of the heat dissipation. Therefore, a power module with an insulated metal substrate (IMS) is proposed. The proposed power module has a smaller volume, better electrical and thermal performance, and high reliability to be utilized in personal vehicles. A fine-quality assembly process is also presented and verified. Furthermore, two different kinds of molding materials that are widely used in power modules, silicone gel, and epoxy, are utilized. The IMS-type module with silicone gel molding fails the temperature cycling test (TCT) with the delamination of the solder layer. The module with epoxy successfully passes the automotive-grade reliability tests, including TCT, highly accelerated stress test, high-temperature reverse bias, and intermittent operational life test according to the standard of AEC-Q101. The finite element analysis for the IMS power module is presented and analyzed under the condition of TCT to estimate the mechanical behavior of the solder layer. The equivalent plastic strain of solder layer with silicone gel and epoxy is 0.76 and 0.08, respectively, after TCT, separately. The main reason can be attributed to the coefficient of thermal expansion between the IMS and molding material. According to the analyzed results, the effect of molding material should not be ignored in the power modulus.

Interface reaction and evolution of micron-sized Ag particles paste joining on electroless Ni-/Pd-/Au-finished DBA and DBC substrates during extreme thermal shock test

The fracture behaviors and Ag–Au joint interface evolution of sintered micron-sized Ag particles paste joined on an electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) and direct-bonded copper (DBC) substrates are evaluated during an extreme thermal shock test (TST) from −50–250 °C. The die shear strength of the as-sintered Ag joint on the DBA substrate is evaluated to be 33.5 MPa at a sintering temperature of 200 °C without any assisted pressure, which decreased gradually with an increase in the thermal cycling number. The shear strength declined slightly but remained at approximately 20 MPa; subsequently, it decreased considerably to 11.2 MPa after 1000 cycles. Coarsening of the sintered Ag layer is observed as the microstructure inhomogeneity and vertical cracks increased after 1000 cycles. In addition, the Al layer induced a greater undulate deformation, resulting in a sintered Ag layer exhibiting partial compression and tension after a TST. The sintered Ag layer became dense with a significant decrease in porosity at the compression parts and large horizontal cracks appeared at the tension parts. Both of horizontal cracks and vertical cracks led to fracture mode change and shear strength decrease. The Ag–Au joint interdiffusion layer became thicker during thermal shock with the gradual diffusion of the Au atoms into the sintered Ag layer. The die shear strength of the as-sintered Ag joint on the DBC substrates is evaluated as 34.4 MPa at 200 °C sintering but decreased to 0 MPa after 250 thermal shock cycles owing to the stress-induced delamination between the Ag and Au interdiffusion layer and sintered Ag layer. This study provides insights into the interface reaction and evolution of sintered Ag on ENEPIG-finished DBA and DBC substrates for applications at high temperatures.